Fix the About panel not always opening on macOS 14.4+.

Also, bump the patch version number and copyright years. (Should have
done that before tagging v0.4.1, so jump to v0.4.2.)

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,50 @@
+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: bug
+assignees: ''
+
+---
+
+## Example bug report template
+
+> Don't worry if you have trouble getting some of this info. Just leave it out.
+
+**Description of the bug**
+> Please don't just say it's "not working".
+
+**Steps to reproduce**
+> Steps to reproduce the bug. This usually doesn't need to be super detailed.
+1. Go to '...'
+2. Click on '...'
+3. See error message '...'
+
+**Versions**
+> Please complete the following information.
+ - Background Music: [e.g. "0.4.0" or "0.4.0-SNAPSHOT-c0ab98b". `Preferences > About Background Music`]
+ - macOS: [e.g. "11.3 Beta (20E5172i)" or "Big Sur". ` > About This Mac`]
+
+**Hardware**
+> Delete this part if you think it's probably not necessary.
+ - Computer: [e.g. "MacBook Pro (13-inch, 2016, Four Thunderbolt 3 Ports)". ` > About This Mac`]
+ - Audio Device: [e.g. "Built-in Output. Manufacturer: Apple Inc. Output Channels: 2 [...]". `System Information app > Hardware > Audio`]
+
+**Debug logs**
+> If you think the developers might not be able to reproduce the bug on their computers, e.g. because an important feature is completely broken and they would have noticed, it can help to include [debug logs](https://github.com/kyleneideck/BackgroundMusic/wiki/Getting-Debug-Logs). This takes a little effort, so feel free to leave it out at first.
+
+[Debug logs attached here](https://github.com/example/background-music-debug-logs.txt)
+
+**Other info**
+> Anything else you want to add?
+
+---
+
+> Tips
+> (Delete this section before posting.)
+>  - https://github.com/kyleneideck/BackgroundMusic#troubleshooting
+>  - Try the latest SNAPSHOT version from https://github.com/kyleneideck/BackgroundMusic/releases
+>     - Especially if you're using macOS Big Sur.
+>  - If your bug is one of common issues, consider leaving a comment or a +1 (👍) on an existing issue:
+>     - Background Music currently only supports audio devices with two channels. Bluetooth devices often only have one.
+>     - Volumes having no effect for certain apps: Microsoft Teams ([workaround](https://github.com/kyleneideck/BackgroundMusic/issues/268#issuecomment-604977210)), Zoom ([workaround](https://github.com/kyleneideck/BackgroundMusic/issues/396#issuecomment-741992157)), Discord ([workaround](https://github.com/kyleneideck/BackgroundMusic/issues/210#issuecomment-507048957), [see also](https://github.com/kyleneideck/BackgroundMusic/issues/267#issuecomment-617327850)), Chrome (sometimes)

.github/ISSUE_TEMPLATE/other.md

@@ -0,0 +1,10 @@
+---
+name: Other
+about: Feature request, question, support request or anything else
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+> There's no template for this issue type. I just wanted to make it clear that it's OK to submit other types of issues.

.github/workflows/build-test-release.yml

@@ -0,0 +1,216 @@
+# TODO: Split this into multiple .yml files? Multiple jobs?
+name: Build, Test and Release
+
+on:
+  push:
+    branches:
+      - '*'
+    tags:
+      - '*'
+  pull_request:
+    branches:
+      - '*'
+
+jobs:
+  # Build and test in the same job because the UI tests expect BGMDriver to be installed.
+  build-and-test:
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 30
+    strategy:
+      matrix:
+        # TODO: Add older macOS versions.
+        os:
+          - macos-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+      - name: Work in a case-sensitive disk image.
+        # This lets us catch failures that only happen on case-sensitive filesystems.
+        run: |
+          hdiutil create \
+            -type SPARSEBUNDLE \
+            -fs 'Case-sensitive Journaled HFS+' \
+            -volname bgmbuild \
+            -nospotlight \
+            -verbose \
+            -attach \
+            -size 100m \
+            bgmbuild.dmg
+          sudo cp -r . /Volumes/bgmbuild
+          cd /Volumes/bgmbuild
+      - name: Install coreutils for actions/runner/issues/884 workaround.
+        # See https://github.com/actions/runner/issues/884#issuecomment-1018851327
+        run: brew install coreutils
+      - name: Build and install Background Music.
+        run: |
+          # `sudo` and `tput` expect this to be set.
+          export TERM=xterm-256color
+          genv --default-signal=PIPE yes | sudo ./build_and_install.sh
+      - name: Print the log file.
+        if: always()
+        run: cat build_and_install.log
+      - name: Log some checksums.
+        run: 'find */build/Release/*/ -type f -exec md5 {} \;'
+      - name: Log the installed audio devices and their IDs.
+        run: |
+          system_profiler SPAudioDataType
+          say -a '?'
+      - name: Check the BGM dirs and files were installed.
+        run: |
+          # These commands fail if the dir/file isn't found.
+          ls -la "/Applications/Background Music.app"
+          ls -la "/Library/Audio/Plug-Ins/HAL/Background Music Device.driver"
+          ls -la "/usr/local/libexec/BGMXPCHelper.xpc" \
+            || ls -la "/Library/Application Support/Background Music/BGMXPCHelper.xpc"
+          ls -la "/Library/LaunchDaemons/com.bearisdriving.BGM.XPCHelper.plist"
+      - name: Close BGMApp (which the install script opened).
+        run: >-
+          osascript -e 'tell application "Background Music" to quit'
+          || killall "Background Music"
+      - name: Skip the UI tests. (They don't work on GitHub Actions yet.)
+        run: BGMApp/BGMAppTests/UITests/skip-ui-tests.py
+      - name: Run the tests.
+        run: |
+          echo '::group::BGMDriver Tests'
+          xcodebuild \
+            -quiet \
+            -workspace BGM.xcworkspace \
+            -scheme 'Background Music Device' \
+            test
+          echo '::endgroup::'
+
+          echo '::group::BGMXPCHelper Tests'
+          xcodebuild \
+            -quiet \
+            -workspace BGM.xcworkspace \
+            -scheme 'BGMXPCHelper' \
+            test
+          echo '::endgroup::'
+
+          # Grant BGMApp authorization to use input devices.
+          # This is necessary for the UI tests because accepting the "Background Music would like to
+          # use the microphone" dialog programmatically isn't reliable.
+          # TODO: Commented out because we would need to generate the csreq (codesign signature)
+          #       value to match the BGMApp bundle the tests will run against.
+          # dbPath="$HOME/Library/Application Support/com.apple.TCC/TCC.db"
+          # values="'kTCCServiceMicrophone','com.bearisdriving.BGM.App',0,2,2,1,X'FADE0C000000004800000001000000070000000800000014545ABE68FAF437700B14984BB24117EDDA1BBF2C0000000800000014386FB63B9CD6BA6E83CEDEAF4EDEE177C1FAEA92',NULL,NULL,'UNUSED',NULL,0,1652845317"
+          # sqlQuery="INSERT OR IGNORE INTO access VALUES($values);"
+          # sqlite3 "$dbPath" "$sqlQuery" || (echo "Failed to modify $dbPath"; exit 1)
+          # # Log the added TCC.db entry.
+          # sqlite3 "$dbPath" "select * from access where client like '%BGM%';"
+
+          echo '::group::BGMApp Tests'
+          # TODO: Commented out in case it uses too much CPU.
+          # log stream --info \
+          #   --predicate 'process == "coreaudiod" or
+          #                process == "Background Music" or
+          #                process == "BGMXPCHelper" or
+          #                composedMessage contains[cd] "Background Music" or
+          #                composedMessage contains "BGM"' > app.log &
+          xcodebuild \
+            -quiet \
+            -workspace BGM.xcworkspace \
+            -scheme 'Background Music' \
+            test
+          echo '::endgroup::'
+      - name: Upload the test results.
+        if: always()
+        uses: actions/upload-artifact@v3
+        with:
+          name: bgm-test-results
+          path: |
+            /Users/runner/Library/Developer/Xcode/DerivedData/*/Logs/Test/*.xcresult
+            app.log
+            /Users/runner/Library/Logs/CrashReporter/*
+            /Users/runner/Library/Logs/DiagnosticReports/*
+      - name: Uninstall Background Music.
+        run: |
+          # `tput` expects this to be set.
+          export TERM=xterm-256color
+          genv --default-signal=PIPE yes | sudo ./uninstall.sh
+      - name: Check the BGM dirs and files were removed.
+        run: |
+          if ls -la "/Applications/Background Music.app"; then exit 1; fi
+          if ls -la "/Library/Audio/Plug-Ins/HAL/Background Music Device.driver"; then exit 1; fi
+          if ls -la "/usr/local/libexec/BGMXPCHelper.xpc"; then exit 1; fi
+          if ls -la "/Library/Application Support/Background Music/BGMXPCHelper.xpc"; then
+            exit 1
+          fi
+          if ls -la "/Library/LaunchDaemons/com.bearisdriving.BGM.XPCHelper.plist"; then exit 1; fi
+  release:
+    runs-on: macos-latest
+    timeout-minutes: 15
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+      - name: Build the .pkg installer.
+        run: |
+          # `sudo` and `tput` expect this to be set.
+          export TERM=xterm-256color
+          # If this build is for a tag with "DEBUG" in its name, build a debug package. (More
+          # detailed logging, no optimization, etc.)
+          if [[ "$GITHUB_REF" =~ .*DEBUG.* ]]; then
+            sudo ./package.sh -d
+          else
+            sudo ./package.sh
+          fi
+      - name: Install the .pkg.
+        # Delete archives/ first because it contains a copy of Background Music.app.
+        # Background Music.app is "relocatable", which means that if the user moves it and then
+        # installs a new version, macOS will put the new version in the same place. This makes sure
+        # the installer puts Background Music.app in /Applications so the build won't fail when we
+        # check that later.
+        #
+        # package.sh puts the archives in a zipfile next to the .pkg, so we can still upload them
+        # after deleting the directory here.
+        #
+        # TODO: On TravisCI, this was failing for debug builds. We couldn't figure out why, so we
+        #       might have to ignore that with
+        #       || [[ "$GITHUB_REF" =~ .*DEBUG.* ]]
+        run: |
+          sudo rm -rf archives
+          sudo installer \
+            -pkg Background-Music-*/BackgroundMusic-*.pkg \
+            -target / \
+            -verbose \
+            -dumplog
+      - name: Print the installer logs.
+        if: always()
+        # This trims the start of the log to save space.
+        run: grep -E -A 9999 -B 20 'Background.?Music' /var/log/install.log
+      - name: Check the BGM dirs and files were installed.
+        if: always()
+        run: |
+          ls -la "/Applications/Background Music.app"
+          ls -la "/Library/Audio/Plug-Ins/HAL/Background Music Device.driver"
+          ls -la "/usr/local/libexec/BGMXPCHelper.xpc" \
+            || ls -la "/Library/Application Support/Background Music/BGMXPCHelper.xpc"
+          ls -la "/Library/LaunchDaemons/com.bearisdriving.BGM.XPCHelper.plist"
+      - name: Upload the .pkg installer and archives.
+        if: always()
+        uses: actions/upload-artifact@v3
+        with:
+          name: pkg-installer
+          path: Background-Music-*
+      - name: Upload the log file from the package.sh build.
+        if: always()
+        uses: actions/upload-artifact@v3
+        with:
+          name: build-and-install-log-for-pkg
+          path: build_and_install.log
+# TODO: Create a GitHub release. This is the Travis YAML that was handling it:
+# deploy:
+#  provider: releases
+#  api_key:
+#    secure: j5Gd[...]
+#  file_glob: true
+#  file: Background-Music-*/*
+#  skip_cleanup: true
+#  name: $TRAVIS_TAG
+#  prerelease: true
+#  draft: true
+#  on:
+#    repo: kyleneideck/BackgroundMusic
+#    tags: true
+#    # TODO: Use "condition" to build master and tags?
+#    condition: $DEPLOY = true

.gitignore

@@ -7,6 +7,9 @@ tags
 cmake-build-debug/
 /Background-Music-*/
 BGM.xcworkspace/xcshareddata/IDEWorkspaceChecks.plist
+Images/*.aux
+Images/*.log
+/archives/
 
 # Everything below is from https://github.com/github/gitignore/blob/master/Objective-C.gitignore
 

.travis.yml

@@ -1,109 +0,0 @@
-language: objective-c
-matrix:
-  include:
-    - os: osx
-      osx_image: xcode10
-      xcode_sdk: macosx10.14
-      sudo: required
-      env: DEPLOY=true
-    - os: osx
-      osx_image: xcode9.4
-      xcode_sdk: macosx10.13
-      sudo: required
-    - os: osx
-      osx_image: xcode9.3
-      xcode_sdk: macosx10.13
-      sudo: required
-    - os: osx
-      osx_image: xcode9.2
-      xcode_sdk: macosx10.13
-      sudo: required
-    - os: osx
-      osx_image: xcode8.3
-      xcode_sdk: macosx10.12
-      sudo: required
-    - os: osx
-      osx_image: xcode7.3
-      xcode_sdk: macosx10.11
-      sudo: required
-      env: PACKAGE=false
-# branches:
-#   only:
-#     - master
-install:
-# Install Apple's NullAudio device. Travis' VMs don't have any audio devices installed.
-  - sudo xcodebuild -project BGMApp/BGMAppTests/NullAudio/AudioDriverExamples.xcodeproj -target NullAudio DSTROOT="/" install
-  - sudo launchctl kickstart -kp system/com.apple.audio.coreaudiod || sudo killall coreaudiod
-script:
-# Build in a case-sensitive disk image to catch failures that only happen on case-sensitive filesystems.
-  - hdiutil create -type SPARSEBUNDLE -fs 'Case-sensitive Journaled HFS+' -volname bgmbuild -nospotlight -verbose -attach -size 100m bgmbuild.dmg
-  - sudo cp -r . /Volumes/bgmbuild
-  - cd /Volumes/bgmbuild
-# Install Background Music.
-  - yes | ./build_and_install.sh
-  - cat build_and_install.log
-  - find */build/Release/*/ -type f -exec md5 {} \;
-# Log the installed audio devices...
-  - system_profiler SPAudioDataType
-# ...and their IDs.
-  - say -a '?'
-# Check the BGM dirs and files were installed. (These fail if the dir/file isn't found.)
-  - ls -la "/Applications/Background Music.app"
-  - ls -la "/Library/Audio/Plug-Ins/HAL/Background Music Device.driver"
-  - ls -la "/usr/local/libexec/BGMXPCHelper.xpc" || ls -la "/Library/Application Support/Background Music/BGMXPCHelper.xpc"
-  - ls -la "/Library/LaunchDaemons/com.bearisdriving.BGM.XPCHelper.plist"
-# Close BGMApp (which the install script opened).
-#
-# The killall fallback command is necessary because the AppleScript gets "user canceled" on Travis'
-# Xcode 9 images for some reason.
-  - osascript -e 'tell application "Background Music" to quit' || killall "Background Music"
-# Skip the UI tests until Travis has support for them.
-  - BGMApp/BGMAppTests/UITests/travis-skip.py
-# Run the tests.
-  - xcodebuild -workspace BGM.xcworkspace -scheme 'Background Music Device' test
-  - xcodebuild -workspace BGM.xcworkspace -scheme 'Background Music' test
-  - xcodebuild -workspace BGM.xcworkspace -scheme 'BGMXPCHelper' test
-# Uninstall Background Music.
-  - yes | ./uninstall.sh
-# Check the BGM dirs and files were removed.
-  - if ls -la "/Applications/Background Music.app"; then false; fi
-  - if ls -la "/Library/Audio/Plug-Ins/HAL/Background Music Device.driver"; then false; fi
-  - if ls -la "/usr/local/libexec/BGMXPCHelper.xpc"; then false; fi
-  - if ls -la "/Library/Application Support/Background Music/BGMXPCHelper.xpc"; then false; fi
-  - if ls -la "/Library/LaunchDaemons/com.bearisdriving.BGM.XPCHelper.plist"; then false; fi
-# Return early if we're not testing packaging on this OS X version.
-  - if [[ "$PACKAGE" == "false" ]]; then exit 0; fi
-# Build the .pkg installer. Print the build logs if it fails. If this build is for a tag with
-# "DEBUG" in its name, build a debug package. (More detailed logging, no optimization, etc.)
-  - if [[ "$TRAVIS_TAG" =~ .*DEBUG.* ]]; then
-        ./package.sh -d || (cat build_and_install.log && travis_terminate 1);
-    else
-        ./package.sh || (cat build_and_install.log && travis_terminate 1);
-    fi
-# Install the .pkg.
-  - sudo installer -pkg Background-Music-*/BackgroundMusic-*.pkg -target / -verbose -dumplog
-# Check the BGM dirs and files were installed again.
-  - ls -la "/Applications/Background Music.app"
-  - ls -la "/Library/Audio/Plug-Ins/HAL/Background Music Device.driver"
-  - ls -la "/usr/local/libexec/BGMXPCHelper.xpc" || ls -la "/Library/Application Support/Background Music/BGMXPCHelper.xpc"
-  - ls -la "/Library/LaunchDaemons/com.bearisdriving.BGM.XPCHelper.plist"
-# Post on IRC when Travis builds finish.
-notifications:
-  irc: "irc.freenode.org#backgroundmusic"
-# Upload the .pkg and dSYM zip to GitHub.
-deploy:
-  provider: releases
-  api_key:
-    secure: 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
-  file_glob: true
-  file: Background-Music-*/*
-  skip_cleanup: true
-  name: $TRAVIS_TAG
-  prerelease: true
-  on:
-    repo: kyleneideck/BackgroundMusic
-    tags: true
-    # TODO: Use "condition" to build master and tags?
-    condition: $DEPLOY = true
-
-

BGMApp/BGMApp.xcodeproj/xcshareddata/xcschemes/BGMXPCHelper.xcscheme

@@ -26,9 +26,42 @@
       buildConfiguration = "Debug"
       selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
       selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
+      shouldUseLaunchSchemeArgsEnv = "YES"
       enableAddressSanitizer = "YES"
-      language = ""
-      shouldUseLaunchSchemeArgsEnv = "YES">
+      enableASanStackUseAfterReturn = "YES"
+      enableUBSanitizer = "YES"
+      codeCoverageEnabled = "YES"
+      onlyGenerateCoverageForSpecifiedTargets = "YES">
+      <MacroExpansion>
+         <BuildableReference
+            BuildableIdentifier = "primary"
+            BlueprintIdentifier = "27379B8E1C7F57DA0084A24C"
+            BuildableName = "BGMXPCHelper.xpc"
+            BlueprintName = "BGMXPCHelper"
+            ReferencedContainer = "container:BGMApp.xcodeproj">
+         </BuildableReference>
+      </MacroExpansion>
+      <AdditionalOptions>
+         <AdditionalOption
+            key = "NSZombieEnabled"
+            value = "YES"
+            isEnabled = "YES">
+         </AdditionalOption>
+         <AdditionalOption
+            key = "MallocScribble"
+            value = ""
+            isEnabled = "YES">
+         </AdditionalOption>
+      </AdditionalOptions>
+      <CodeCoverageTargets>
+         <BuildableReference
+            BuildableIdentifier = "primary"
+            BlueprintIdentifier = "27379B8E1C7F57DA0084A24C"
+            BuildableName = "BGMXPCHelper.xpc"
+            BlueprintName = "BGMXPCHelper"
+            ReferencedContainer = "container:BGMApp.xcodeproj">
+         </BuildableReference>
+      </CodeCoverageTargets>
       <Testables>
          <TestableReference
             skipped = "NO">
@@ -41,24 +74,14 @@
             </BuildableReference>
          </TestableReference>
       </Testables>
-      <MacroExpansion>
-         <BuildableReference
-            BuildableIdentifier = "primary"
-            BlueprintIdentifier = "27379B8E1C7F57DA0084A24C"
-            BuildableName = "BGMXPCHelper.xpc"
-            BlueprintName = "BGMXPCHelper"
-            ReferencedContainer = "container:BGMApp.xcodeproj">
-         </BuildableReference>
-      </MacroExpansion>
-      <AdditionalOptions>
-      </AdditionalOptions>
    </TestAction>
    <LaunchAction
       buildConfiguration = "Debug"
       selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
       selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
       enableAddressSanitizer = "YES"
-      language = ""
+      enableASanStackUseAfterReturn = "YES"
+      enableUBSanitizer = "YES"
       launchStyle = "0"
       useCustomWorkingDirectory = "NO"
       ignoresPersistentStateOnLaunch = "NO"
@@ -82,6 +105,16 @@
          </EnvironmentVariable>
       </EnvironmentVariables>
       <AdditionalOptions>
+         <AdditionalOption
+            key = "NSZombieEnabled"
+            value = "YES"
+            isEnabled = "YES">
+         </AdditionalOption>
+         <AdditionalOption
+            key = "MallocScribble"
+            value = ""
+            isEnabled = "YES">
+         </AdditionalOption>
       </AdditionalOptions>
    </LaunchAction>
    <ProfileAction

BGMApp/BGMApp.xcodeproj/xcshareddata/xcschemes/Background Music.xcscheme

@@ -26,11 +26,47 @@
       buildConfiguration = "Debug"
       selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
       selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
+      shouldUseLaunchSchemeArgsEnv = "YES"
       enableAddressSanitizer = "YES"
-      shouldUseLaunchSchemeArgsEnv = "YES">
+      enableASanStackUseAfterReturn = "YES"
+      enableUBSanitizer = "YES"
+      codeCoverageEnabled = "YES"
+      onlyGenerateCoverageForSpecifiedTargets = "YES">
+      <MacroExpansion>
+         <BuildableReference
+            BuildableIdentifier = "primary"
+            BlueprintIdentifier = "1CB8B3351BBA75EF000E2DD1"
+            BuildableName = "Background Music.app"
+            BlueprintName = "Background Music"
+            ReferencedContainer = "container:BGMApp.xcodeproj">
+         </BuildableReference>
+      </MacroExpansion>
+      <AdditionalOptions>
+         <AdditionalOption
+            key = "NSZombieEnabled"
+            value = "YES"
+            isEnabled = "YES">
+         </AdditionalOption>
+         <AdditionalOption
+            key = "MallocScribble"
+            value = ""
+            isEnabled = "YES">
+         </AdditionalOption>
+      </AdditionalOptions>
+      <CodeCoverageTargets>
+         <BuildableReference
+            BuildableIdentifier = "primary"
+            BlueprintIdentifier = "1CB8B3351BBA75EF000E2DD1"
+            BuildableName = "Background Music.app"
+            BlueprintName = "Background Music"
+            ReferencedContainer = "container:BGMApp.xcodeproj">
+         </BuildableReference>
+      </CodeCoverageTargets>
       <Testables>
          <TestableReference
-            skipped = "NO">
+            skipped = "NO"
+            parallelizable = "YES"
+            testExecutionOrdering = "random">
             <BuildableReference
                BuildableIdentifier = "primary"
                BlueprintIdentifier = "2743C9F51D86CFF90089613B"
@@ -50,23 +86,14 @@
             </BuildableReference>
          </TestableReference>
       </Testables>
-      <MacroExpansion>
-         <BuildableReference
-            BuildableIdentifier = "primary"
-            BlueprintIdentifier = "1CB8B3351BBA75EF000E2DD1"
-            BuildableName = "Background Music.app"
-            BlueprintName = "Background Music"
-            ReferencedContainer = "container:BGMApp.xcodeproj">
-         </BuildableReference>
-      </MacroExpansion>
-      <AdditionalOptions>
-      </AdditionalOptions>
    </TestAction>
    <LaunchAction
       buildConfiguration = "Debug"
       selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
       selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
       enableAddressSanitizer = "YES"
+      enableASanStackUseAfterReturn = "YES"
+      enableUBSanitizer = "YES"
       launchStyle = "0"
       useCustomWorkingDirectory = "NO"
       ignoresPersistentStateOnLaunch = "NO"
@@ -101,6 +128,16 @@
          </EnvironmentVariable>
       </EnvironmentVariables>
       <AdditionalOptions>
+         <AdditionalOption
+            key = "NSZombieEnabled"
+            value = "YES"
+            isEnabled = "YES">
+         </AdditionalOption>
+         <AdditionalOption
+            key = "MallocScribble"
+            value = ""
+            isEnabled = "YES">
+         </AdditionalOption>
       </AdditionalOptions>
    </LaunchAction>
    <ProfileAction

BGMApp/BGMApp/BGMApp-Debug.entitlements

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>com.apple.security.automation.apple-events</key>
+	<true/>
+	<key>com.apple.security.device.audio-input</key>
+	<true/>
+    <!--
+    Without this key, AddressSanitizer and the UI tests would only work when BGMApp is code signed.
+    -->
+	<key>com.apple.security.cs.disable-library-validation</key>
+	<true/>
+</dict>
+</plist>

BGMApp/BGMApp/BGMApp.entitlements

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>com.apple.security.automation.apple-events</key>
+	<true/>
+	<key>com.apple.security.device.audio-input</key>
+	<true/>
+</dict>
+</plist>

BGMApp/BGMApp/BGMAppDelegate.h

@@ -17,13 +17,15 @@
 //  BGMAppDelegate.h
 //  BGMApp
 //
-//  Copyright © 2016, 2017 Kyle Neideck
+//  Copyright © 2016, 2017, 2020 Kyle Neideck
+//  Copyright © 2021 Marcus Wu
 //
 //  Sets up and tears down the app.
 //
 
 // Local Includes
 #import "BGMAudioDeviceManager.h"
+#import "BGMAppVolumesController.h"
 
 // System Includes
 #import <Cocoa/Cocoa.h>
@@ -50,8 +52,10 @@ static NSInteger const kSeparatorBelowVolumesMenuItemTag = 4;
 @property (unsafe_unretained) IBOutlet NSTextView* aboutPanelLicenseView;
 
 @property (weak) IBOutlet NSMenuItem* autoPauseMenuItemUnwrapped;
+@property (weak) IBOutlet NSMenuItem* debugLoggingMenuItemUnwrapped;
 
 @property (readonly) BGMAudioDeviceManager* audioDevices;
+@property BGMAppVolumesController* appVolumes;
 
 @end
 

BGMApp/BGMApp/BGMAppDelegate.mm

@@ -17,59 +17,68 @@
 //  BGMAppDelegate.mm
 //  BGMApp
 //
-//  Copyright © 2016-2018 Kyle Neideck
+//  Copyright © 2016-2022 Kyle Neideck
+//  Copyright © 2021 Marcus Wu
 //
 
-// Self Includes
+// Self Include
 #import "BGMAppDelegate.h"
 
 // Local Includes
 #import "BGM_Utils.h"
-#import "BGMUserDefaults.h"
-#import "BGMMusicPlayers.h"
+#import "BGMAppVolumes.h"
+#import "BGMAppVolumesController.h"
 #import "BGMAutoPauseMusic.h"
 #import "BGMAutoPauseMenuItem.h"
-#import "BGMSystemSoundsVolume.h"
-#import "BGMAppVolumesController.h"
-#import "BGMPreferencesMenu.h"
-#import "BGMXPCListener.h"
+#import "BGMDebugLoggingMenuItem.h"
+#import "BGMMusicPlayers.h"
+#import "BGMOutputDeviceMenuSection.h"
 #import "BGMOutputVolumeMenuItem.h"
+#import "BGMPreferencesMenu.h"
+#import "BGMPreferredOutputDevices.h"
+#import "BGMStatusBarItem.h"
+#import "BGMSystemSoundsVolume.h"
 #import "BGMTermination.h"
+#import "BGMUserDefaults.h"
+#import "BGMXPCListener.h"
 #import "SystemPreferences.h"
 
-// PublicUtility Includes
-#import "CAPropertyAddress.h"
-
 // System Includes
 #import <AVFoundation/AVCaptureDevice.h>
 
 
 #pragma clang assume_nonnull begin
 
-static float const     kStatusBarIconPadding = 0.25;
 static NSString* const kOptNoPersistentData  = @"--no-persistent-data";
 static NSString* const kOptShowDockIcon      = @"--show-dock-icon";
 
 @implementation BGMAppDelegate {
-    // The button in the system status bar (the bar with volume, battery, clock, etc.) to show the main menu
-    // for the app. These are called "menu bar extras" in the Human Interface Guidelines.
-    NSStatusItem* statusBarItem;
+    // The button in the system status bar that shows the main menu.
+    BGMStatusBarItem* statusBarItem;
     
     // Only show the 'BGMXPCHelper is missing' error dialog once.
     BOOL haveShownXPCHelperErrorMessage;
-    
+
+    // Persistently stores user settings and data.
+    BGMUserDefaults* userDefaults;
+
     BGMAutoPauseMusic* autoPauseMusic;
     BGMAutoPauseMenuItem* autoPauseMenuItem;
     BGMMusicPlayers* musicPlayers;
     BGMSystemSoundsVolume* systemSoundsVolume;
-    BGMAppVolumesController* appVolumes;
+    BGMOutputDeviceMenuSection* outputDeviceMenuSection;
     BGMPreferencesMenu* prefsMenu;
+    BGMDebugLoggingMenuItem* debugLoggingMenuItem;
     BGMXPCListener* xpcListener;
+    BGMPreferredOutputDevices* preferredOutputDevices;
 }
 
 @synthesize audioDevices = audioDevices;
+@synthesize appVolumes = appVolumes;
 
 - (void) awakeFromNib {
+    [super awakeFromNib];
+    
     // Show BGMApp in the dock, if the command-line option for that was passed. This is used by the
     // UI tests.
     if ([NSProcessInfo.processInfo.arguments indexOfObject:kOptShowDockIcon] != NSNotFound) {
@@ -78,72 +87,19 @@ static NSString* const kOptShowDockIcon      = @"--show-dock-icon";
     
     haveShownXPCHelperErrorMessage = NO;
 
-    [self initStatusBarItem];
-}
-
-// Set up the status bar item. (The thing you click to show BGMApp's UI.)
-- (void) initStatusBarItem {
-    statusBarItem = [[NSStatusBar systemStatusBar] statusItemWithLength:NSSquareStatusItemLength];
-
-    // NSStatusItem doesn't have the "button" property on OS X 10.9.
-    BOOL buttonAvailable = (floor(NSAppKitVersionNumber) >= NSAppKitVersionNumber10_10);
-
-    // Set the title/tooltip to "Background Music".
-    statusBarItem.title = [NSRunningApplication currentApplication].localizedName;
-    statusBarItem.toolTip = statusBarItem.title;
-
-    if (buttonAvailable) {
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wpartial-availability"
-        statusBarItem.button.accessibilityLabel = statusBarItem.title;
-#pragma clang diagnostic pop
+    // Set up audioDevices, which coordinates BGMDevice and the output device. It manages
+    // playthrough, volume/mute controls, etc.
+    if (![self initAudioDeviceManager]) {
+        return;
     }
 
-    // Set the icon.
-    NSImage* icon = [NSImage imageNamed:@"FermataIcon"];
+    // Stored user settings
+    userDefaults = [self createUserDefaults];
 
-    if (icon != nil) {
-        NSRect statusBarItemFrame;
-
-        if (buttonAvailable) {
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wpartial-availability"
-            statusBarItemFrame = statusBarItem.button.frame;
-#pragma clang diagnostic pop
-        } else {
-            // OS X 10.9 fallback. I haven't tested this (or anything else on 10.9).
-            statusBarItemFrame = statusBarItem.view.frame;
-        }
-
-        CGFloat lengthMinusPadding = statusBarItemFrame.size.height * (1 - kStatusBarIconPadding);
-        [icon setSize:NSMakeSize(lengthMinusPadding, lengthMinusPadding)];
-
-        // Make the icon a "template image" so it gets drawn colour-inverted when it's highlighted or the status
-        // bar's in dark mode
-        [icon setTemplate:YES];
-
-        if (buttonAvailable) {
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wpartial-availability"
-            statusBarItem.button.image = icon;
-#pragma clang diagnostic pop
-        } else {
-            statusBarItem.image = icon;
-        }
-    } else {
-        // If our icon is missing for some reason, fallback to a fermata character (1D110)
-        if (buttonAvailable) {
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wpartial-availability"
-            statusBarItem.button.title = @"𝄐";
-#pragma clang diagnostic pop
-        } else {
-            statusBarItem.title = @"𝄐";
-        }
-    }
-    
-    // Set the main menu
-    statusBarItem.menu = self.bgmMenu;
+    // Add the status bar item. (The thing you click to show BGMApp's main menu.)
+    statusBarItem = [[BGMStatusBarItem alloc] initWithMenu:self.bgmMenu
+                                              audioDevices:audioDevices
+                                              userDefaults:userDefaults];
 }
 
 - (void) applicationDidFinishLaunching:(NSNotification*)aNotification {
@@ -158,63 +114,10 @@ static NSString* const kOptShowDockIcon      = @"--show-dock-icon";
           NSBundle.mainBundle.infoDictionary[@"CFBundleShortVersionString"],
           NSBundle.mainBundle.infoDictionary[@"CFBundleVersion"]);
 
-    // Set up audioDevices, which coordinates BGMDevice and the output device. It manages
-    // playthrough, volume/mute controls, etc.
-    if (![self initAudioDeviceManager]) {
-        return;
-    }
-
-    // Make BGMDevice the default device.
-    [self setBGMDeviceAsDefault];
-
-    // Handle some of the unusual reasons BGMApp might have to exit, mostly crashes.
-    BGMTermination::SetUpTerminationCleanUp(audioDevices);
-
-    // Set up the rest of the UI and other external interfaces.
-    BGMUserDefaults* userDefaults = [self createUserDefaults];
-
-    musicPlayers = [[BGMMusicPlayers alloc] initWithAudioDevices:audioDevices
-                                                    userDefaults:userDefaults];
-
-    autoPauseMusic = [[BGMAutoPauseMusic alloc] initWithAudioDevices:audioDevices
-                                                        musicPlayers:musicPlayers];
-    
-    [self setUpMainMenu:userDefaults];
-    
-    xpcListener = [[BGMXPCListener alloc] initWithAudioDevices:audioDevices
-                                  helperConnectionErrorHandler:^(NSError* error) {
-                                      NSLog(@"BGMAppDelegate::applicationDidFinishLaunching: (helperConnectionErrorHandler) "
-                                             "BGMXPCHelper connection error: %@",
-                                            error);
-                                      [self showXPCHelperErrorMessage:error];
-                                  }];
-}
-
-// Returns NO if (and only if) BGMApp is about to terminate because of a fatal error.
-- (BOOL) initAudioDeviceManager {
-    NSError* error;
-    audioDevices = [[BGMAudioDeviceManager alloc] initWithError:&error];
-
-    if (!audioDevices) {
-        [self showDeviceNotFoundErrorMessageAndExit:error.code];
-        return NO;
-    }
-
-    return YES;
-}
-
-// Sets the "Background Music" virtual audio device (BGMDevice) as the user's default audio device.
-- (void) setBGMDeviceAsDefault {
-    void (^setDefaultDevice)() = ^{
-        NSError* error = [audioDevices setBGMDeviceAsOSDefault];
-
-        if (error) {
-            [self showSetDeviceAsDefaultError:error
-                                      message:@"Could not set the Background Music device as your"
-                                               "default audio device."
-                              informativeText:@"You might be able to change it yourself."];
-        }
-    };
+    // Handles changing (or not changing) the output device when devices are added or removed. Must
+    // be initialised before calling setBGMDeviceAsDefault.
+    preferredOutputDevices =
+        [[BGMPreferredOutputDevices alloc] initWithDevices:audioDevices userDefaults:userDefaults];
 
     // Skip this if we're compiling on a version of macOS before 10.14 as won't compile and it
     // isn't needed.
@@ -225,35 +128,174 @@ static NSString* const kOptShowDockIcon      = @"--show-dock-icon";
         // before making BGMDevice the default device. This way, if the user is playing audio when
         // they open Background Music, we won't interrupt it while we're waiting for them to click
         // OK.
-        //
-        // TODO: This isn't a perfect solution because, if the user takes too long to accept,
-        //       BGMPlayThrough will try to use BGMDevice again and log some errors.
         [AVCaptureDevice requestAccessForMediaType:AVMediaTypeAudio
                                  completionHandler:^(BOOL granted) {
-                                     if (granted) {
-                                         DebugMsg("BGMAppDelegate::setBGMDeviceAsDefault: "
-                                                  "Permission granted");
-                                         setDefaultDevice();
-                                     } else {
-                                         NSLog(@"BGMAppDelegate::setBGMDeviceAsDefault: "
-                                               "Permission denied");
-                                         // TODO: If they don't accept, Background Music won't work
-                                         //       at all and the only way to fix it is in System
-                                         //       Preferences, so we should show an error dialog
-                                         //       with instructions.
-                                     }
-                                 }];
+            dispatch_async(dispatch_get_main_queue(), ^{
+                if (granted) {
+                    DebugMsg("BGMAppDelegate::applicationDidFinishLaunching: Permission granted");
+                    [self continueLaunchAfterInputDevicePermissionGranted];
+                } else {
+                    NSLog(@"BGMAppDelegate::applicationDidFinishLaunching: Permission denied");
+                    // If they don't accept, Background Music won't work at all and the only way to
+                    // fix it is in System Preferences, so show an error dialog with instructions.
+                    //
+                    // TODO: It would be nice if this dialog had a shortcut to open the System
+                    //       Preferences panel. See showSetDeviceAsDefaultError.
+                    [self showErrorMessage:@"Background Music needs permission to use microphones."
+                           informativeText:@"It uses a virtual microphone to access your system's "
+                                            "audio.\n\nYou can grant the permission by going to "
+                                            "System Preferences > Security and Privacy > "
+                                            "Microphone and checking the box for Background Music."
+                 exitAfterMessageDismissed:YES];
+                }
+            });
+        }];
     }
     else
 #endif
     {
         // We can change the device immediately on older versions of macOS because they don't
         // require user permission for input devices.
-        setDefaultDevice();
+        [self continueLaunchAfterInputDevicePermissionGranted];
     }
 }
 
-- (void) setUpMainMenu:(BGMUserDefaults*)userDefaults {
+- (void) continueLaunchAfterInputDevicePermissionGranted {
+    // Choose an output device for BGMApp to use to play audio.
+    if (![self setInitialOutputDevice]) {
+        return;
+    }
+
+    // Make BGMDevice the default device.
+    [self setBGMDeviceAsDefault];
+
+    // Handle some of the unusual reasons BGMApp might have to exit, mostly crashes.
+    BGMTermination::SetUpTerminationCleanUp(audioDevices);
+
+    // Set up the rest of the UI and other external interfaces.
+    musicPlayers = [[BGMMusicPlayers alloc] initWithAudioDevices:audioDevices
+                                                    userDefaults:userDefaults];
+
+    autoPauseMusic = [[BGMAutoPauseMusic alloc] initWithAudioDevices:audioDevices
+                                                        musicPlayers:musicPlayers];
+
+    [self setUpMainMenu];
+
+    xpcListener = [[BGMXPCListener alloc] initWithAudioDevices:audioDevices
+                                  helperConnectionErrorHandler:^(NSError* error) {
+        NSLog(@"BGMAppDelegate::continueLaunchAfterInputDevicePermissionGranted: "
+              "(helperConnectionErrorHandler) BGMXPCHelper connection error: %@",
+              error);
+        [self showXPCHelperErrorMessage:error];
+    }];
+}
+
+// Returns NO if (and only if) BGMApp is about to terminate because of a fatal error.
+- (BOOL) initAudioDeviceManager {
+    audioDevices = [BGMAudioDeviceManager new];
+
+    if (!audioDevices) {
+        [self showBGMDeviceNotFoundErrorMessageAndExit];
+        return NO;
+    }
+
+    return YES;
+}
+
+// Returns NO if (and only if) BGMApp is about to terminate because of a fatal error.
+- (BOOL) setInitialOutputDevice {
+    AudioObjectID preferredDevice = [preferredOutputDevices findPreferredDevice];
+
+    if (preferredDevice != kAudioObjectUnknown) {
+        NSError* __nullable error = [audioDevices setOutputDeviceWithID:preferredDevice
+                                                        revertOnFailure:NO];
+        if (error) {
+            // Show the error message.
+            [self showFailedToSetOutputDeviceErrorMessage:BGMNN(error)
+                                          preferredDevice:preferredDevice];
+        }
+    } else {
+        // We couldn't find a device to use, so show an error message and quit.
+        [self showOutputDeviceNotFoundErrorMessageAndExit];
+        return NO;
+    }
+
+    return YES;
+}
+
+// Sets the "Background Music" virtual audio device (BGMDevice) as the user's default audio device.
+- (void) setBGMDeviceAsDefault {
+    NSError* error = [audioDevices setBGMDeviceAsOSDefault];
+
+    if (error) {
+        [self showSetDeviceAsDefaultError:error
+                                  message:@"Could not set the Background Music device as your"
+                                           "default audio device."
+                          informativeText:@"You might be able to change it yourself."];
+    }
+}
+
+- (void) menuWillOpen:(NSMenu*)menu {
+    if (@available(macOS 10.16, *)) {
+        // Set menu offset and check for any active menu items
+        float menuOffset = 12.0;
+        for (NSMenuItem* menuItem in self.bgmMenu.itemArray) {
+            if (menuItem.state == NSControlStateValueOn && menuItem.indentationLevel == 0) {
+                menuOffset += 10;
+                break;
+            }
+        }
+        
+        // Align volume output device and slider
+        for (NSView* subview in self.outputVolumeView.subviews) {
+            CGRect newSubview = subview.frame;
+            newSubview.origin.x = menuOffset;
+            subview.frame = newSubview;
+        }
+
+        // Align system sounds and app volumes
+        double appIconTitleOffset = 0;
+        for (NSMenuItem* menuItem in self.bgmMenu.itemArray) {
+            if (menuItem.view.subviews.count == 7 || menuItem.view.subviews.count == 3) {
+                NSTextField* appTitle;
+                NSImageView* appIcon;
+                
+                for (NSView* subview in menuItem.view.subviews) {
+                    if (menuItem.view.subviews.count == 3) {
+                        // System sounds
+                        if ([subview isKindOfClass:[NSTextField class]]) {
+                            appTitle = (NSTextField*)subview;
+                        }
+                        if ([subview isKindOfClass:[NSImageView class]]) {
+                            appIcon = (NSImageView*)subview;
+                        }
+                    } else if (menuItem.view.subviews.count == 7) {
+                        // App volumes
+                        if ([subview isKindOfClass:[BGMAVM_AppNameLabel class]]) {
+                            appTitle = (NSTextField*)subview;
+                        }
+                        if ([subview isKindOfClass:[BGMAVM_AppIcon class]]) {
+                            appIcon = (NSImageView*)subview;
+                        }
+                    }
+                }
+ 
+                if (appIconTitleOffset == 0) {
+                    appIconTitleOffset = appTitle.frame.origin.x - appIcon.frame.origin.x;
+                }
+                
+                CGRect newAppIcon = appIcon.frame;
+                newAppIcon.origin.x = menuOffset;
+                appIcon.frame = newAppIcon;
+                CGRect newAppTitle = appTitle.frame;
+                newAppTitle.origin.x = menuOffset + appIconTitleOffset;
+                appTitle.frame = newAppTitle;
+            }
+        }
+    }
+}
+
+- (void) setUpMainMenu {
     autoPauseMenuItem =
         [[BGMAutoPauseMenuItem alloc] initWithMenuItem:self.autoPauseMenuItemUnwrapped
                                         autoPauseMusic:autoPauseMusic
@@ -262,12 +304,26 @@ static NSString* const kOptShowDockIcon      = @"--show-dock-icon";
 
     [self initVolumesMenuSection];
 
+    // Output device selection.
+    outputDeviceMenuSection =
+            [[BGMOutputDeviceMenuSection alloc] initWithBGMMenu:self.bgmMenu
+                                                   audioDevices:audioDevices
+                                               preferredDevices:preferredOutputDevices];
+    [audioDevices setOutputDeviceMenuSection:outputDeviceMenuSection];
+
+    // Preferences submenu.
     prefsMenu = [[BGMPreferencesMenu alloc] initWithBGMMenu:self.bgmMenu
                                                audioDevices:audioDevices
                                                musicPlayers:musicPlayers
+                                              statusBarItem:statusBarItem
                                                  aboutPanel:self.aboutPanel
                                       aboutPanelLicenseView:self.aboutPanelLicenseView];
 
+    // Enable/disable debug logging. Hidden unless you option-click the status bar icon.
+    debugLoggingMenuItem =
+        [[BGMDebugLoggingMenuItem alloc] initWithMenuItem:self.debugLoggingMenuItemUnwrapped];
+    [statusBarItem setDebugLoggingMenuItem:debugLoggingMenuItem];
+
     // Handle events about the main menu. (See the NSMenuDelegate methods below.)
     self.bgmMenu.delegate = self;
 }
@@ -326,30 +382,48 @@ static NSString* const kOptShowDockIcon      = @"--show-dock-icon";
 
 #pragma mark Error messages
 
-- (void) showDeviceNotFoundErrorMessageAndExit:(NSInteger)code {
-    // Show an error dialog and exit if either BGMDevice wasn't found on the system or we couldn't find any output devices
-    
-    // NSAlert should only be used on the main thread.
-    dispatch_async(dispatch_get_main_queue(), ^{
-        NSAlert* alert = [NSAlert new];
-        
-        if (code == kBGMErrorCode_BGMDeviceNotFound) {
-            // TODO: Check whether the driver files are in /Library/Audio/Plug-Ins/HAL and offer to install them if not. Also,
-            //       it would be nice if we could restart coreaudiod automatically (using launchd).
-            [alert setMessageText:@"Could not find the Background Music virtual audio device."];
-            [alert setInformativeText:@"Make sure you've installed Background Music Device.driver to /Library/Audio/Plug-Ins/HAL and restarted coreaudiod (e.g. \"sudo killall coreaudiod\")."];
-        } else if (code == kBGMErrorCode_OutputDeviceNotFound) {
-            [alert setMessageText:@"Could not find an audio output device."];
-            [alert setInformativeText:@"If you do have one installed, this is probably a bug. Sorry about that. Feel free to file an issue on GitHub."];
-        }
+- (void) showBGMDeviceNotFoundErrorMessageAndExit {
+    // BGMDevice wasn't found on the system. Most likely, BGMDriver isn't installed. Show an error
+    // dialog and exit.
+    //
+    // TODO: Check whether the driver files are in /Library/Audio/Plug-Ins/HAL? Might even want to
+    //       offer to install them if not.
+    [self showErrorMessage:@"Could not find the Background Music virtual audio device."
+           informativeText:@"Make sure you've installed Background Music Device.driver to "
+                            "/Library/Audio/Plug-Ins/HAL and restarted coreaudiod (e.g. \"sudo "
+                            "killall coreaudiod\")."
+ exitAfterMessageDismissed:YES];
+}
+
+- (void) showFailedToSetOutputDeviceErrorMessage:(NSError*)error
+                                 preferredDevice:(BGMAudioDevice)device {
+    NSLog(@"Failed to set initial output device. Error: %@", error);
+
+    dispatch_async(dispatch_get_main_queue(), ^{
+        NSAlert* alert = [NSAlert alertWithError:BGMNN(error)];
+        alert.messageText = @"Failed to set the output device.";
+
+        NSString* __nullable name = nil;
+        BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
+            name = (__bridge NSString* __nullable)device.CopyName();
+        });
+
+        alert.informativeText =
+                [NSString stringWithFormat:@"Could not start the device '%@'. (Error: %ld)",
+                        name, error.code];
 
-        // This crashes if built with Xcode 9.0.1, but works with versions of Xcode before 9 and
-        // with 9.1.
         [alert runModal];
-        [NSApp terminate:self];
     });
 }
 
+- (void) showOutputDeviceNotFoundErrorMessageAndExit {
+    // We couldn't find any output devices. Show an error dialog and exit.
+    [self showErrorMessage:@"Could not find an audio output device."
+           informativeText:@"If you do have one installed, this is probably a bug. Sorry about "
+                            "that. Feel free to file an issue on GitHub."
+ exitAfterMessageDismissed:YES];
+}
+
 - (void) showXPCHelperErrorMessage:(NSError*)error {
     if (!haveShownXPCHelperErrorMessage) {
         haveShownXPCHelperErrorMessage = YES;
@@ -373,6 +447,25 @@ static NSString* const kOptShowDockIcon      = @"--show-dock-icon";
     }
 }
 
+- (void) showErrorMessage:(NSString*)message
+          informativeText:(NSString*)informativeText
+exitAfterMessageDismissed:(BOOL)fatal {
+    // NSAlert should only be used on the main thread.
+    dispatch_async(dispatch_get_main_queue(), ^{
+        NSAlert* alert = [NSAlert new];
+        [alert setMessageText:message];
+        [alert setInformativeText:informativeText];
+
+        // This crashes if built with Xcode 9.0.1, but works with versions of Xcode before 9 and
+        // with 9.1.
+        [alert runModal];
+
+        if (fatal) {
+            [NSApp terminate:self];
+        }
+    });
+}
+
 - (void) showSetDeviceAsDefaultError:(NSError*)error
                              message:(NSString*)msg
                      informativeText:(NSString*)info {

BGMApp/BGMApp/BGMAppVolumes.h

@@ -18,6 +18,7 @@
 //  BGMApp
 //
 //  Copyright © 2016, 2017 Kyle Neideck
+//  Copyright © 2021 Marcus Wu
 //
 
 // Local Includes
@@ -35,7 +36,7 @@
                   bgmMenu:(NSMenu*)inMenu
             appVolumeView:(NSView*)inView;
 
-// Pass -1 for initialVolume or initialPan to leave the volume/pan at its default level.
+// Pass -1 for initialVolume or kAppPanNoValue for initialPan to leave the volume/pan at its default level.
 - (void) insertMenuItemForApp:(NSRunningApplication*)app
                 initialVolume:(int)volume
                    initialPan:(int)pan;
@@ -44,6 +45,9 @@
 
 - (void) removeAllAppVolumeMenuItems;
 
+- (BGMAppVolumeAndPan) getVolumeAndPanForApp:(NSRunningApplication*)app;
+- (void) setVolumeAndPan:(BGMAppVolumeAndPan)volumeAndPan forApp:(NSRunningApplication*)app;
+
 @end
 
 // Protocol for the UI custom classes

BGMApp/BGMApp/BGMAppVolumes.m

@@ -17,8 +17,10 @@
 //  BGMAppVolumes.m
 //  BGMApp
 //
-//  Copyright © 2016-2018 Kyle Neideck
+//  Copyright © 2016-2020 Kyle Neideck
 //  Copyright © 2017 Andrew Tonner
+//  Copyright © 2021 Marcus Wu
+//  Copyright © 2022 Jon Egan
 //
 
 // Self Include
@@ -124,6 +126,79 @@ static NSString* const kMoreAppsMenuTitle          = @"More Apps";
     }
 }
 
+- (NSMenuItem*) getMenuItemForApp:(NSRunningApplication*)app {
+    NSInteger lastAppVolumeMenuItemIndex = [self lastMenuItemIndex] - 2;
+
+    for (NSInteger i = [self firstMenuItemIndex]; i <= lastAppVolumeMenuItemIndex; i++) {
+        NSMenuItem* item = [bgmMenu itemAtIndex:i];
+        NSRunningApplication* itemApp = item.representedObject;
+        BGMAssert(itemApp, "!itemApp for %s", item.title.UTF8String);
+
+        if ([itemApp isEqual:app]) {
+            return item;
+        }
+    }
+    for (NSInteger i = 0; i < [moreAppsMenu numberOfItems]; i++) {
+        NSMenuItem* item = [moreAppsMenu itemAtIndex:i];
+        NSRunningApplication* itemApp = item.representedObject;
+        BGMAssert(itemApp, "!itemApp for %s", item.title.UTF8String);
+
+        if ([itemApp isEqual:app]) {
+            return item;
+        }
+    }
+
+    return nil;
+}
+
+- (BGMAppVolumeAndPan) getVolumeAndPanForApp:(NSRunningApplication*)app {
+    BGMAppVolumeAndPan result = {
+        .volume = -1,
+        .pan = kAppPanNoValue
+    };
+
+    NSMenuItem *item = [self getMenuItemForApp:app];
+
+    if (item == nil) {
+        return result;
+    }
+
+    for (NSView* subview in item.view.subviews) {
+        // Get the volume.
+        if ([subview isKindOfClass:[BGMAVM_VolumeSlider class]]) {
+            result.volume = [(BGMAVM_VolumeSlider*)subview intValue];
+        }
+
+        // Get the pan position.
+        if ([subview isKindOfClass:[BGMAVM_PanSlider class]]) {
+            result.pan = [(BGMAVM_PanSlider*)subview intValue];
+        }
+    }
+
+    return result;
+}
+
+- (void) setVolumeAndPan:(BGMAppVolumeAndPan)volumeAndPan forApp:(NSRunningApplication*)app {
+    NSMenuItem *item = [self getMenuItemForApp:app];
+
+    if (item == nil) {
+        return;
+    }
+
+    for (NSView* subview in item.view.subviews) {
+        // Set the volume.
+        if (volumeAndPan.volume != -1 && [subview isKindOfClass:[BGMAVM_VolumeSlider class]]) {
+            [(BGMAVM_VolumeSlider*)subview setRelativeVolume:volumeAndPan.volume];
+        }
+
+        // Set the pan position.
+        if (volumeAndPan.pan != kAppPanNoValue && [subview isKindOfClass:[BGMAVM_PanSlider class]]) {
+            [(BGMAVM_PanSlider*)subview setPanPosition:volumeAndPan.pan];
+        }
+    }
+
+}
+
 // Create a blank menu item to copy as a template.
 - (NSMenuItem*) createBlankAppVolumeMenuItem {
     NSMenuItem* menuItem = [[NSMenuItem alloc] initWithTitle:@"" action:nil keyEquivalent:@""];
@@ -143,7 +218,7 @@ static NSString* const kMoreAppsMenuTitle          = @"More Apps";
         }
 
         // Set the pan position.
-        if (pan != -1 && [subview isKindOfClass:[BGMAVM_PanSlider class]]) {
+        if (pan != kAppPanNoValue && [subview isKindOfClass:[BGMAVM_PanSlider class]]) {
             [(BGMAVM_PanSlider*)subview setPanPosition:pan];
         }
     }
@@ -196,20 +271,32 @@ static NSString* const kMoreAppsMenuTitle          = @"More Apps";
     
     BGMAssert(button, "!button");
     BGMAssert(menuItem, "!menuItem");
-    
+
     CGFloat width = menuItem.view.frame.size.width;
-    CGFloat height = menuItem.view.frame.size.height;
-    
 #if DEBUG
-    const char* appName = [((NSRunningApplication*)menuItem.representedObject).localizedName UTF8String];
+    CGFloat height = menuItem.view.frame.size.height;
 #endif
 
+    const char* appName =
+        [((NSRunningApplication*)menuItem.representedObject).localizedName UTF8String];
+
     // Using this function (instead of just ==) shouldn't be necessary, but just in case.
+#if DEBUG
     BOOL(^nearEnough)(CGFloat x, CGFloat y) = ^BOOL(CGFloat x, CGFloat y) {
         return fabs(x - y) < 0.01;  // We don't need much precision.
     };
+#endif
     
-    if (nearEnough(button.frameCenterRotation, 0.0)) {
+    bool allSubviewsShowing = true;
+    for (NSView* subview in menuItem.view.subviews) {
+        if (subview.hidden) {
+            allSubviewsShowing = false;
+            break;
+        }
+        //DebugMsg("BGMAppVolumes:: subview hash / hidden: (%lu) / (%hhd)", (unsigned long)subview.hash, subview.hidden);
+    }
+
+    if (allSubviewsShowing) {
         // Hide extra controls
         DebugMsg("BGMAppVolumes::showHideExtraControls: Hiding extra controls (%s)", appName);
         
@@ -241,7 +328,7 @@ static NSString* const kMoreAppsMenuTitle          = @"More Apps";
         menuItem.view.frameSize = NSMakeSize(width, appVolumeViewFullHeight);
         // Turn the button rightside up so the arrowhead points up.
         button.frameCenterRotation = 0.0;
-        // Move the button down slightly, back to it's original position.
+        // Move the button down slightly, back to its original position.
         [button setFrameOrigin:NSMakePoint(button.frame.origin.x, button.frame.origin.y + 1)];
 
         // Set all of the UI elements in the menu item to "not hidden" for accessibility clients.
@@ -446,4 +533,3 @@ static NSString* const kMoreAppsMenuTitle          = @"More Apps";
 }
 
 @end
-

BGMApp/BGMApp/BGMAppVolumesController.h

@@ -18,6 +18,7 @@
 //  BGMApp
 //
 //  Copyright © 2017 Kyle Neideck
+//  Copyright © 2021 Marcus Wu
 //
 
 // Local Includes
@@ -29,6 +30,11 @@
 
 #pragma clang assume_nonnull begin
 
+typedef struct BGMAppVolumeAndPan {
+    int volume;
+    int pan;
+} BGMAppVolumeAndPan;
+
 @interface BGMAppVolumesController : NSObject
 
 - (id) initWithMenu:(NSMenu*)menu
@@ -45,6 +51,9 @@ forAppWithProcessID:(pid_t)processID
     forAppWithProcessID:(pid_t)processID
                bundleID:(NSString* __nullable)bundleID;
 
+- (BGMAppVolumeAndPan) getVolumeAndPanForApp:(NSRunningApplication *)app;
+- (void) setVolumeAndPan:(BGMAppVolumeAndPan)volumeAndPan forApp:(NSRunningApplication*)app;
+
 @end
 
 #pragma clang assume_nonnull end

BGMApp/BGMApp/BGMAppVolumesController.mm

@@ -19,6 +19,7 @@
 //
 //  Copyright © 2017, 2018 Kyle Neideck
 //  Copyright © 2017 Andrew Tonner
+//  Copyright © 2021 Marcus Wu
 //
 
 // Self Include
@@ -40,11 +41,6 @@
 
 #pragma clang assume_nonnull begin
 
-typedef struct BGMAppVolumeAndPan {
-    int volume;
-    int pan;
-} BGMAppVolumeAndPan;
-
 @implementation BGMAppVolumesController {
     // The App Volumes UI.
     BGMAppVolumes* appVolumes;
@@ -104,11 +100,25 @@ typedef struct BGMAppVolumeAndPan {
     }
 }
 
+- (BGMAppVolumeAndPan) getVolumeAndPanForApp:(NSRunningApplication *)app {
+    return [appVolumes getVolumeAndPanForApp:app];
+}
+
+- (void) setVolumeAndPan:(BGMAppVolumeAndPan)volumeAndPan forApp:(NSRunningApplication*)app {
+    [appVolumes setVolumeAndPan:volumeAndPan forApp:app];
+    if (volumeAndPan.volume != -1) {
+        [self setVolume:volumeAndPan.volume forAppWithProcessID:app.processIdentifier bundleID:app.bundleIdentifier];
+    }
+    if (volumeAndPan.pan != kAppPanNoValue) {
+        [self setPanPosition:volumeAndPan.pan forAppWithProcessID:app.processIdentifier bundleID:app.bundleIdentifier];
+    }
+}
+
 - (BGMAppVolumeAndPan) getVolumeAndPanForApp:(NSRunningApplication*)app
                                  fromVolumes:(const CACFArray&)volumes {
     BGMAppVolumeAndPan volumeAndPan = {
         .volume = -1,
-        .pan = -1
+        .pan = kAppPanNoValue
     };
 
     for (UInt32 i = 0; i < volumes.GetNumberItems(); i++) {

BGMApp/BGMApp/BGMAppWatcher.h

@@ -0,0 +1,49 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMAppWatcher.h
+//  BGMApp
+//
+//  Copyright © 2019 Kyle Neideck
+//
+//  Calls callback functions when a given application is launched or terminated. Starts watching
+//  after being initialised, stops after being destroyed.
+//
+
+// System Includes
+#import <Foundation/Foundation.h>
+
+
+#pragma clang assume_nonnull begin
+
+@interface BGMAppWatcher : NSObject
+
+// appLaunched will be called when the application is launched and appTerminated will be called when
+// it's terminated. Background apps, status bar apps, etc. are ignored.
+- (instancetype) initWithBundleID:(NSString*)bundleID
+                      appLaunched:(void(^)(void))appLaunched
+                    appTerminated:(void(^)(void))appTerminated;
+
+// With this constructor, when an application is launched or terminated, isMatchingBundleID will be
+// called first to decide whether or not the callback should be called.
+- (instancetype) initWithAppLaunched:(void(^)(void))appLaunched
+                       appTerminated:(void(^)(void))appTerminated
+                  isMatchingBundleID:(BOOL(^)(NSString* appBundleID))isMatchingBundleID;
+
+@end
+
+#pragma clang assume_nonnull end
+

BGMApp/BGMApp/BGMAppWatcher.m

@@ -0,0 +1,109 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMAppWatcher.m
+//  BGMApp
+//
+//  Copyright © 2019 Kyle Neideck
+//
+
+// Self Include
+#import "BGMAppWatcher.h"
+
+// System Includes
+#import <Cocoa/Cocoa.h>
+
+
+#pragma clang assume_nonnull begin
+
+@implementation BGMAppWatcher {
+    // Tokens for the notification observers so we can remove them in dealloc.
+    id<NSObject> didLaunchToken;
+    id<NSObject> didTerminateToken;
+}
+
+- (instancetype) initWithBundleID:(NSString*)bundleID
+                      appLaunched:(void(^)(void))appLaunched
+                    appTerminated:(void(^)(void))appTerminated {
+    return [self initWithAppLaunched:appLaunched
+                       appTerminated:appTerminated
+                  isMatchingBundleID:^BOOL(NSString* appBundleID) {
+                      return [bundleID isEqualToString:appBundleID];
+                  }];
+}
+
+- (instancetype) initWithAppLaunched:(void(^)(void))appLaunched
+                       appTerminated:(void(^)(void))appTerminated
+                  isMatchingBundleID:(BOOL(^)(NSString*))isMatchingBundleID
+{
+    if ((self = [super init])) {
+        NSNotificationCenter* center = [NSWorkspace sharedWorkspace].notificationCenter;
+
+        didLaunchToken =
+            [center addObserverForName:NSWorkspaceDidLaunchApplicationNotification
+                                object:nil
+                                 queue:nil
+                            usingBlock:^(NSNotification* notification) {
+                                if ([BGMAppWatcher shouldBeHandled:notification
+                                                isMatchingBundleID:isMatchingBundleID]) {
+                                    appLaunched();
+                                }
+                            }];
+
+        didTerminateToken =
+            [center addObserverForName:NSWorkspaceDidTerminateApplicationNotification
+                                object:nil
+                                 queue:nil
+                            usingBlock:^(NSNotification* notification) {
+                                if ([BGMAppWatcher shouldBeHandled:notification
+                                                isMatchingBundleID:isMatchingBundleID]) {
+                                    appTerminated();
+                                }
+                            }];
+    }
+
+    return self;
+}
+
+// Returns YES if we should call the app launch/termination callback for this NSNotification.
++ (BOOL) shouldBeHandled:(NSNotification*)notification
+      isMatchingBundleID:(BOOL(^)(NSString*))isMatchingBundleID {
+    NSString* __nullable notifiedBundleID =
+        [notification.userInfo[NSWorkspaceApplicationKey] bundleIdentifier];
+
+    // Ignore the notification if the app doesn't have a bundle ID or isMatchingBundleID returns NO.
+    return notifiedBundleID && isMatchingBundleID((NSString*)notifiedBundleID);
+}
+
+- (void) dealloc {
+    // Remove the application launch/termination observers.
+    NSNotificationCenter* center = [NSWorkspace sharedWorkspace].notificationCenter;
+
+    if (didLaunchToken) {
+        [center removeObserver:didLaunchToken];
+        didLaunchToken = nil;
+    }
+
+    if (didTerminateToken) {
+        [center removeObserver:didTerminateToken];
+        didTerminateToken = nil;
+    }
+}
+
+@end
+
+#pragma clang assume_nonnull end
+

BGMApp/BGMApp/BGMAudioDevice.cpp

@@ -48,7 +48,7 @@ BGMAudioDevice::BGMAudioDevice(const CAHALAudioDevice& inDevice)
 :
     BGMAudioDevice(inDevice.GetObjectID())
 {
-};
+}
 
 BGMAudioDevice::~BGMAudioDevice()
 {
@@ -81,7 +81,7 @@ bool    BGMAudioDevice::HasSettableMasterVolume(AudioObjectPropertyScope inScope
 bool    BGMAudioDevice::HasSettableVirtualMasterVolume(AudioObjectPropertyScope inScope) const
 {
     AudioObjectPropertyAddress virtualMasterVolumeAddress = {
-        kAudioHardwareServiceDeviceProperty_VirtualMasterVolume,
+        kAudioHardwareServiceDeviceProperty_VirtualMainVolume,
         inScope,
         kAudioObjectPropertyElementMaster
     };
@@ -220,7 +220,7 @@ bool    BGMAudioDevice::GetVirtualMasterVolumeScalar(AudioObjectPropertyScope in
                                                      Float32& outVirtualMasterVolume) const
 {
     AudioObjectPropertyAddress virtualMasterVolumeAddress = {
-        kAudioHardwareServiceDeviceProperty_VirtualMasterVolume,
+        kAudioHardwareServiceDeviceProperty_VirtualMainVolume,
         inScope,
         kAudioObjectPropertyElementMaster
     };
@@ -265,7 +265,7 @@ bool    BGMAudioDevice::SetVirtualMasterVolumeScalar(AudioObjectPropertyScope in
         bool didGetVirtualMasterBalance = GetVirtualMasterBalance(inScope, virtualMasterBalance);
 
         AudioObjectPropertyAddress virtualMasterVolumeAddress = {
-            kAudioHardwareServiceDeviceProperty_VirtualMasterVolume,
+            kAudioHardwareServiceDeviceProperty_VirtualMainVolume,
             inScope,
             kAudioObjectPropertyElementMaster
         };
@@ -277,7 +277,7 @@ bool    BGMAudioDevice::SetVirtualMasterVolumeScalar(AudioObjectPropertyScope in
 
         // Reset the balance
         AudioObjectPropertyAddress virtualMasterBalanceAddress = {
-            kAudioHardwareServiceDeviceProperty_VirtualMasterBalance,
+            kAudioHardwareServiceDeviceProperty_VirtualMainBalance,
             inScope,
             kAudioObjectPropertyElementMaster
         };
@@ -310,7 +310,7 @@ bool    BGMAudioDevice::GetVirtualMasterBalance(AudioObjectPropertyScope inScope
                                                 Float32& outVirtualMasterBalance) const
 {
     AudioObjectPropertyAddress virtualMasterBalanceAddress = {
-        kAudioHardwareServiceDeviceProperty_VirtualMasterBalance,
+        kAudioHardwareServiceDeviceProperty_VirtualMainBalance,
         inScope,
         kAudioObjectPropertyElementMaster
     };

BGMApp/BGMApp/BGMAudioDevice.h

@@ -16,7 +16,7 @@
 //  BGMAudioDevice.h
 //  BGMApp
 //
-//  Copyright © 2017 Kyle Neideck
+//  Copyright © 2017, 2020 Kyle Neideck
 //
 //  A HAL audio device. Note that this class's only state is the AudioObjectID of the device.
 //
@@ -59,7 +59,8 @@ public:
                        operator AudioObjectID() const { return GetObjectID(); }
 
     /*!
-     @return True if this device is BGMDevice. (Specifically, the main instance of BGMDevice.)
+     @return True if this device is BGMDevice. (Specifically, the main instance of BGMDevice, not
+             the instance used for UI sounds.)
      @throws CAException If the HAL returns an error when queried.
      */
     bool               IsBGMDevice() const { return IsBGMDevice(false); };

BGMApp/BGMApp/BGMAudioDeviceManager.h

@@ -17,7 +17,7 @@
 //  BGMAudioDeviceManager.h
 //  BGMApp
 //
-//  Copyright © 2016, 2017 Kyle Neideck
+//  Copyright © 2016-2018 Kyle Neideck
 //
 //  Manages BGMDevice and the output device. Sets the system's current default device as the output
 //  device on init, then starts playthrough and mirroring the devices' controls.
@@ -39,21 +39,25 @@
 
 // Forward Declarations
 @class BGMOutputVolumeMenuItem;
+@class BGMOutputDeviceMenuSection;
 
 
 #pragma clang assume_nonnull begin
 
-static const int kBGMErrorCode_BGMDeviceNotFound    = 1;
-static const int kBGMErrorCode_OutputDeviceNotFound = 2;
-static const int kBGMErrorCode_ReturningEarly       = 3;
+static const int kBGMErrorCode_OutputDeviceNotFound = 1;
+static const int kBGMErrorCode_ReturningEarly       = 2;
 
 @interface BGMAudioDeviceManager : NSObject
 
-- (instancetype) initWithError:(NSError**)error;
+// Returns nil if BGMDevice isn't installed.
+- (instancetype) init;
 
 // Set the BGMOutputVolumeMenuItem to be notified when the output device is changed.
 - (void) setOutputVolumeMenuItem:(BGMOutputVolumeMenuItem*)item;
 
+// Set the BGMOutputDeviceMenuSection to be notified when the output device is changed.
+- (void) setOutputDeviceMenuSection:(BGMOutputDeviceMenuSection*)menuSection;
+
 // Set BGMDevice as the default audio device for all processes
 - (NSError* __nullable) setBGMDeviceAsOSDefault;
 // Replace BGMDevice as the default device with the output device

BGMApp/BGMApp/BGMAudioDeviceManager.mm

@@ -26,16 +26,17 @@
 // Local Includes
 #import "BGM_Types.h"
 #import "BGM_Utils.h"
-#import "BGMDeviceControlSync.h"
-#import "BGMPlayThrough.h"
 #import "BGMAudioDevice.h"
-#import "BGMXPCProtocols.h"
+#import "BGMDeviceControlSync.h"
+#import "BGMOutputDeviceMenuSection.h"
 #import "BGMOutputVolumeMenuItem.h"
+#import "BGMPlayThrough.h"
+#import "BGMXPCProtocols.h"
 
 // PublicUtility Includes
-#import "CAHALAudioSystemObject.h"
-#import "CAAutoDisposer.h"
 #import "CAAtomic.h"
+#import "CAAutoDisposer.h"
+#import "CAHALAudioSystemObject.h"
 
 
 #pragma clang assume_nonnull begin
@@ -60,41 +61,25 @@
     NSXPCConnection* __nullable bgmXPCHelperConnection;
 
     BGMOutputVolumeMenuItem* __nullable outputVolumeMenuItem;
+    BGMOutputDeviceMenuSection* __nullable outputDeviceMenuSection;
 
     NSRecursiveLock* stateLock;
 }
 
 #pragma mark Construction/Destruction
 
-- (instancetype) initWithError:(NSError** __nullable)error {
+- (instancetype) init {
     if ((self = [super init])) {
         stateLock = [NSRecursiveLock new];
         bgmXPCHelperConnection = nil;
         outputVolumeMenuItem = nil;
+        outputDeviceMenuSection = nil;
+        outputDevice = kAudioObjectUnknown;
 
         try {
             bgmDevice = new BGMBackgroundMusicDevice;
         } catch (const CAException& e) {
-            LogError("BGMAudioDeviceManager::initWithError: BGMDevice not found. (%d)", e.GetError());
-            
-            if (error) {
-                *error = [NSError errorWithDomain:@kBGMAppBundleID code:kBGMErrorCode_BGMDeviceNotFound userInfo:nil];
-            }
-            
-            self = nil;
-            return self;
-        }
-
-        try {
-            [self initOutputDevice];
-        } catch (const CAException& e) {
-            LogError("BGMAudioDeviceManager::initWithError: failed to init output device (%d)",
-                     e.GetError());
-
-            if (error) {
-                *error = [NSError errorWithDomain:@kBGMAppBundleID code:kBGMErrorCode_OutputDeviceNotFound userInfo:nil];
-            }
-            
+            LogError("BGMAudioDeviceManager::init: BGMDevice not found. (%d)", e.GetError());
             self = nil;
             return self;
         }
@@ -116,90 +101,14 @@
     }
 }
 
-// Throws a CAException if it fails to set the output device.
-- (void) initOutputDevice {
-    CAHALAudioSystemObject audioSystem;
-    // outputDevice = BGMAudioDevice(CFSTR("AppleHDAEngineOutput:1B,0,1,1:0"));
-    BGMAudioDevice defaultDevice = audioSystem.GetDefaultAudioDevice(false, false);
-
-    if (defaultDevice.IsBGMDeviceInstance()) {
-        // BGMDevice is already the default (it could have been set manually or BGMApp could have
-        // failed to change it back the last time it closed), so just pick the device with the
-        // lowest latency.
-        //
-        // TODO: Temporarily disable BGMDevice so we can find out what the previous default was and
-        //       use that instead.
-        [self setOutputDeviceByLatency];
-    } else {
-        // TODO: Return the error from setOutputDeviceWithID so it can be returned by initWithError.
-        [self setOutputDeviceWithID:defaultDevice revertOnFailure:NO];
-    }
-
-    if (outputDevice == kAudioObjectUnknown) {
-        LogError("BGMAudioDeviceManager::initOutputDevice: Failed to set output device");
-        Throw(CAException(kAudioHardwareUnspecifiedError));
-    }
-
-    if (outputDevice.IsBGMDeviceInstance()) {
-        LogError("BGMAudioDeviceManager::initOutputDevice: Failed to change output device from "
-                 "BGMDevice");
-        Throw(CAException(kAudioHardwareUnspecifiedError));
-    }
-    
-    // Log message
-    CFStringRef outputDeviceUID = outputDevice.CopyDeviceUID();
-    DebugMsg("BGMAudioDeviceManager::initOutputDevice: Set output device to %s",
-             CFStringGetCStringPtr(outputDeviceUID, kCFStringEncodingUTF8));
-    CFRelease(outputDeviceUID);
-}
-
-- (void) setOutputDeviceByLatency {
-    CAHALAudioSystemObject audioSystem;
-    UInt32 numDevices = audioSystem.GetNumberAudioDevices();
-
-    if (numDevices > 0) {
-        BGMAudioDevice minLatencyDevice = kAudioObjectUnknown;
-        UInt32 minLatency = UINT32_MAX;
-
-        CAAutoArrayDelete<AudioObjectID> devices(numDevices);
-        audioSystem.GetAudioDevices(numDevices, devices);
-
-        for (UInt32 i = 0; i < numDevices; i++) {
-            BGMAudioDevice device(devices[i]);
-
-            if (!device.IsBGMDeviceInstance()) {
-                BOOL hasOutputChannels = NO;
-
-                BGMLogAndSwallowExceptionsMsg("BGMAudioDeviceManager::setOutputDeviceByLatency",
-                                              "GetTotalNumberChannels", ([&] {
-                    hasOutputChannels = device.GetTotalNumberChannels(/* inIsInput = */ false) > 0;
-                }));
-
-                if (hasOutputChannels) {
-                    BGMLogAndSwallowExceptionsMsg("BGMAudioDeviceManager::setOutputDeviceByLatency",
-                                                  "GetLatency", ([&] {
-                        UInt32 latency = device.GetLatency(false);
-
-                        if (latency < minLatency) {
-                            minLatencyDevice = devices[i];
-                            minLatency = latency;
-                        }
-                    }));
-                }
-            }
-        }
-
-        if (minLatencyDevice != kAudioObjectUnknown) {
-            // TODO: On error, try a different output device.
-            [self setOutputDeviceWithID:minLatencyDevice revertOnFailure:NO];
-        }
-    }
-}
-
 - (void) setOutputVolumeMenuItem:(BGMOutputVolumeMenuItem*)item {
     outputVolumeMenuItem = item;
 }
 
+- (void) setOutputDeviceMenuSection:(BGMOutputDeviceMenuSection*)menuSection {
+    outputDeviceMenuSection = menuSection;
+}
+
 #pragma mark Systemwide Default Device
 
 // Note that there are two different "default" output devices on OS X: "output" and "system output". See
@@ -319,29 +228,9 @@
         AudioDeviceID currentDeviceID = outputDevice.GetObjectID();  // (Doesn't throw.)
 
         try {
-            if (newDeviceID != currentDeviceID) {
-                BGMAudioDevice newOutputDevice(newDeviceID);
-                [self setOutputDeviceForPlaythroughAndControlSync:newOutputDevice];
-                outputDevice = newOutputDevice;
-            }
-            
-            // Set the output device to use the new data source.
-            if (dataSourceID) {
-                // TODO: If this fails, ideally we'd still start playthrough and return an error, but not
-                //       revert the device. It would probably be a bit awkward, though.
-                [self setDataSource:*dataSourceID device:outputDevice];
-            }
-            
-            if (newDeviceID != currentDeviceID) {
-                // We successfully changed to the new device. Start playthrough on it, since audio might be
-                // playing. (If we only changed the data source, playthrough will already be running if it
-                // needs to be.)
-                playThrough.Start();
-                playThrough_UISounds.Start();
-                // But stop playthrough if audio isn't playing, since it uses CPU.
-                playThrough.StopIfIdle();
-                playThrough_UISounds.StopIfIdle();
-            }
+            [self setOutputDeviceWithIDImpl:newDeviceID
+                               dataSourceID:dataSourceID
+                            currentDeviceID:currentDeviceID];
         } catch (const CAException& e) {
             BGMAssert(e.GetError() != kAudioHardwareNoError,
                       "CAException with kAudioHardwareNoError");
@@ -355,6 +244,7 @@
                                         revertTo:(revertOnFailure ? &currentDeviceID : nullptr)];
         }
 
+        // Tell other classes and BGMXPCHelper that we changed the output device.
         [self propagateOutputDeviceChange];
     } @finally {
         [stateLock unlock];
@@ -363,6 +253,41 @@
     return nil;
 }
 
+// Throws CAException.
+- (void) setOutputDeviceWithIDImpl:(AudioObjectID)newDeviceID
+                      dataSourceID:(UInt32* __nullable)dataSourceID
+                   currentDeviceID:(AudioObjectID)currentDeviceID {
+    if (newDeviceID != currentDeviceID) {
+        BGMAudioDevice newOutputDevice(newDeviceID);
+        [self setOutputDeviceForPlaythroughAndControlSync:newOutputDevice];
+        outputDevice = newOutputDevice;
+    }
+
+    // Set the output device to use the new data source.
+    if (dataSourceID) {
+        // TODO: If this fails, ideally we'd still start playthrough and return an error, but not
+        //       revert the device. It would probably be a bit awkward, though.
+        [self setDataSource:*dataSourceID device:outputDevice];
+    }
+
+    if (newDeviceID != currentDeviceID) {
+        // We successfully changed to the new device. Start playthrough on it, since audio might be
+        // playing. (If we only changed the data source, playthrough will already be running if it
+        // needs to be.)
+        playThrough.Start();
+        playThrough_UISounds.Start();
+        // But stop playthrough if audio isn't playing, since it uses CPU.
+        playThrough.StopIfIdle();
+        playThrough_UISounds.StopIfIdle();
+    }
+
+    CFStringRef outputDeviceUID = outputDevice.CopyDeviceUID();
+    DebugMsg("BGMAudioDeviceManager::setOutputDeviceWithIDImpl: Set output device to %s (%d)",
+             CFStringGetCStringPtr(outputDeviceUID, kCFStringEncodingUTF8),
+             outputDevice.GetObjectID());
+    CFRelease(outputDeviceUID);
+}
+
 // Changes the output device that playthrough plays audio to and that BGMDevice's controls are
 // kept in sync with. Throws CAException.
 - (void) setOutputDeviceForPlaythroughAndControlSync:(const BGMAudioDevice&)newOutputDevice {
@@ -406,6 +331,7 @@
 
     // Update the menu item for the volume of the output device.
     [outputVolumeMenuItem outputDeviceDidChange];
+    [outputDeviceMenuSection outputDeviceDidChange];
 }
 
 - (NSError*) failedToSetOutputDevice:(AudioDeviceID)deviceID
@@ -452,8 +378,14 @@
     
     @try {
         gotLock = [stateLock tryLock];
-        
-        if (gotLock) {
+
+        BOOL isBigSur = NO;
+        if (@available(macOS 11.0, *)) {
+            isBigSur = YES;
+        }
+
+        // Always start playthrough asynchronously. Temp workaround for deadlock on Big Sur.
+        if (!isBigSur && gotLock) {
             BGMPlayThrough& pt = (forUISoundsDevice ? playThrough_UISounds : playThrough);
 
             // Playthrough might not have been notified that BGMDevice is starting yet, so make sure

BGMApp/BGMApp/BGMAutoPauseMenuItem.m

@@ -17,7 +17,7 @@
 //  BGMAutoPauseMenuItem.m
 //  BGMApp
 //
-//  Copyright © 2016 Kyle Neideck
+//  Copyright © 2016, 2019 Kyle Neideck
 //  Copyright © 2016 Tanner Hoke
 //
 
@@ -25,7 +25,7 @@
 #import "BGMAutoPauseMenuItem.h"
 
 // Local Includes
-#import "BGMMusicPlayer.h"
+#import "BGMAppWatcher.h"
 
 
 #pragma clang assume_nonnull begin
@@ -41,7 +41,7 @@ static SInt64 const kMenuItemUpdateWaitTime = 1;
     NSMenuItem* menuItem;
     BGMAutoPauseMusic* autoPauseMusic;
     BGMMusicPlayers* musicPlayers;
-    id<NSObject> didLaunchToken, didTerminateToken;
+    BGMAppWatcher* appWatcher;
 }
 
 - (instancetype) initWithMenuItem:(NSMenuItem*)item
@@ -66,53 +66,39 @@ static SInt64 const kMenuItemUpdateWaitTime = 1;
         // Toggle auto-pause when the menu item is clicked.
         menuItem.target = self;
         menuItem.action = @selector(toggleAutoPauseMusic);
-        
-        [self updateMenuItemTitle];
-        [self initMusicPlayerObservers];
+
+        [self initMenuItemTitle];
     }
     
     return self;
 }
 
-- (void) initMusicPlayerObservers {
-    // Add observers that enable/disable the Auto-pause Music menu item when the music player is launched/terminated.
-    NSNotificationCenter* center = [[NSWorkspace sharedWorkspace] notificationCenter];
-    
-    id<NSObject> (^addObserver)(NSString*) = ^(NSString* name) {
-        return [center addObserverForName:name
-                                   object:nil
-                                    queue:nil
-                               usingBlock:^(NSNotification* note) {
-                                   NSString* appBundleID = [note.userInfo[NSWorkspaceApplicationKey] bundleIdentifier];
-                                   BOOL isAboutThisMusicPlayer = musicPlayers.selectedMusicPlayer.bundleID &&
-                                       [appBundleID isEqualToString:(NSString*)musicPlayers.selectedMusicPlayer.bundleID];
-                                   
-                                   if (isAboutThisMusicPlayer) {
-                                       dispatch_after(dispatch_time(DISPATCH_TIME_NOW,
-                                                                    kMenuItemUpdateWaitTime * NSEC_PER_SEC),
-                                                      dispatch_get_main_queue(),
-                                                      ^{
-                                                          [self updateMenuItemTitle];
-                                                      });
-                                   }
-                               }];
-    };
-    
-    didLaunchToken = addObserver(NSWorkspaceDidLaunchApplicationNotification);
-    didTerminateToken = addObserver(NSWorkspaceDidTerminateApplicationNotification);
-}
+- (void) initMenuItemTitle {
+    // Set the initial text, tool-tip, state, etc.
+    [self updateMenuItemTitle];
 
-- (void) dealloc {
-    // Remove the application launch/termination observers.
-    NSNotificationCenter* center = [[NSWorkspace sharedWorkspace] notificationCenter];
-    
-    if (didLaunchToken) {
-        [center removeObserver:didLaunchToken];
-    }
-    
-    if (didTerminateToken) {
-        [center removeObserver:didTerminateToken];
-    }
+    // Avoid retain cycles in case we ever want to destroy instances of this class.
+    BGMAutoPauseMenuItem* __weak weakSelf = self;
+
+    // Add a callback that enables/disables the Auto-pause Music menu item when the music player
+    // is launched/terminated.
+    void (^callback)(void) = ^{
+        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, kMenuItemUpdateWaitTime * NSEC_PER_SEC),
+                       dispatch_get_main_queue(),
+                       ^{
+                           BGMAutoPauseMenuItem* strongSelf = weakSelf;
+                           [strongSelf updateMenuItemTitle];
+                       });
+    };
+
+    appWatcher = [[BGMAppWatcher alloc] initWithAppLaunched:callback
+                                              appTerminated:callback
+                                         isMatchingBundleID:^BOOL(NSString* appBundleID) {
+        BGMAutoPauseMenuItem* strongSelf = weakSelf;
+        NSString* __nullable playerBundleID =
+                strongSelf->musicPlayers.selectedMusicPlayer.bundleID;
+        return playerBundleID && [appBundleID isEqualToString:(NSString*)playerBundleID];
+    }];
 }
 
 - (void) toggleAutoPauseMusic {
@@ -143,7 +129,7 @@ static SInt64 const kMenuItemUpdateWaitTime = 1;
     //
     // We don't actually disable it just in case the user decides to disable auto-pause and their music player isn't
     // running. E.g. someone who only recently installed Background Music and doesn't want to use auto-pause at all.
-    if (musicPlayers.selectedMusicPlayer.isRunning) {
+    if (musicPlayers.selectedMusicPlayer.running) {
         menuItem.attributedTitle = nil;
         menuItem.toolTip = nil;
     } else {

BGMApp/BGMApp/BGMBackgroundMusicDevice.cpp

@@ -17,7 +17,7 @@
 //  BGMBackgroundMusicDevice.cpp
 //  BGMApp
 //
-//  Copyright © 2016, 2017 Kyle Neideck
+//  Copyright © 2016-2019 Kyle Neideck
 //  Copyright © 2017 Andrew Tonner
 //
 
@@ -239,7 +239,11 @@ BGMBackgroundMusicDevice::ResponsibleBundleIDsOf(CACFString inParentBundleID)
         // Discord
         { "com.hnc.Discord", { "com.hnc.Discord.helper" } },
         // Skype
-        { "com.skype.skype", { "com.skype.skype.Helper" } }
+        { "com.skype.skype", { "com.skype.skype.Helper" } },
+        // Google Chrome
+        { "com.google.Chrome", { "com.google.Chrome.helper" } },
+        // Microsoft Edge
+        { "com.microsoft.edgemac", { "com.microsoft.edgemac.helper" } }
     };
 
     // Parallels' VM "dock helper" apps have bundle IDs like

BGMApp/BGMApp/BGMDebugLoggingMenuItem.h

@@ -0,0 +1,48 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMDebugLoggingMenuItem.h
+//  BGMApp
+//
+//  Copyright © 2020 Kyle Neideck
+//
+//  A menu item in the main menu that enables/disables debug logging. Only visible if you hold the
+//  option down when you click the status bar icon to reveal the main menu.
+//
+//  TODO: It would be better to have this menu item in the Preferences menu (maybe in an Advanced
+//        section) and always visible, but first we'd need to add something that tells the user how
+//        to view the log messages. Or better yet, something that automatically opens them.
+//
+
+// System Includes
+#import <Cocoa/Cocoa.h>
+
+
+#pragma clang assume_nonnull begin
+
+@interface BGMDebugLoggingMenuItem : NSObject
+
+- (instancetype) initWithMenuItem:(NSMenuItem*)menuItem;
+
+// True if the main menu is showing hidden items/options because the user held the option key when
+// they clicked the icon. This class makes the debug logging menu item visible if this property has
+// been set true or if debug logging is enabled.
+@property (nonatomic) BOOL menuShowingExtraOptions;
+
+@end
+
+#pragma clang assume_nonnull end
+

BGMApp/BGMApp/BGMDebugLoggingMenuItem.m

@@ -0,0 +1,73 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMDebugLoggingMenuItem.m
+//  BGMApp
+//
+//  Copyright © 2020 Kyle Neideck
+//
+
+// Self Include
+#import "BGMDebugLoggingMenuItem.h"
+
+// PublicUtility Includes
+#import "BGMDebugLogging.h"
+#import "CADebugMacros.h"
+
+
+#pragma clang assume_nonnull begin
+
+@implementation BGMDebugLoggingMenuItem {
+    NSMenuItem* _menuItem;
+    BOOL _menuShowingExtraOptions;
+}
+
+- (instancetype) initWithMenuItem:(NSMenuItem*)menuItem {
+    if ((self = [super init])) {
+        _menuItem = menuItem;
+        _menuItem.state =
+                BGMDebugLoggingIsEnabled() ? NSControlStateValueOn : NSControlStateValueOff;
+
+        [self setMenuShowingExtraOptions:NO];
+
+        // Enable/disable debug logging when the menu item is clicked.
+        menuItem.target = self;
+        menuItem.action = @selector(toggleDebugLogging);
+    }
+
+    return self;
+}
+
+- (void) setMenuShowingExtraOptions:(BOOL)showingExtra {
+    _menuShowingExtraOptions = showingExtra;
+    _menuItem.hidden = !BGMDebugLoggingIsEnabled() && !showingExtra;
+
+    DebugMsg("BGMDebugLoggingMenuItem::menuShowingExtraOptions: %s the menu item",
+             _menuItem.hidden ? "Hiding" : "Showing");
+}
+
+- (void) toggleDebugLogging {
+    BGMSetDebugLoggingEnabled(!BGMDebugLoggingIsEnabled());
+    _menuItem.state = BGMDebugLoggingIsEnabled() ? NSControlStateValueOn : NSControlStateValueOff;
+
+    DebugMsg("BGMDebugLoggingMenuItem::toggleDebugLogging: Debug logging %s",
+             BGMDebugLoggingIsEnabled() ? "enabled" : "disabled");
+}
+
+@end
+
+#pragma clang assume_nonnull end
+

BGMApp/BGMApp/BGMOutputDeviceMenuSection.h

@@ -0,0 +1,46 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMOutputDeviceMenuSection.h
+//  BGMApp
+//
+//  Copyright © 2016, 2018 Kyle Neideck
+//
+
+// Local Includes
+#import "BGMAudioDeviceManager.h"
+#import "BGMPreferredOutputDevices.h"
+
+// System Includes
+#import <AppKit/AppKit.h>
+
+
+#pragma clang assume_nonnull begin
+
+@interface BGMOutputDeviceMenuSection : NSObject
+
+- (instancetype) initWithBGMMenu:(NSMenu*)inBGMMenu
+                    audioDevices:(BGMAudioDeviceManager*)inAudioDevices
+                preferredDevices:(BGMPreferredOutputDevices*)inPreferredDevices;
+
+// To be called when BGMApp has been set to use a different output device. For example, when a new
+// device is connected and BGMPreferredOutputDevices decides BGMApp should switch to it.
+- (void) outputDeviceDidChange;
+
+@end
+
+#pragma clang assume_nonnull end
+

BGMApp/BGMApp/BGMOutputDeviceMenuSection.mm

@@ -14,14 +14,14 @@
 // along with Background Music. If not, see <http://www.gnu.org/licenses/>.
 
 //
-//  BGMOutputDevicePrefs.mm
+//  BGMOutputDeviceMenuSection.mm
 //  BGMApp
 //
-//  Copyright © 2016, 2017 Kyle Neideck
+//  Copyright © 2016-2018 Kyle Neideck
 //
 
 // Self Include
-#import "BGMOutputDevicePrefs.h"
+#import "BGMOutputDeviceMenuSection.h"
 
 // Local Includes
 #import "BGM_Utils.h"
@@ -29,33 +29,112 @@
 #import "BGMAudioDevice.h"
 
 // PublicUtility Includes
-#include "CAHALAudioSystemObject.h"
-#include "CAHALAudioDevice.h"
-#include "CAAutoDisposer.h"
+#import "CAAutoDisposer.h"
+#import "CAHALAudioDevice.h"
+#import "CAHALAudioSystemObject.h"
+#import "CAPropertyAddress.h"
+
+// STL Includes
+#import <set>
 
 
 #pragma clang assume_nonnull begin
 
-static NSInteger const kOutputDeviceMenuItemTag = 2;
+static NSInteger const kOutputDeviceMenuItemTag = 5;
 
-@implementation BGMOutputDevicePrefs {
+@implementation BGMOutputDeviceMenuSection {
+    NSMenu* bgmMenu;
     BGMAudioDeviceManager* audioDevices;
+    BGMPreferredOutputDevices* preferredDevices;
     NSMutableArray<NSMenuItem*>* outputDeviceMenuItems;
+    // Called when a CoreAudio property has changed and we might need to update the menu. For
+    // example, when a device is connected or disconnected.
+    AudioObjectPropertyListenerBlock refreshNeededListener;
+    // The devices we've added refreshNeededListener to. Used to avoid adding it to a device twice
+    // for the same property and to remove it from all devices in dealloc.
+    std::set<BGMAudioDevice> listenedDevices_kAudioDevicePropertyDataSources;
+    std::set<BGMAudioDevice> listenedDevices_kAudioDevicePropertyDataSource;
 }
 
-- (id) initWithAudioDevices:(BGMAudioDeviceManager*)inAudioDevices {
+- (instancetype) initWithBGMMenu:(NSMenu*)inBGMMenu
+                    audioDevices:(BGMAudioDeviceManager*)inAudioDevices
+                preferredDevices:(BGMPreferredOutputDevices*)inPreferredDevices {
     if ((self = [super init])) {
+        bgmMenu = inBGMMenu;
         audioDevices = inAudioDevices;
+        preferredDevices = inPreferredDevices;
         outputDeviceMenuItems = [NSMutableArray new];
+
+        [self listenForDevicesAddedOrRemoved];
+        [self populateBGMMenu];
     }
     
     return self;
 }
 
-- (void) populatePreferencesMenu:(NSMenu*)prefsMenu {
+- (void) dealloc {
+    // Tell CoreAudio not to call the listener block anymore. This probably isn't necessary.
+    //
+    // I think it's safe to do this without dispatching to the main queue because dealloc and
+    // refreshNeededListener should be essentially mutually exclusive. If refreshNeededListener is
+    // invoked and gets a value for weakSelf, it holds the strong ref until it returns, so dealloc
+    // won't be called. If refreshNeededListener is invoked and deallocation has started, it will
+    // get nil for weakSelf and just return.
+    auto removeListener = [&] (CAHALAudioObject audioObject, AudioObjectPropertySelector prop) {
+        BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
+            // Check the object still exists first to reduce unnecessary error logs.
+            if (CAHALAudioObject::ObjectExists(audioObject.GetObjectID())) {
+                audioObject.RemovePropertyListenerBlock(CAPropertyAddress(prop),
+                                                        dispatch_get_main_queue(),
+                                                        refreshNeededListener);
+            }
+        });
+    };
+
+    // Remove the listener from each audio object we added it to.
+    removeListener(CAHALAudioSystemObject(), kAudioHardwarePropertyDevices);
+
+    for (auto device : listenedDevices_kAudioDevicePropertyDataSources) {
+        removeListener(device, kAudioDevicePropertyDataSources);
+    }
+
+    for (auto device : listenedDevices_kAudioDevicePropertyDataSource) {
+        removeListener(device, kAudioDevicePropertyDataSource);
+    }
+}
+
+- (void) listenForDevicesAddedOrRemoved {
+    // Create the block that will run when a device is added or removed.
+    BGMOutputDeviceMenuSection* __weak weakSelf = self;
+
+    refreshNeededListener = ^(UInt32 inNumberAddresses,
+                              const AudioObjectPropertyAddress* inAddresses) {
+        #pragma unused (inNumberAddresses, inAddresses)
+
+        BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
+            [weakSelf populateBGMMenu];
+        });
+    };
+
+    // Register the listener block to be called when devices are connected or disconnected.
+    BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
+        CAHALAudioSystemObject().AddPropertyListenerBlock(
+            CAPropertyAddress(kAudioHardwarePropertyDevices),
+            dispatch_get_main_queue(),
+            refreshNeededListener);
+    });
+}
+
+- (void) populateBGMMenu {
+    // TODO: Technically, we should assert we're on the main queue rather than just the main thread.
+    BGMAssert([NSThread isMainThread],
+              "BGMOutputDeviceMenuSection::populateBGMMenu called on non-main thread");
+
     // Remove existing menu items
     for (NSMenuItem* item in outputDeviceMenuItems) {
-        [prefsMenu removeItem:item];
+        DebugMsg("BGMOutputDeviceMenuSection::populateBGMMenu: Removing %s",
+                 item.description.UTF8String);
+        [bgmMenu removeItem:item];
     }
     
     [outputDeviceMenuItems removeAllObjects];
@@ -69,25 +148,51 @@ static NSInteger const kOutputDeviceMenuItemTag = 2;
         audioSystem.GetAudioDevices(numDevices, devices);
         
         for (UInt32 i = 0; i < numDevices; i++) {
-            [self insertMenuItemsForDevice:devices[i] preferencesMenu:prefsMenu];
+            [self insertMenuItemsForDevice:devices[i]];
         }
     }
 }
 
-- (void) insertMenuItemsForDevice:(BGMAudioDevice)device preferencesMenu:(NSMenu*)prefsMenu {
+- (void) insertMenuItemsForDevice:(BGMAudioDevice)device {
     // Insert menu items after the item for the "Output Device" heading.
-    const NSInteger menuItemsIdx = [prefsMenu indexOfItemWithTag:kOutputDeviceMenuItemTag] + 1;
+    const NSInteger menuItemsIdx = [bgmMenu indexOfItemWithTag:kOutputDeviceMenuItemTag] + 1;
 
     BOOL canBeOutputDevice = YES;
-    BGMLogAndSwallowExceptions("BGMOutputDevicePrefs::insertMenuItemsForDevice", ([&] {
+    BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
         canBeOutputDevice = device.CanBeOutputDeviceInBGMApp();
-    }));
+    });
 
     if (canBeOutputDevice) {
         for (NSMenuItem* item : [self createMenuItemsForDevice:device]) {
-            [prefsMenu insertItem:item atIndex:menuItemsIdx];
+            DebugMsg("BGMOutputDeviceMenuSection::insertMenuItemsForDevice: Inserting %s",
+                     item.description.UTF8String);
+            [bgmMenu insertItem:item atIndex:menuItemsIdx];
             [outputDeviceMenuItems addObject:item];
         }
+
+        // Add listeners to update the menu when the device's data source changes or it changes its
+        // list of data sources. We do this so that, for example, when you plug headphones into the
+        // built-in jack, the menu item for the built-in device will change from "Internal Speakers"
+        // to "Headphones".
+        if (listenedDevices_kAudioDevicePropertyDataSources.count(device) == 0) {
+            BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
+                device.AddPropertyListenerBlock(CAPropertyAddress(kAudioDevicePropertyDataSources,
+                                                                  kAudioDevicePropertyScopeOutput),
+                                                dispatch_get_main_queue(),
+                                                refreshNeededListener);
+                listenedDevices_kAudioDevicePropertyDataSources.insert(device);
+            });
+        };
+
+        if (listenedDevices_kAudioDevicePropertyDataSource.count(device) == 0) {
+            BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
+                device.AddPropertyListenerBlock(CAPropertyAddress(kAudioDevicePropertyDataSource,
+                                                                  kAudioDevicePropertyScopeOutput),
+                                                dispatch_get_main_queue(),
+                                                refreshNeededListener);
+                listenedDevices_kAudioDevicePropertyDataSource.insert(device);
+            });
+        };
     }
 }
 
@@ -97,7 +202,7 @@ static NSInteger const kOutputDeviceMenuItemTag = 2;
     NSMutableArray<NSMenuItem*>* items = [NSMutableArray new];
 
     AudioObjectPropertyScope scope = kAudioObjectPropertyScopeOutput;
-    UInt32 channel = 0;  // 0 is the master channel.
+    UInt32 channel = kAudioObjectPropertyElementMaster;
     
     // If the device has data sources, create a menu item for each. Otherwise, create a single menu item
     // for the device. This way the menu items' titles will be, for example, "Internal Speakers" rather
@@ -106,8 +211,8 @@ static NSInteger const kOutputDeviceMenuItemTag = 2;
     // TODO: Handle data destinations as well? I don't have (or know of) any hardware with them.
     // TODO: Use the current data source's name when the control isn't settable, but only add one menu item.
     UInt32 numDataSources = 0;
-    
-    BGMLogAndSwallowExceptions("BGMOutputDevicePrefs::createMenuItemsForDevice", [&] {
+
+    BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
         if (device.HasDataSourceControl(scope, channel) &&
                 device.DataSourceControlIsSettable(scope, channel)) {
             numDataSources = device.GetNumberAvailableDataSources(scope, channel);
@@ -120,32 +225,32 @@ static NSInteger const kOutputDeviceMenuItemTag = 2;
         device.GetAvailableDataSources(scope, channel, numDataSources, dataSourceIDs);
         
         for (UInt32 i = 0; i < numDataSources; i++) {
-            DebugMsg("BGMOutputDevicePrefs::createMenuItemsForDevice: Creating item. %s%u %s%u",
+            DebugMsg("BGMOutputDeviceMenuSection::createMenuItemsForDevice: "
+                     "Creating item. %s%u %s%u",
                      "Device ID:", device.GetObjectID(),
                      ", Data source ID:", dataSourceIDs[i]);
-            
-            BGMLogAndSwallowExceptionsMsg("BGMOutputDevicePrefs::createMenuItemsForDevice", "(DS)", [&]() {
-                NSNumber* dataSourceID = [NSNumber numberWithUnsignedInt:dataSourceIDs[i]];
+
+            BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, "(DS)", [&] {
                 NSString* dataSourceName =
                     CFBridgingRelease(device.CopyDataSourceNameForID(scope, channel, dataSourceIDs[i]));
                 NSString* deviceName = CFBridgingRelease(device.CopyName());
                 
                 [items addObject:[self createMenuItemForDevice:device
-                                                  dataSourceID:dataSourceID
+                                                  dataSourceID:@(dataSourceIDs[i])
                                                          title:dataSourceName
                                                        toolTip:deviceName]];
             });
         }
     } else {
-        DebugMsg("BGMOutputDevicePrefs::createMenuItemsForDevice: Creating item. %s%u",
+        DebugMsg("BGMOutputDeviceMenuSection::createMenuItemsForDevice: Creating item. %s%u",
                  "Device ID:", device.GetObjectID());
-        
-        BGMLogAndSwallowExceptions("BGMOutputDevicePrefs::createMenuItemsForDevice", ([&] {
+
+        BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
             [items addObject:[self createMenuItemForDevice:device
                                               dataSourceID:nil
                                                      title:CFBridgingRelease(device.CopyName())
                                                    toolTip:nil]];
-        }));
+        });
     }
     
     return items;
@@ -161,13 +266,13 @@ static NSInteger const kOutputDeviceMenuItemTag = 2;
     }
     
     NSMenuItem* item = [[NSMenuItem alloc] initWithTitle:BGMNN(title)
-                                                  action:@selector(outputDeviceWasChanged:)
+                                                  action:@selector(outputDeviceMenuItemSelected:)
                                            keyEquivalent:@""];
     
     // Add the AirPlay icon to the labels of AirPlay devices.
     //
     // TODO: Test this with real hardware that supports AirPlay. (I don't have any.)
-    BGMLogAndSwallowExceptions("BGMOutputDevicePrefs::createMenuItemForDevice", [&] {
+    BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
         if (device.GetTransportType() == kAudioDeviceTransportTypeAirPlay) {
             item.image = [NSImage imageNamed:@"AirPlayIcon"];
             
@@ -189,12 +294,30 @@ static NSInteger const kOutputDeviceMenuItemTag = 2;
     item.indentationLevel = 1;
     item.representedObject = @{ @"deviceID": @(device.GetObjectID()),
                                 @"dataSourceID": dataSourceID ? BGMNN(dataSourceID) : [NSNull null] };
+
+#if __clang_major__ >= 9
+    if (@available(macOS 10.10, *)) {
+        // Used for UI tests.
+        item.accessibilityIdentifier = @"output-device";
+    }
+#endif
     
     return item;
 }
 
-- (void) outputDeviceWasChanged:(NSMenuItem*)menuItem {
-    DebugMsg("BGMOutputDevicePrefs::outputDeviceWasChanged: '%s' menu item selected",
+// Called by BGMAudioDeviceManager to tell us a different device has been set as the output device.
+- (void) outputDeviceDidChange {
+    BGMOutputDeviceMenuSection* __weak weakSelf = self;
+
+    dispatch_async(dispatch_get_main_queue(), ^{
+        BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
+            [weakSelf populateBGMMenu];
+        });
+    });
+}
+
+- (void) outputDeviceMenuItemSelected:(NSMenuItem*)menuItem {
+    DebugMsg("BGMOutputDeviceMenuSection::outputDeviceMenuItemSelected: '%s' menu item selected",
              [menuItem.title UTF8String]);
     
     // Make sure the menu item is actually for an output device.
@@ -217,6 +340,11 @@ static NSInteger const kOutputDeviceMenuItemTag = 2;
                 [NSString stringWithFormat:@"%@ (%@)", menuItem.title, menuItem.toolTip] :
                 menuItem.title;
 
+        if (changingDevice) {
+            // Add the new output device to the list of preferred devices.
+            [preferredDevices userChangedOutputDeviceTo:newDeviceID];
+        }
+
         // Dispatched because it usually blocks. (Note that we're using
         // DISPATCH_QUEUE_PRIORITY_HIGH, which is the second highest priority.)
         dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{

BGMApp/BGMApp/BGMOutputVolumeMenuItem.mm

@@ -17,7 +17,7 @@
 //  BGMOutputVolumeMenuItem.mm
 //  BGMApp
 //
-//  Copyright © 2017, 2018 Kyle Neideck
+//  Copyright © 2017-2019 Kyle Neideck
 //
 
 // Self Include
@@ -26,6 +26,7 @@
 // Local Includes
 #import "BGM_Utils.h"
 #import "BGMAudioDevice.h"
+#import "BGMVolumeChangeListener.h"
 
 // PublicUtility Includes
 #import "CAException.h"
@@ -46,7 +47,7 @@ NSString* const __nonnull      kGenericOutputDeviceName = @"Output Device";
     NSTextField* deviceLabel;
     NSSlider* volumeSlider;
     BGMAudioDevice outputDevice;
-    AudioObjectPropertyListenerBlock updateSliderListenerBlock;
+    BGMVolumeChangeListener* volumeChangeListener;
     AudioObjectPropertyListenerBlock updateLabelListenerBlock;
 }
 
@@ -66,9 +67,8 @@ NSString* const __nonnull      kGenericOutputDeviceName = @"Output Device";
         volumeSlider = slider;
         outputDevice = audioDevices.outputDevice;
 
-        // These are initialised in the methods called below.
-        updateSliderListenerBlock = nil;
-        updateLabelListenerBlock = nil;
+        // volumeChangeListener and updateLabelListenerBlock are initialised in the methods called
+        // below.
 
         // Apply our custom view from MainMenu.xib.
         self.view = view;
@@ -84,27 +84,11 @@ NSString* const __nonnull      kGenericOutputDeviceName = @"Output Device";
     return self;
 }
 
-// We currently only use one instance of this class and it's never deallocated, but it's probably
-// good practice to define dealloc anyway.
 - (void) dealloc {
-    dispatch_sync(dispatch_get_main_queue(), ^{
-        // Remove the audio property listeners.
-        [self removeOutputDeviceDataSourceListener];
-
-        BGMLogAndSwallowExceptions("BGMOutputVolumeMenuItem::dealloc", ([&] {
-            audioDevices.bgmDevice.RemovePropertyListenerBlock(
-                CAPropertyAddress(kAudioDevicePropertyVolumeScalar, kScope),
-                dispatch_get_main_queue(),
-                updateSliderListenerBlock);
-        }));
-
-        BGMLogAndSwallowExceptions("BGMOutputVolumeMenuItem::dealloc", ([&] {
-            audioDevices.bgmDevice.RemovePropertyListenerBlock(
-                CAPropertyAddress(kAudioDevicePropertyMute, kScope),
-                dispatch_get_main_queue(),
-                updateSliderListenerBlock);
-        }));
-    });
+    // Remove the audio property listeners.
+    // TODO: This call isn't thread safe. (But currently this dealloc method is only called if
+    //       there's an error.)
+    [self removeOutputDeviceDataSourceListener];
 }
 
 - (void) initSlider {
@@ -120,32 +104,9 @@ NSString* const __nonnull      kGenericOutputDeviceName = @"Output Device";
     // Register a listener that will update the slider when the user changes the volume or
     // mutes/unmutes their audio.
     BGMOutputVolumeMenuItem* __weak weakSelf = self;
-
-    updateSliderListenerBlock =
-        ^(UInt32 inNumberAddresses, const AudioObjectPropertyAddress* inAddresses) {
-            // The docs for AudioObjectPropertyListenerBlock say inAddresses will always contain
-            // at least one property the block is listening to, so there's no need to check it.
-            #pragma unused (inNumberAddresses, inAddresses)
-            [weakSelf updateVolumeSlider];
-        };
-
-    // Instead of swallowing exceptions, we could try again later, but I doubt it would be worth the
-    // effort. And the documentation doesn't actually explain what could cause this to fail.
-    BGMLogAndSwallowExceptions("BGMOutputVolumeMenuItem::initSlider", ([&] {
-        // Register the listener to receive volume notifications.
-        audioDevices.bgmDevice.AddPropertyListenerBlock(
-            CAPropertyAddress(kAudioDevicePropertyVolumeScalar, kScope),
-            dispatch_get_main_queue(),
-            updateSliderListenerBlock);
-    }));
-
-    BGMLogAndSwallowExceptions("BGMOutputVolumeMenuItem::initSlider", ([&] {
-        // Register the same listener for mute/unmute notifications.
-        audioDevices.bgmDevice.AddPropertyListenerBlock(
-            CAPropertyAddress(kAudioDevicePropertyMute, kScope),
-            dispatch_get_main_queue(),
-            updateSliderListenerBlock);
-    }));
+    volumeChangeListener = new BGMVolumeChangeListener(audioDevices.bgmDevice, [=] {
+        [weakSelf updateVolumeSlider];
+    });
 }
 
 // Updates the value of the output volume slider. Should only be called on the main thread because
@@ -180,7 +141,7 @@ NSString* const __nonnull      kGenericOutputDeviceName = @"Output Device";
             volumeSlider.doubleValue = 0.0;
         }
     }));
-};
+}
 
 - (void) addOutputDeviceDataSourceListener {
     // Create the block that updates deviceLabel when the output device's data source changes, e.g.
@@ -212,10 +173,20 @@ NSString* const __nonnull      kGenericOutputDeviceName = @"Output Device";
 - (void) removeOutputDeviceDataSourceListener {
     BGMLogAndSwallowExceptions("BGMOutputVolumeMenuItem::removeOutputDeviceDataSourceListener",
                                ([&] {
-        outputDevice.RemovePropertyListenerBlock(
-            CAPropertyAddress(kAudioDevicePropertyDataSource, kScope),
-            dispatch_get_main_queue(),
-            updateLabelListenerBlock);
+        // Technically, there's a race here in that the device could be removed after we check it
+        // exists, but before we try to remove the listener. We could check the error code of the
+        // exception and not log an error message if the code is kAudioHardwareBadObjectError or
+        // kAudioHardwareBadDeviceError, but it probably wouldn't be worth the effort.
+        //
+        // So for now the main reason for checking the device exists here is that it makes debug
+        // builds much less likely to crash here. (They crash/break when an error is logged so it
+        // will be noticed.)
+        if (CAHALAudioObject::ObjectExists(outputDevice)) {
+            outputDevice.RemovePropertyListenerBlock(
+                CAPropertyAddress(kAudioDevicePropertyDataSource, kScope),
+                dispatch_get_main_queue(),
+                updateLabelListenerBlock);
+        }
     }));
 }
 
@@ -240,36 +211,53 @@ NSString* const __nonnull      kGenericOutputDeviceName = @"Output Device";
 // datasource, the device's name is set as this menu item's tooltip. Falls back to a generic name if
 // the device returns an error when queried.
 - (void) updateLabelAndToolTip {
-    BOOL didSetLabel = NO;
-
-    try {
-        if (outputDevice.HasDataSourceControl(kScope, kMasterChannel)) {
-            // The device has datasources, so use the current datasource's name like macOS does.
-            UInt32 dataSourceID = outputDevice.GetCurrentDataSourceID(kScope, kMasterChannel);
-
-            deviceLabel.stringValue =
-                (__bridge_transfer NSString*)outputDevice.CopyDataSourceNameForID(kScope,
-                                                                                  kMasterChannel,
-                                                                                  dataSourceID);
-            didSetLabel = YES;  // So we know not to change the text if setting the tooltip fails.
-
-            // Set the tooltip of the menu item (the container) rather than the label because menu
-            // items' tooltips will still appear when a different app is focused and, as far as I
-            // know, BGMApp should never be the foreground app.
-            self.toolTip = (__bridge_transfer NSString*)outputDevice.CopyName();
-        } else {
-            deviceLabel.stringValue = (__bridge_transfer NSString*)outputDevice.CopyName();
-            self.toolTip = nil;
-        }
-    } catch (const CAException& e) {
-        BGMLogException(e);
-
-        // The device returned an error, so set the label to a generic device name, since we don't
-        // want to leave it set to the previous device's name.
+    if (outputDevice.GetObjectID() == kAudioObjectUnknown) {
+        DebugMsg("BGMOutputVolumeMenuItem::updateLabelAndToolTip: Output device unknown. Using the "
+                 "generic label.");
         self.toolTip = nil;
+        deviceLabel.stringValue = kGenericOutputDeviceName;
+    } else {
+        BOOL didSetLabel = NO;
 
-        if (!didSetLabel) {
-            deviceLabel.stringValue = kGenericOutputDeviceName;
+        DebugMsg("BGMOutputVolumeMenuItem::updateLabelAndToolTip: Output device: %u",
+                 outputDevice.GetObjectID());
+
+        try {
+            if (outputDevice.HasDataSourceControl(kScope, kMasterChannel)) {
+                DebugMsg("BGMOutputVolumeMenuItem::updateLabelAndToolTip: Getting data source ID");
+                // The device has datasources, so use the current datasource's name like macOS does.
+                UInt32 dataSourceID = outputDevice.GetCurrentDataSourceID(kScope, kMasterChannel);
+
+                DebugMsg("BGMOutputVolumeMenuItem::updateLabelAndToolTip: "
+                         "Getting name for data source %u",
+                         dataSourceID);
+                deviceLabel.stringValue =
+                    (__bridge_transfer NSString*)outputDevice.CopyDataSourceNameForID(
+                        kScope, kMasterChannel, dataSourceID);
+
+                // So we know not to change the text if setting the tooltip fails.
+                didSetLabel = YES;
+
+                DebugMsg("BGMOutputVolumeMenuItem::updateLabelAndToolTip: Getting device name");
+                // Set the tooltip of the menu item (the container) rather than the label because
+                // menu items' tooltips will still appear when a different app is focused and, as
+                // far as I know, BGMApp should never be the foreground app.
+                self.toolTip = (__bridge_transfer NSString*)outputDevice.CopyName();
+            } else {
+                DebugMsg("BGMOutputVolumeMenuItem::updateLabelAndToolTip: Getting device name");
+                deviceLabel.stringValue = (__bridge_transfer NSString*)outputDevice.CopyName();
+                self.toolTip = nil;
+            }
+        } catch (const CAException& e) {
+            BGMLogException(e);
+
+            // The device returned an error, so set the label to a generic device name, since we
+            // don't want to leave it set to the previous device's name.
+            self.toolTip = nil;
+
+            if (!didSetLabel) {
+                deviceLabel.stringValue = kGenericOutputDeviceName;
+            }
         }
     }
 

BGMApp/BGMApp/BGMPlayThrough.cpp

@@ -17,7 +17,7 @@
 //  BGMPlayThrough.cpp
 //  BGMApp
 //
-//  Copyright © 2016, 2017 Kyle Neideck
+//  Copyright © 2016, 2017, 2020 Kyle Neideck
 //
 
 // Self Include
@@ -28,7 +28,6 @@
 #include "BGM_Utils.h"
 
 // PublicUtility Includes
-#include "CAAtomic.h"
 #include "CAHALAudioSystemObject.h"
 #include "CAPropertyAddress.h"
 
@@ -57,9 +56,20 @@ BGMPlayThrough::BGMPlayThrough(BGMAudioDevice inInputDevice, BGMAudioDevice inOu
 
 BGMPlayThrough::~BGMPlayThrough()
 {
+    CAMutex::Locker stateLocker(mStateMutex);
+
     BGMLogAndSwallowExceptionsMsg("BGMPlayThrough::~BGMPlayThrough", "Deactivate", [&]() {
         Deactivate();
     });
+
+    // If one of the IOProcs failed to stop, CoreAudio could (at least in theory) still call it
+    // after this point. This isn't a solution, but calling DeallocateBuffer instead of letting it
+    // deallocate itself should at least make the error less likely to cause a segfault, since
+    // DeallocateBuffer takes the buffer locks and sets mBuffer to null.
+    //
+    // TODO: It probably wouldn't be too hard to fix this properly by giving the IOProcs weak refs
+    //       to the BGMPlayThrough object instead of raw pointers.
+    DeallocateBuffer();
     
     if(mOutputDeviceIOProcSemaphore != SEMAPHORE_NULL)
     {
@@ -97,7 +107,7 @@ void    BGMPlayThrough::Init(BGMAudioDevice inInputDevice, BGMAudioDevice inOutp
     catch (...)
     {
         // Clean up.
-        mBuffer.Deallocate();
+        DeallocateBuffer();
         throw;
     }
 }
@@ -178,7 +188,7 @@ void    BGMPlayThrough::Deactivate()
         DebugMsg("BGMPlayThrough::Deactivate: Deactivating playthrough");
         
         bool inputDeviceIsBGMDevice = true;
-        
+
         CATry
         inputDeviceIsBGMDevice = mInputDevice.IsBGMDeviceInstance();
         CACatch
@@ -231,13 +241,29 @@ void    BGMPlayThrough::AllocateBuffer()
         Throw(CAException(kAudioHardwareUnsupportedOperationError));
     }
     
+    // Need to lock the buffer mutexes to make sure the IOProcs aren't accessing it. The order is
+    // important here. We always lock them in the same order to prevent deadlocks.
+    CAMutex::Locker lockerInput(mBufferInputMutex);
+    CAMutex::Locker lockerOutput(mBufferOutputMutex);
+
+    mBuffer = std::unique_ptr<CARingBuffer>(new CARingBuffer);
+
     // The calculation for the size of the buffer is from Apple's CAPlayThrough.cpp sample code
     //
     // TODO: Test playthrough with hardware with more than 2 channels per frame, a sample (virtual) format other than
     //       32-bit floats and/or an IO buffer size other than 512 frames
-    mBuffer.Allocate(outputFormat[0].mChannelsPerFrame,
-                     outputFormat[0].mBytesPerFrame,
-                     mOutputDevice.GetIOBufferSize() * 20);
+    mBuffer->Allocate(outputFormat[0].mChannelsPerFrame,
+                      outputFormat[0].mBytesPerFrame,
+                      mOutputDevice.GetIOBufferSize() * 20);
+}
+
+void    BGMPlayThrough::DeallocateBuffer()
+{
+    // Need to lock the buffer mutexes to make sure the IOProcs aren't accessing it. The order is
+    // important here. We always lock them in the same order to prevent deadlocks.
+    CAMutex::Locker lockerInput(mBufferInputMutex);
+    CAMutex::Locker lockerOutput(mBufferOutputMutex);
+    mBuffer = nullptr;  // Note that the buffer's destructor will deallocate it.
 }
 
 void    BGMPlayThrough::CreateIOProcIDs()
@@ -569,20 +595,21 @@ OSStatus    BGMPlayThrough::WaitForOutputDeviceToStart() noexcept
     while((theError != KERN_SUCCESS) &&         // Signalled from the IOProc.
           (state == IOState::Starting) &&       // IO state changed.
           (waitedNsec < kStartIOTimeoutNsec));  // Timed out.
-    
-#if DEBUG
-    UInt64 startedBy = mach_absolute_time();
-    
-    struct mach_timebase_info baseInfo = { 0, 0 };
-    mach_timebase_info(&baseInfo);
-    UInt64 base = baseInfo.numer / baseInfo.denom;
-    
-    DebugMsg("BGMPlayThrough::WaitForOutputDeviceToStart: Started %f ms after notification, %f ms "
-             "after entering WaitForOutputDeviceToStart.",
-             static_cast<Float64>(startedBy - mToldOutputDeviceToStartAt) * base / NSEC_PER_MSEC,
-             static_cast<Float64>(startedBy - startedAt) * base / NSEC_PER_MSEC);
-#endif
-    
+
+    if(BGMDebugLoggingIsEnabled())
+    {
+        UInt64 startedBy = mach_absolute_time();
+
+        struct mach_timebase_info baseInfo = { 0, 0 };
+        mach_timebase_info(&baseInfo);
+        UInt64 base = baseInfo.numer / baseInfo.denom;
+
+        DebugMsg("BGMPlayThrough::WaitForOutputDeviceToStart: Started %f ms after notification, %f "
+                 "ms after entering WaitForOutputDeviceToStart.",
+                 static_cast<Float64>(startedBy - mToldOutputDeviceToStartAt) * base / NSEC_PER_MSEC,
+                 static_cast<Float64>(startedBy - startedAt) * base / NSEC_PER_MSEC);
+    }
+
     // Figure out which error code to return.
     switch (theError)
     {
@@ -604,7 +631,7 @@ OSStatus    BGMPlayThrough::WaitForOutputDeviceToStart() noexcept
 
 // Release any threads waiting for the output device to start. This function doesn't take mStateMutex
 // because it gets called on the IO thread, which is realtime priority.
-void    BGMPlayThrough::ReleaseThreadsWaitingForOutputToStart() const
+void    BGMPlayThrough::ReleaseThreadsWaitingForOutputToStart()
 {
     if(mActive)
     {
@@ -612,13 +639,10 @@ void    BGMPlayThrough::ReleaseThreadsWaitingForOutputToStart() const
         
         if(semaphore != SEMAPHORE_NULL)
         {
-            DebugMsg("BGMPlayThrough::ReleaseThreadsWaitingForOutputToStart: Releasing waiting threads");
+            mRTLogger.LogReleasingWaitingThreads();
+
             kern_return_t theError = semaphore_signal_all(semaphore);
-            
-            // TODO: Tell another thread to log this error, since we might be on a realtime thread.
-            BGM_Utils::LogIfMachError("BGMPlayThrough::ReleaseThreadsWaitingForOutputToStart",
-                                      "semaphore_signal_all",
-                                      theError);
+            mRTLogger.LogIfMachError_ReleaseWaitingThreadsSignal(theError);
         }
     }
 }
@@ -638,21 +662,16 @@ OSStatus    BGMPlayThrough::Stop()
         bool outputDeviceAlive = false;
         
         CATry
-        inputDeviceAlive = mInputDevice.IsAlive();
+        inputDeviceAlive = CAHALAudioObject::ObjectExists(mInputDevice) && mInputDevice.IsAlive();
         CACatch
         
         CATry
-        outputDeviceAlive = mOutputDevice.IsAlive();
+        outputDeviceAlive =
+            CAHALAudioObject::ObjectExists(mOutputDevice) && mOutputDevice.IsAlive();
         CACatch
-        
-        if(inputDeviceAlive)
-        {
-            mInputDeviceIOProcState = IOState::Stopping;
-        }
-        if(outputDeviceAlive)
-        {
-            mOutputDeviceIOProcState = IOState::Stopping;
-        }
+
+        mInputDeviceIOProcState = inputDeviceAlive ? IOState::Stopping : IOState::Stopped;
+        mOutputDeviceIOProcState = outputDeviceAlive ? IOState::Stopping : IOState::Stopped;
         
         // Wait for the IOProcs to stop themselves. This is so the IOProcs don't get called after the BGMPlayThrough instance
         // (pointed to by the client data they get from the HAL) is deallocated.
@@ -662,14 +681,28 @@ OSStatus    BGMPlayThrough::Stop()
         //     when you make the call from outside of your IOProc. However, if you call AudioDeviceStop() from inside your IOProc,
         //     you do get the guarantee that your IOProc will not get called again after the IOProc has returned.
         UInt64 totalWaitNs = 0;
-        BGMLogAndSwallowExceptions("BGMPlayThrough::Stop", [&]() {
-            Float64 expectedInputCycleNs =
-                mInputDevice.GetIOBufferSize() * (1 / mInputDevice.GetNominalSampleRate()) * NSEC_PER_SEC;
-            Float64 expectedOutputCycleNs =
-                mOutputDevice.GetIOBufferSize() * (1 / mOutputDevice.GetNominalSampleRate()) * NSEC_PER_SEC;
+        BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&]() {
+            Float64 expectedInputCycleNs = 0;
+
+            if(inputDeviceAlive)
+            {
+                expectedInputCycleNs =
+                    mInputDevice.GetIOBufferSize() * (1 / mInputDevice.GetNominalSampleRate()) *
+                            NSEC_PER_SEC;
+            }
+
+            Float64 expectedOutputCycleNs = 0;
+
+            if(outputDeviceAlive)
+            {
+                expectedOutputCycleNs =
+                    mOutputDevice.GetIOBufferSize() * (1 / mOutputDevice.GetNominalSampleRate()) *
+                            NSEC_PER_SEC;
+            }
+
             UInt64 expectedMaxCycleNs =
                 static_cast<UInt64>(std::max(expectedInputCycleNs, expectedOutputCycleNs));
-            
+
             while((mInputDeviceIOProcState == IOState::Stopping || mOutputDeviceIOProcState == IOState::Stopping)
                   && (totalWaitNs < kStopIOProcTimeoutInIOCycles * expectedMaxCycleNs))
             {
@@ -858,12 +891,15 @@ void    BGMPlayThrough::HandleBGMDeviceIsRunning(BGMPlayThrough* refCon)
                 isRunningSomewhereOtherThanBGMApp =
                     IsRunningSomewhereOtherThanBGMApp(refCon->mInputDevice);
             });
+
+            DebugMsg("BGMPlayThrough::HandleBGMDeviceIsRunning: "
+                     "BGMDevice is %srunning somewhere other than BGMApp",
+                     isRunningSomewhereOtherThanBGMApp ? "" : " not");
             
             if(isRunningSomewhereOtherThanBGMApp)
             {
-#if DEBUG
                 refCon->mToldOutputDeviceToStartAt = mach_absolute_time();
-#endif
+
                 // TODO: Handle expected exceptions (mostly CAExceptions from PublicUtility classes) in Start.
                 //       For any that can't be handled sensibly in Start, catch them here and retry a few
                 //       times (with a very short delay) before handling them by showing an unobtrusive error
@@ -904,8 +940,8 @@ bool    BGMPlayThrough::IsRunningSomewhereOtherThanBGMApp(const BGMAudioDevice&
 
 #pragma mark IOProcs
 
-// Note that the IOProcs will very likely not run on the same thread and that they intentionally don't
-// lock any mutexes.
+// Note that the IOProcs will very likely not run on the same thread and that they intentionally
+// only lock mutexes around their use of mBuffer.
 
 // static
 OSStatus    BGMPlayThrough::InputDeviceIOProc(AudioObjectID           inDevice,
@@ -923,6 +959,7 @@ OSStatus    BGMPlayThrough::InputDeviceIOProc(AudioObjectID           inDevice,
     
     IOState state;
     UpdateIOProcState("InputDeviceIOProc",
+                      refCon->mRTLogger,
                       refCon->mInputDeviceIOProcState,
                       refCon->mInputDeviceIOProcID,
                       refCon->mInputDevice,
@@ -943,16 +980,30 @@ OSStatus    BGMPlayThrough::InputDeviceIOProc(AudioObjectID           inDevice,
     
     UInt32 framesToStore = inInputData->mBuffers[0].mDataByteSize / (SizeOf32(Float32) * 2);
 
-    CARingBufferError err =
-        refCon->mBuffer.Store(inInputData,
-                              framesToStore,
-                              static_cast<CARingBuffer::SampleTime>(inInputTime->mSampleTime));
-    
-    HandleRingBufferError(err, "InputDeviceIOProc", "mBuffer.Store");
-    
-    CAMemoryBarrier();
-    refCon->mLastInputSampleTime = inInputTime->mSampleTime;
-    
+    // See the comments in OutputDeviceIOProc where it locks mBufferOutputMutex.
+    CAMutex::Tryer tryer(refCon->mBufferInputMutex);
+
+    // Disable a warning about accessing mBuffer without holding both mBufferInputMutex and
+    // mBufferOutputMutex. Explained further in OutputDeviceIOProc.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wthread-safety"
+    if(tryer.HasLock() && refCon->mBuffer)
+    {
+        CARingBufferError err =
+                refCon->mBuffer->Store(inInputData,
+                                       framesToStore,
+                                       static_cast<CARingBuffer::SampleTime>(
+                                               inInputTime->mSampleTime));
+#pragma clang diagnostic pop
+        refCon->mRTLogger.LogIfRingBufferError_Store(err);
+
+        refCon->mLastInputSampleTime = inInputTime->mSampleTime;
+    }
+    else
+    {
+        refCon->mRTLogger.LogRingBufferUnavailable("InputDeviceIOProc", tryer.HasLock());
+    }
+
     return noErr;
 }
 
@@ -965,13 +1016,14 @@ OSStatus    BGMPlayThrough::OutputDeviceIOProc(AudioObjectID           inDevice,
                                                const AudioTimeStamp*   inOutputTime,
                                                void* __nullable        inClientData)
 {
-    #pragma unused (inDevice, inNow, inInputData, inInputTime, inOutputTime)
+    #pragma unused (inDevice, inNow, inInputData, inInputTime)
     
     // refCon (reference context) is the instance that created the IOProc
     BGMPlayThrough* const refCon = static_cast<BGMPlayThrough*>(inClientData);
     
     IOState state;
     const bool didChangeState = UpdateIOProcState("OutputDeviceIOProc",
+                                                  refCon->mRTLogger,
                                                   refCon->mOutputDeviceIOProcState,
                                                   refCon->mOutputDeviceIOProcID,
                                                   refCon->mOutputDevice,
@@ -980,6 +1032,7 @@ OSStatus    BGMPlayThrough::OutputDeviceIOProc(AudioObjectID           inDevice,
     if(state == IOState::Stopped || state == IOState::Stopping)
     {
         // Return early, since we just asked to stop. (Or something really weird is going on.)
+        FillWithSilence(outOutputData);
         return noErr;
     }
     
@@ -999,8 +1052,7 @@ OSStatus    BGMPlayThrough::OutputDeviceIOProc(AudioObjectID           inDevice,
     if(refCon->mLastInputSampleTime == -1)
     {
         // Return early, since we don't have any data to output yet.
-        //
-        // TODO: Write silence to outOutputData here
+        FillWithSilence(outOutputData);
         return noErr;
     }
     
@@ -1011,13 +1063,8 @@ OSStatus    BGMPlayThrough::OutputDeviceIOProc(AudioObjectID           inDevice,
         refCon->mInToOutSampleOffset = inOutputTime->mSampleTime - refCon->mLastInputSampleTime;
         
         // Log if we dropped frames
-        if(refCon->mFirstInputSampleTime != refCon->mLastInputSampleTime)
-        {
-            DebugMsg("BGMPlayThrough::OutputDeviceIOProc: Dropped %f frames before output started. %s%f %s%f",
-                     (refCon->mLastInputSampleTime - refCon->mFirstInputSampleTime),
-                     "mFirstInputSampleTime=", refCon->mFirstInputSampleTime,
-                     "mLastInputSampleTime=", refCon->mLastInputSampleTime);
-        }
+        refCon->mRTLogger.LogIfDroppedFrames(refCon->mFirstInputSampleTime,
+                                             refCon->mLastInputSampleTime);
     }
     
     CARingBuffer::SampleTime readHeadSampleTime =
@@ -1026,47 +1073,93 @@ OSStatus    BGMPlayThrough::OutputDeviceIOProc(AudioObjectID           inDevice,
         static_cast<CARingBuffer::SampleTime>(refCon->mLastInputSampleTime);
     
     UInt32 framesToOutput = outOutputData->mBuffers[0].mDataByteSize / (SizeOf32(Float32) * 2);
-    
-    // Very occasionally (at least for me) our read head gets ahead of input, i.e. we haven't received any new input since
-    // this IOProc was last called, and we have to recalculate its position. I figure this might be caused by clock drift
-    // but I'm really not sure. It also happens if the input or output sample times are restarted from zero.
-    //
-    // We also recalculate the offset if the read head is outside of the ring buffer. This happens for example when you plug
-    // in or unplug headphones, which causes the output sample times to be restarted from zero.
-    //
-    // The vast majority of the time, just using lastInputSampleTime as the read head time instead of the one we calculate
-    // would work fine (and would also account for the above).
-    SInt64 bufferStartTime, bufferEndTime;
-    CARingBufferError err = refCon->mBuffer.GetTimeBounds(bufferStartTime, bufferEndTime);
-    bool outOfBounds = false;
-    if(err == kCARingBufferError_OK)
-    {
-        outOfBounds = (readHeadSampleTime < bufferStartTime) || (readHeadSampleTime - framesToOutput > bufferEndTime);
-    }
-    if(lastInputSampleTime < readHeadSampleTime || outOfBounds)
-    {
-        DebugMsg("BGMPlayThrough::OutputDeviceIOProc: No input samples ready at output sample time. %s%lld %s%lld %s%f",
-                 "lastInputSampleTime=", lastInputSampleTime,
-                 "readHeadSampleTime=", readHeadSampleTime,
-                 "mInToOutSampleOffset=", refCon->mInToOutSampleOffset);
-        
-        // Recalculate the in-to-out offset and read head
-        refCon->mInToOutSampleOffset = inOutputTime->mSampleTime - lastInputSampleTime;
-        readHeadSampleTime = static_cast<CARingBuffer::SampleTime>(inOutputTime->mSampleTime - refCon->mInToOutSampleOffset);
-    }
 
-    // Copy the frames from the ring buffer
-    err = refCon->mBuffer.Fetch(outOutputData, framesToOutput, readHeadSampleTime);
-    
-    HandleRingBufferError(err, "OutputDeviceIOProc", "mBuffer.Fetch");
-    
+    // When the input and output devices are set, during start up or because the user changed the
+    // output device, this class (re)allocates the ring buffer (mBuffer). We try to take this
+    // lock before accessing the buffer to make sure it's allocated.
+    //
+    // If we don't get the lock, another thread must be allocating or deallocating it, so we just
+    // give up. We can't avoid audio glitches while changing devices anyway. This class tries to
+    // make sure the IOProcs aren't running when it allocates the buffer, but it can't guarantee
+    // that.
+    //
+    // Note that this is only realtime safe because we only try to lock the mutex. If another
+    // thread has the mutex, it will be a non-realtime thread, so we can't wait for it.
+    CAMutex::Tryer tryer(refCon->mBufferOutputMutex);
+
+    // Disable a warning about accessing mBuffer without holding both mBufferInputMutex and
+    // mBufferOutputMutex. The input IOProc always writes ahead of where the output IOProc will read
+    // in a given IO cycle, so it's safe for them to read and write at the same time.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wthread-safety"
+    if(tryer.HasLock() && refCon->mBuffer)
+    {
+        // Very occasionally (at least for me) our read head gets ahead of input, i.e. we haven't
+        // received any new input since this IOProc was last called, and we have to recalculate its
+        // position. I figure this might be caused by clock drift but I'm really not sure. It also
+        // happens if the input or output sample times are restarted from zero.
+        //
+        // We also recalculate the offset if the read head is outside of the ring buffer. This
+        // happens for example when you plug in or unplug headphones, which causes the output sample
+        // times to be restarted from zero.
+        //
+        // The vast majority of the time, just using lastInputSampleTime as the read head time
+        // instead of the one we calculate would work fine (and would also account for the above).
+        SInt64 bufferStartTime, bufferEndTime;
+        CARingBufferError err = refCon->mBuffer->GetTimeBounds(bufferStartTime, bufferEndTime);
+        bool outOfBounds = false;
+
+        if(err == kCARingBufferError_OK)
+        {
+            outOfBounds = (readHeadSampleTime < bufferStartTime)
+                    || (readHeadSampleTime - framesToOutput > bufferEndTime);
+        }
+
+        if(lastInputSampleTime < readHeadSampleTime || outOfBounds)
+        {
+            refCon->mRTLogger.LogNoSamplesReady(lastInputSampleTime,
+                                                readHeadSampleTime,
+                                                refCon->mInToOutSampleOffset);
+
+            // Recalculate the in-to-out offset and read head.
+            refCon->mInToOutSampleOffset = inOutputTime->mSampleTime - lastInputSampleTime;
+            readHeadSampleTime = static_cast<CARingBuffer::SampleTime>(
+                    inOutputTime->mSampleTime - refCon->mInToOutSampleOffset);
+        }
+
+        // Copy the frames from the ring buffer.
+        err = refCon->mBuffer->Fetch(outOutputData, framesToOutput, readHeadSampleTime);
+        refCon->mRTLogger.LogIfRingBufferError_Fetch(err);
+
+        if(err != kCARingBufferError_OK)
+        {
+            FillWithSilence(outOutputData);
+        }
+    }
+    else
+    {
+        refCon->mRTLogger.LogRingBufferUnavailable("OutputDeviceIOProc", tryer.HasLock());
+        FillWithSilence(outOutputData);
+    }
+#pragma clang diagnostic pop
+
     refCon->mLastOutputSampleTime = inOutputTime->mSampleTime;
     
     return noErr;
 }
 
 // static
-bool    BGMPlayThrough::UpdateIOProcState(const char* __nullable callerName,
+inline void BGMPlayThrough::FillWithSilence(AudioBufferList* ioBuffer)
+{
+    for(UInt32 i = 0; i < ioBuffer->mNumberBuffers; i++)
+    {
+        memset(ioBuffer->mBuffers[i].mData, 0, ioBuffer->mBuffers[i].mDataByteSize);
+    }
+}
+
+// static
+bool    BGMPlayThrough::UpdateIOProcState(const char* inCallerName,
+                                          BGMPlayThroughRTLogger& inRTLogger,
                                           std::atomic<IOState>& inState,
                                           AudioDeviceIOProcID __nullable inIOProcID,
                                           BGMAudioDevice& inDevice,
@@ -1079,6 +1172,9 @@ bool    BGMPlayThrough::UpdateIOProcState(const char* __nullable callerName,
     //
     // compare_exchange_strong will return true iff it changed inState from Starting to Running.
     // Otherwise it will set prevState to the current value of inState.
+    //
+    // TODO: We probably don't actually need memory_order_seq_cst (the default). Would it be worth
+    //       changing? Might be worth checking for the other atomics/barriers in this class, too.
     IOState prevState = IOState::Starting;
     bool didChangeState = inState.compare_exchange_strong(prevState, IOState::Running);
 
@@ -1096,21 +1192,28 @@ bool    BGMPlayThrough::UpdateIOProcState(const char* __nullable callerName,
         {
             // The IOProc isn't Starting or Running, so it must be Stopping. That is, it's been
             // told to stop itself.
-            
             BGMAssert(outNewState == IOState::Stopping,
                       "BGMPlayThrough::UpdateIOProcState: Unexpected state: %d",
                       outNewState);
             
             bool stoppedSuccessfully = false;
-            BGMLogAndSwallowExceptionsMsg("BGMPlayThrough::UpdateIOProcState", callerName, [&]() {
-                // TODO: If this throws, tell another thread to log the exception rather than
-                //       logging it from a real-time thread.
+
+            try
+            {
                 inDevice.StopIOProc(inIOProcID);
 
                 // StopIOProc didn't throw, so the IOProc won't be called again until the next
                 // time playthrough is started.
                 stoppedSuccessfully = true;
-            });
+            }
+            catch(CAException e)
+            {
+                inRTLogger.LogExceptionStoppingIOProc(inCallerName, e.GetError());
+            }
+            catch(...)
+            {
+                inRTLogger.LogExceptionStoppingIOProc(inCallerName);
+            }
 
             if(stoppedSuccessfully)
             {
@@ -1123,7 +1226,7 @@ bool    BGMPlayThrough::UpdateIOProcState(const char* __nullable callerName,
                 //
                 // Stop won't return until the IOProc has changed inState to Stopped, unless it
                 // times out, so Stop should still be waiting. And since Start and Stop are
-                // mutually exclusive, so this should be safe.
+                // mutually exclusive, this should be safe.
                 //
                 // But if Stop has timed out and inState has changed, we leave it in its new
                 // state (unless there's some ABA problem thing happening), which I suspect is
@@ -1136,8 +1239,8 @@ bool    BGMPlayThrough::UpdateIOProcState(const char* __nullable callerName,
                 }
                 else
                 {
-                    DebugMsg("BGMPlayThrough::UpdateIOProcState: inState changed since last read "
-                             "outNewState = %d", outNewState);
+                    inRTLogger.LogUnexpectedIOStateAfterStopping(inCallerName,
+                                                                 static_cast<int>(outNewState));
                 }
             }
         }
@@ -1146,30 +1249,3 @@ bool    BGMPlayThrough::UpdateIOProcState(const char* __nullable callerName,
     return didChangeState;
 }
 
-// static
-void    BGMPlayThrough::HandleRingBufferError(CARingBufferError inErr,
-                                              const char* inMethodName,
-                                              const char* inCallReturningErr)
-{
-#if DEBUG
-    if(inErr != kCARingBufferError_OK)
-    {
-        const char* errStr = (inErr == kCARingBufferError_TooMuch ? "kCARingBufferError_TooMuch" :
-                              (inErr == kCARingBufferError_CPUOverload ? "kCARingBufferError_CPUOverload" : "unknown error"));
-
-        DebugMsg("BGMPlayThrough::%s: %s returned %s (%d)", inMethodName, inCallReturningErr, errStr, inErr);
-        
-        // kCARingBufferError_CPUOverload wouldn't mean we have a bug, but I think kCARingBufferError_TooMuch would
-        if(inErr != kCARingBufferError_CPUOverload)
-        {
-            Throw(CAException(inErr));
-        }
-    }
-#else
-    // Not sure what we should do to handle these errors in release builds, if anything.
-    // TODO: There's code in Apple's CAPlayThrough.cpp sample code that handles them. (Look for "kCARingBufferError_OK"
-    //       around line 707.) Should be easy enough to use, but it's more complicated that just directly copying it.
-    #pragma unused (inErr, inMethodName, inCallReturningErr)
-#endif
-}
-

BGMApp/BGMApp/BGMPlayThrough.h

@@ -17,7 +17,7 @@
 //  BGMPlayThrough.h
 //  BGMApp
 //
-//  Copyright © 2016, 2017 Kyle Neideck
+//  Copyright © 2016, 2017, 2020 Kyle Neideck
 //
 //  Reads audio from an input device and immediately writes it to an output device. We currently use this class with the input
 //  device always set to BGMDevice and the output device set to the one selected in the preferences menu.
@@ -41,14 +41,17 @@
 
 // Local Includes
 #include "BGMAudioDevice.h"
+#include "BGMPlayThroughRTLogger.h"
 
 // PublicUtility Includes
-#include "CARingBuffer.h"
 #include "CAMutex.h"
+#include "CARingBuffer.h"
+#include "BGMThreadSafetyAnalysis.h"
 
 // STL Includes
 #include <atomic>
 #include <algorithm>
+#include <memory>
 
 // System Includes
 #include <mach/semaphore.h>
@@ -78,7 +81,8 @@ public:
     
 private:
     /*! @throws CAException */
-    void                Init(BGMAudioDevice inInputDevice, BGMAudioDevice inOutputDevice);
+    void                Init(BGMAudioDevice inInputDevice, BGMAudioDevice inOutputDevice)
+                            REQUIRES(mStateMutex);
 
 public:
     /*! @throws CAException */
@@ -87,7 +91,8 @@ public:
     void                Deactivate();
 
 private:
-    void                AllocateBuffer();
+    void                AllocateBuffer() REQUIRES(mStateMutex);
+    void                DeallocateBuffer();
 
     /*! @throws CAException */
     void                CreateIOProcIDs();
@@ -97,7 +102,7 @@ private:
         @return True if both IOProcs are stopped.
         @nonthreadsafe
      */
-    bool                CheckIOProcsAreStopped() const noexcept; // TODO: REQUIRES(mStateMutex);
+    bool                CheckIOProcsAreStopped() const noexcept REQUIRES(mStateMutex);
 	
 public:
     /*!
@@ -115,7 +120,8 @@ public:
     OSStatus            WaitForOutputDeviceToStart() noexcept;
     
 private:
-    void                ReleaseThreadsWaitingForOutputToStart() const;
+    /*! Real-time safe. */
+    void                ReleaseThreadsWaitingForOutputToStart();
     
 public:
     OSStatus            Stop();
@@ -146,7 +152,10 @@ private:
                                            AudioBufferList*        outOutputData,
                                            const AudioTimeStamp*   inOutputTime,
                                            void* __nullable        inClientData);
-    
+
+    /*! Fills the given ABL with zeroes to make it silent. */
+    static inline void  FillWithSilence(AudioBufferList* ioBuffer);
+
     // The state of an IOProc. Used by the IOProc to tell other threads when it's finished starting. Used by other
     // threads to tell the IOProc to stop itself. (Probably used for other things as well.)
     enum class          IOState
@@ -156,27 +165,45 @@ private:
     
     // The IOProcs call this to update their IOState member. Also stops the IOProc if its state has been set to Stopping.
     // Returns true if it changes the state.
-    static bool         UpdateIOProcState(const char* __nullable callerName,
+    static bool         UpdateIOProcState(const char* inCallerName,
+                                          BGMPlayThroughRTLogger& inRTLogger,
                                           std::atomic<IOState>& inState,
                                           AudioDeviceIOProcID __nullable inIOProcID,
                                           BGMAudioDevice& inDevice,
                                           IOState& outNewState);
     
-    static void         HandleRingBufferError(CARingBufferError err,
-                                              const char* methodName,
-                                              const char* callReturningErr);
-    
 private:
-    CARingBuffer        mBuffer;
+    std::unique_ptr<CARingBuffer>    mBuffer PT_GUARDED_BY(mBufferInputMutex)
+                                        PT_GUARDED_BY(mBufferOutputMutex) { nullptr };
     
     AudioDeviceIOProcID __nullable mInputDeviceIOProcID { nullptr };
     AudioDeviceIOProcID __nullable mOutputDeviceIOProcID { nullptr };
     
     BGMAudioDevice      mInputDevice { kAudioObjectUnknown };
     BGMAudioDevice      mOutputDevice { kAudioObjectUnknown };
-    
-    CAMutex             mStateMutex { "Playthrough state" };
-    
+
+    // mStateMutex is the general purpose mutex. mBufferInputMutex and mBufferOutputMutex are
+    // just used to make sure mBuffer, the ring buffer, is allocated when the IOProcs access it. See
+    // the comments in the IOProcs for details.
+    //
+    // If a thread might lock more than one of these mutexes, it *must* take them in this order:
+    //     1. mStateMutex
+    //     2. mBufferInputMutex
+    //     3. mBufferOutputMutex
+    //
+    // The ACQUIRED_BEFORE annotations don't do anything yet. From clang's docs: "ACQUIRED_BEFORE(…)
+    // and ACQUIRED_AFTER(…) are currently unimplemented. To be fixed in a future update." After
+    // they've fixed that, the compiler will enforce the ordering statically.
+    //
+    // TODO: We can't use std::shared_lock because we're still on C++11, but we could use std::lock
+    //       to help ensure the locks are always taken in the right order.
+    // TODO: It would be better to have a separate class for the buffer and its mutexes.
+    CAMutex             mStateMutex ACQUIRED_BEFORE(mBufferInputMutex)
+                            ACQUIRED_BEFORE(mBufferOutputMutex) { "Playthrough state" };
+    CAMutex             mBufferInputMutex ACQUIRED_BEFORE(mBufferOutputMutex)
+                            { "Playthrough ring buffer input" };
+    CAMutex             mBufferOutputMutex { "Playthrough ring buffer output" };
+
     // Signalled when the output IOProc runs. We use it to tell BGMDriver when the output device is ready to receive audio data.
     semaphore_t         mOutputDeviceIOProcSemaphore { SEMAPHORE_NULL };
     
@@ -200,7 +227,9 @@ private:
     
     // Subtract this from the output time to get the input time.
     Float64             mInToOutSampleOffset { 0.0 };
-    
+
+    BGMPlayThroughRTLogger mRTLogger;
+
 };
 
 #pragma clang assume_nonnull end

BGMApp/BGMApp/BGMPlayThroughRTLogger.cpp

@@ -0,0 +1,521 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMPlayThroughRTLogger.cpp
+//  BGMApp
+//
+//  Copyright © 2020 Kyle Neideck
+//
+
+// Self Include
+#include "BGMPlayThroughRTLogger.h"
+
+// Local Includes
+#include "BGM_Utils.h"
+
+// PublicUtility Includes
+#include "CADebugMacros.h"
+
+// STL Includes
+#include <atomic>
+
+// System Includes
+#include <CoreAudio/CoreAudio.h>
+#include <mach/mach_init.h>
+#include <mach/task.h>
+#include <unistd.h>
+
+
+#pragma clang assume_nonnull begin
+
+// Track the number of messages logged when built for the unit tests.
+#if BGM_UnitTest
+    #define LogSync_Debug(inFormat, ...) do { \
+                mNumDebugMessagesLogged++; \
+                DebugMsg(inFormat, ## __VA_ARGS__); \
+            } while (0)
+#else
+    #define LogSync_Debug(inFormat, ...) DebugMsg(inFormat, ## __VA_ARGS__)
+#endif
+
+#pragma mark Construction/Destruction
+
+BGMPlayThroughRTLogger::BGMPlayThroughRTLogger()
+{
+    // Create the semaphore we use to wake up the logging thread when it has messages to log.
+    mWakeUpLoggingThreadSemaphore = CreateSemaphore();
+
+    // Create the logging thread last because it starts immediately and expects the other member
+    // variables to be initialised.
+    mLoggingThread = std::thread(&BGMPlayThroughRTLogger::LoggingThreadEntry, this);
+}
+
+// static
+semaphore_t BGMPlayThroughRTLogger::CreateSemaphore()
+{
+    // TODO: Make a BGMMachSemaphore class to reduce some of this repetitive semaphore code.
+
+    // Create the semaphore.
+    semaphore_t semaphore;
+    kern_return_t error =
+            semaphore_create(mach_task_self(), &semaphore, SYNC_POLICY_FIFO, 0);
+
+    // Check the error code.
+    BGM_Utils::ThrowIfMachError("BGMPlayThroughRTLogger::CreateSemaphore",
+                                "semaphore_create",
+                                error);
+    ThrowIf(semaphore == SEMAPHORE_NULL,
+            CAException(kAudioHardwareUnspecifiedError),
+            "BGMPlayThroughRTLogger::CreateSemaphore: Failed to create semaphore");
+
+    return semaphore;
+}
+
+BGMPlayThroughRTLogger::~BGMPlayThroughRTLogger()
+{
+    // Stop the logging thread.
+    mLoggingThreadShouldExit = true;
+    kern_return_t error = semaphore_signal(mWakeUpLoggingThreadSemaphore);
+
+    BGM_Utils::LogIfMachError("BGMPlayThroughRTLogger::~BGMPlayThroughRTLogger",
+                              "semaphore_signal",
+                              error);
+
+    if(error == KERN_SUCCESS)
+    {
+        // Wait for it to stop.
+        mLoggingThread.join();
+
+        // Destroy the semaphore.
+        error = semaphore_destroy(mach_task_self(), mWakeUpLoggingThreadSemaphore);
+        BGM_Utils::LogIfMachError("BGMPlayThroughRTLogger::~BGMPlayThroughRTLogger",
+                                  "semaphore_destroy",
+                                  error);
+    }
+    else
+    {
+        // If we couldn't tell it to wake up, it's not safe to wait for it to stop or to destroy the
+        // semaphore. We have to detach it so its destructor doesn't cause a crash.
+        mLoggingThread.detach();
+    }
+}
+
+#pragma mark Log Messages
+
+void BGMPlayThroughRTLogger::LogReleasingWaitingThreads()
+{
+    if(!BGMDebugLoggingIsEnabled())
+    {
+        return;
+    }
+
+    if(!mLogReleasingWaitingThreadsMsg.is_lock_free())
+    {
+        // Modifying mLogReleasingWaitingThreadsMsg might cause the thread to lock a mutex that
+        // isn't safe to lock on a realtime thread, so just give up.
+        return;
+    }
+
+    // Set the flag that tells the logging thread to log the message.
+    mLogReleasingWaitingThreadsMsg = true;
+
+    // Wake the logging thread so it can log the message.
+    WakeLoggingThread();
+}
+
+void BGMPlayThroughRTLogger::LogIfMachError_ReleaseWaitingThreadsSignal(mach_error_t inError)
+{
+    if(inError == KERN_SUCCESS)
+    {
+        // No error.
+        return;
+    }
+
+    if(!mReleaseWaitingThreadsSignalError.is_lock_free())
+    {
+        // Modifying mReleaseWaitingThreadsSignalError might cause the thread to lock a mutex that
+        // isn't safe to lock on a realtime thread, so just give up.
+        return;
+    }
+
+    mReleaseWaitingThreadsSignalError = inError;
+    WakeLoggingThread();
+}
+
+void BGMPlayThroughRTLogger::LogIfDroppedFrames(Float64 inFirstInputSampleTime,
+                                                Float64 inLastInputSampleTime)
+{
+    if(inFirstInputSampleTime == inLastInputSampleTime || !BGMDebugLoggingIsEnabled())
+    {
+        // Either we didn't drop any initial frames or we don't need to log a message about it.
+        return;
+    }
+
+    LogAsync(mDroppedFrames, [&]()
+    {
+        // Store the data to include in the log message.
+        mDroppedFrames.firstInputSampleTime = inFirstInputSampleTime;
+        mDroppedFrames.lastInputSampleTime = inLastInputSampleTime;
+    });
+}
+
+void BGMPlayThroughRTLogger::LogNoSamplesReady(CARingBuffer::SampleTime inLastInputSampleTime,
+                                               CARingBuffer::SampleTime inReadHeadSampleTime,
+                                               Float64 inInToOutSampleOffset)
+{
+    if(!BGMDebugLoggingIsEnabled())
+    {
+        return;
+    }
+
+    LogAsync(mNoSamplesReady, [&]()
+    {
+        // Store the data to include in the log message.
+        mNoSamplesReady.lastInputSampleTime = inLastInputSampleTime;
+        mNoSamplesReady.readHeadSampleTime = inReadHeadSampleTime;
+        mNoSamplesReady.inToOutSampleOffset = inInToOutSampleOffset;
+    });
+}
+
+void BGMPlayThroughRTLogger::LogExceptionStoppingIOProc(const char* inCallerName,
+                                                        OSStatus inError,
+                                                        bool inErrorKnown)
+{
+    LogAsync(mExceptionStoppingIOProc, [&]()
+    {
+        // Store the data to include in the log message.
+        mExceptionStoppingIOProc.callerName = inCallerName;
+        mExceptionStoppingIOProc.error = inError;
+        mExceptionStoppingIOProc.errorKnown = inErrorKnown;
+    });
+}
+
+void BGMPlayThroughRTLogger::LogUnexpectedIOStateAfterStopping(const char* inCallerName,
+                                                               int inIOState)
+{
+    LogAsync(mUnexpectedIOStateAfterStopping, [&]()
+    {
+        // Store the data to include in the log message.
+        mUnexpectedIOStateAfterStopping.callerName = inCallerName;
+        mUnexpectedIOStateAfterStopping.ioState = inIOState;
+    });
+}
+
+void BGMPlayThroughRTLogger::LogRingBufferUnavailable(const char* inCallerName, bool inGotLock)
+{
+    LogAsync(mRingBufferUnavailable, [&]()
+    {
+        // Store the data to include in the log message.
+        mRingBufferUnavailable.callerName = inCallerName;
+        mRingBufferUnavailable.gotLock = inGotLock;
+    });
+}
+
+void BGMPlayThroughRTLogger::LogIfRingBufferError(CARingBufferError inError,
+                                                  std::atomic<CARingBufferError>& outError)
+{
+    if(inError == kCARingBufferError_OK)
+    {
+        // No error.
+        return;
+    }
+
+    if(!outError.is_lock_free())
+    {
+        // Modifying outError might cause the thread to lock a mutex that isn't safe to lock on
+        // a realtime thread, so just give up.
+        return;
+    }
+
+    // Store the error.
+    outError = inError;
+
+    // Wake the logging thread so it can log the error.
+    WakeLoggingThread();
+}
+
+template <typename T, typename F>
+void BGMPlayThroughRTLogger::LogAsync(T& inMessageData, F&& inStoreMessageData)
+{
+    if(!inMessageData.shouldLogMessage.is_lock_free())
+    {
+        // Modifying shouldLogMessage might cause the thread to lock a mutex that isn't safe to
+        // lock on a realtime thread, so just give up.
+        return;
+    }
+
+    if(inMessageData.shouldLogMessage)
+    {
+        // The logging thread could be reading inMessageData.
+        return;
+    }
+
+    // Store the data to include in the log message.
+    //
+    // std::forward lets the compiler treat inStoreMessageData as an rvalue if the caller gave it as
+    // an rvalue. No idea if that actually does anything.
+    std::forward<F>(inStoreMessageData)();
+
+    // shouldLogMessage is a std::atomic, so this store also makes sure that the non-atomic stores
+    // in inStoreMessageData will be visible to the logger thread (since the default memory order is
+    // memory_order_seq_cst).
+    inMessageData.shouldLogMessage = true;
+
+    WakeLoggingThread();
+}
+
+void BGMPlayThroughRTLogger::LogSync_Warning(const char* inFormat, ...)
+{
+    va_list args;
+    va_start(args, inFormat);
+
+#if BGM_UnitTest
+    mNumWarningMessagesLogged++;
+#endif
+
+    vLogWarning(inFormat, args);
+
+    va_end(args);
+}
+
+void BGMPlayThroughRTLogger::LogSync_Error(const char* inFormat, ...)
+{
+    va_list args;
+    va_start(args, inFormat);
+
+#if BGM_UnitTest
+    mNumErrorMessagesLogged++;
+
+    if(!mContinueOnErrorLogged)
+    {
+        vLogError(inFormat, args);
+    }
+#else
+    vLogError(inFormat, args);
+#endif
+
+    va_end(args);
+}
+
+#pragma mark Logging Thread
+
+void BGMPlayThroughRTLogger::WakeLoggingThread()
+{
+    kern_return_t error = semaphore_signal(mWakeUpLoggingThreadSemaphore);
+
+    BGMAssert(error == KERN_SUCCESS, "semaphore_signal (%d)", error);
+
+    // We can't do anything useful with the error in release builds. At least, not easily.
+    (void)error;
+}
+
+void BGMPlayThroughRTLogger::LogMessages()
+{
+    // Log the messages/errors from the realtime threads (if any).
+    LogSync_ReleasingWaitingThreads();
+    LogSync_ReleaseWaitingThreadsSignalError();
+    LogSync_DroppedFrames();
+    LogSync_NoSamplesReady();
+    LogSync_ExceptionStoppingIOProc();
+    LogSync_UnexpectedIOStateAfterStopping();
+    LogSync_RingBufferUnavailable();
+    LogSync_RingBufferError(mRingBufferStoreError, "InputDeviceIOProc");
+    LogSync_RingBufferError(mRingBufferFetchError, "OutputDeviceIOProc");
+}
+
+void BGMPlayThroughRTLogger::LogSync_ReleasingWaitingThreads()
+{
+    if(mLogReleasingWaitingThreadsMsg)
+    {
+        LogSync_Debug("BGMPlayThrough::ReleaseThreadsWaitingForOutputToStart: "
+                      "Releasing waiting threads");
+        // Reset it.
+        mLogReleasingWaitingThreadsMsg = false;
+    }
+}
+
+void BGMPlayThroughRTLogger::LogSync_ReleaseWaitingThreadsSignalError()
+{
+    if(mReleaseWaitingThreadsSignalError != KERN_SUCCESS)
+    {
+        BGM_Utils::LogIfMachError("BGMPlayThrough::ReleaseThreadsWaitingForOutputToStart",
+                                  "semaphore_signal_all",
+                                  mReleaseWaitingThreadsSignalError);
+        // Reset it.
+        mReleaseWaitingThreadsSignalError = KERN_SUCCESS;
+    }
+}
+
+void BGMPlayThroughRTLogger::LogSync_DroppedFrames()
+{
+    if(mDroppedFrames.shouldLogMessage)
+    {
+        LogSync_Debug("BGMPlayThrough::OutputDeviceIOProc: "
+                      "Dropped %f frames before output started. %s%f %s%f",
+                      (mDroppedFrames.lastInputSampleTime - mDroppedFrames.firstInputSampleTime),
+                      "mFirstInputSampleTime=",
+                      mDroppedFrames.firstInputSampleTime,
+                      "mLastInputSampleTime=",
+                      mDroppedFrames.lastInputSampleTime);
+        mDroppedFrames.shouldLogMessage = false;
+    }
+}
+
+void BGMPlayThroughRTLogger::LogSync_NoSamplesReady()
+{
+    if(mNoSamplesReady.shouldLogMessage)
+    {
+        LogSync_Debug("BGMPlayThrough::OutputDeviceIOProc: "
+                      "No input samples ready at output sample time. %s%lld %s%lld %s%f",
+                      "lastInputSampleTime=", mNoSamplesReady.lastInputSampleTime,
+                      "readHeadSampleTime=", mNoSamplesReady.readHeadSampleTime,
+                      "mInToOutSampleOffset=", mNoSamplesReady.inToOutSampleOffset);
+        mNoSamplesReady.shouldLogMessage = false;
+    }
+}
+
+void BGMPlayThroughRTLogger::LogSync_ExceptionStoppingIOProc()
+{
+    if(mExceptionStoppingIOProc.shouldLogMessage)
+    {
+        const char error4CC[5] = CA4CCToCString(mExceptionStoppingIOProc.error);
+        LogSync_Error("BGMPlayThrough::UpdateIOProcState: "
+                      "Exception while stopping IOProc %s: %s (%d)",
+                      mExceptionStoppingIOProc.callerName,
+                      mExceptionStoppingIOProc.errorKnown ? error4CC : "unknown",
+                      mExceptionStoppingIOProc.error);
+        mExceptionStoppingIOProc.shouldLogMessage = false;
+    }
+}
+
+void BGMPlayThroughRTLogger::LogSync_UnexpectedIOStateAfterStopping()
+{
+    if(mUnexpectedIOStateAfterStopping.shouldLogMessage)
+    {
+        LogSync_Warning("BGMPlayThrough::UpdateIOProcState: "
+                        "%s IO state changed since last read. state = %d",
+                        mUnexpectedIOStateAfterStopping.callerName,
+                        mUnexpectedIOStateAfterStopping.ioState);
+        mUnexpectedIOStateAfterStopping.shouldLogMessage = false;
+    }
+}
+
+void BGMPlayThroughRTLogger::LogSync_RingBufferUnavailable()
+{
+    if(mRingBufferUnavailable.shouldLogMessage)
+    {
+        LogSync_Warning("BGMPlayThrough::%s: Ring buffer unavailable. %s",
+                        mRingBufferUnavailable.callerName,
+                        mRingBufferUnavailable.gotLock ?
+                            "No buffer currently allocated." :
+                            "Buffer locked for allocation/deallocation by another thread.");
+        mRingBufferUnavailable.shouldLogMessage = false;
+    }
+}
+
+void BGMPlayThroughRTLogger::LogSync_RingBufferError(
+        std::atomic<CARingBufferError>& ioRingBufferError,
+        const char* inMethodName)
+{
+    CARingBufferError error = ioRingBufferError;
+
+    switch(error)
+    {
+        case kCARingBufferError_OK:
+            // No error.
+            return;
+        case kCARingBufferError_CPUOverload:
+            // kCARingBufferError_CPUOverload might not be our fault, so just log a warning.
+            LogSync_Warning("BGMPlayThrough::%s: Ring buffer error: "
+                            "kCARingBufferError_CPUOverload (%d)",
+                            inMethodName,
+                            error);
+            break;
+        default:
+            // Other types of CARingBuffer errors should never occur. This will crash debug builds.
+            LogSync_Error("BGMPlayThrough::%s: Ring buffer error: %s (%d)",
+                          inMethodName,
+                          (error == kCARingBufferError_TooMuch ?
+                                  "kCARingBufferError_TooMuch" :
+                                  "unknown error"),
+                          error);
+            break;
+    };
+
+    // Reset it.
+    ioRingBufferError = kCARingBufferError_OK;
+}
+
+// static
+void* __nullable BGMPlayThroughRTLogger::LoggingThreadEntry(BGMPlayThroughRTLogger* inRefCon)
+{
+    DebugMsg("BGMPlayThroughRTLogger::IOProcLoggingThreadEntry: "
+             "Starting the IOProc logging thread");
+
+    while(!inRefCon->mLoggingThreadShouldExit)
+    {
+        // Log the messages, if there are any to log.
+        inRefCon->LogMessages();
+
+        // Wait until woken up.
+        kern_return_t error = semaphore_wait(inRefCon->mWakeUpLoggingThreadSemaphore);
+        BGM_Utils::LogIfMachError("BGMPlayThroughRTLogger::IOProcLoggingThreadEntry",
+                                  "semaphore_wait",
+                                  error);
+    }
+
+    DebugMsg("BGMPlayThroughRTLogger::IOProcLoggingThreadEntry: IOProc logging thread exiting");
+
+    return nullptr;
+}
+
+#if BGM_UnitTest
+
+#pragma mark Test Helpers
+
+bool BGMPlayThroughRTLogger::WaitUntilLoggerThreadIdle()
+{
+    int msWaited = 0;
+
+    while(mLogReleasingWaitingThreadsMsg ||
+            mReleaseWaitingThreadsSignalError != KERN_SUCCESS ||
+            mDroppedFrames.shouldLogMessage ||
+            mNoSamplesReady.shouldLogMessage ||
+            mUnexpectedIOStateAfterStopping.shouldLogMessage ||
+            mRingBufferUnavailable.shouldLogMessage ||
+            mExceptionStoppingIOProc.shouldLogMessage ||
+            mRingBufferStoreError != kCARingBufferError_OK ||
+            mRingBufferFetchError != kCARingBufferError_OK)
+    {
+        // Poll until the logger thread has nothing left to log. (Ideally we'd use a semaphore
+        // instead of polling, but it isn't worth the effort at this point.)
+        usleep(10 * 1000);
+        msWaited += 10;
+
+        // Time out after 5 seconds.
+        if(msWaited > 5000)
+        {
+            return false;
+        }
+    }
+
+    return true;
+}
+
+#endif /* BGM_UnitTest */
+
+#pragma clang assume_nonnull end
+

BGMApp/BGMApp/BGMPlayThroughRTLogger.h

@@ -0,0 +1,227 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMPlayThroughRTLogger.h
+//  BGMApp
+//
+//  Copyright © 2020 Kyle Neideck
+//
+//  A real-time safe logger for BGMPlayThrough. The messages are logged asynchronously by a
+//  non-realtime thread.
+//
+//  For the sake of simplicity, this class is very closely coupled with BGMPlayThrough and its
+//  methods make assumptions about where they will be called. Also, if the same logging method is
+//  called multiple times before the logging thread next checks for messages, it will only log the
+//  message for one of those calls and ignore the others.
+//
+//  This class's methods are real-time safe in that they return in a bounded amount of time and we
+//  think they're probably fast enough that the callers won't miss their deadlines, but we don't try
+//  to guarantee it. Some of them should only be called in unusual cases where it's worth increasing
+//  the risk of a thread missing its deadline.
+//
+
+#ifndef BGMApp__BGMPlayThroughRTLogger
+#define BGMApp__BGMPlayThroughRTLogger
+
+// PublicUtility Includes
+#include "CARingBuffer.h"
+
+// STL Includes
+#include <thread>
+
+// System Includes
+#include <mach/error.h>
+#include <mach/semaphore.h>
+
+
+#pragma clang assume_nonnull begin
+
+class BGMPlayThroughRTLogger
+{
+
+#pragma mark Construction/Destruction
+
+public:
+                            BGMPlayThroughRTLogger();
+                            ~BGMPlayThroughRTLogger();
+                            BGMPlayThroughRTLogger(const BGMPlayThroughRTLogger&) = delete;
+                            BGMPlayThroughRTLogger& operator=(
+                                    const BGMPlayThroughRTLogger&) = delete;
+private:
+    static semaphore_t      CreateSemaphore();
+
+#pragma mark Log Messages
+
+public:
+    /*! For BGMPlayThrough::ReleaseThreadsWaitingForOutputToStart. */
+    void                    LogReleasingWaitingThreads();
+    /*! For BGMPlayThrough::ReleaseThreadsWaitingForOutputToStart. */
+    void                    LogIfMachError_ReleaseWaitingThreadsSignal(mach_error_t inError);
+
+    /*! For BGMPlayThrough::OutputDeviceIOProc. Not thread-safe. */
+    void                    LogIfDroppedFrames(Float64 inFirstInputSampleTime,
+                                               Float64 inLastInputSampleTime);
+    /*! For BGMPlayThrough::OutputDeviceIOProc. Not thread-safe. */
+    void                    LogNoSamplesReady(CARingBuffer::SampleTime inLastInputSampleTime,
+                                              CARingBuffer::SampleTime inReadHeadSampleTime,
+                                              Float64 inInToOutSampleOffset);
+
+    /*! For BGMPlayThrough::UpdateIOProcState. Not thread-safe. */
+    void                    LogExceptionStoppingIOProc(const char* inCallerName)
+                            {
+                                LogExceptionStoppingIOProc(inCallerName, noErr, false);
+                            }
+    /*! For BGMPlayThrough::UpdateIOProcState. Not thread-safe. */
+    void                    LogExceptionStoppingIOProc(const char* inCallerName, OSStatus inError)
+                            {
+                                LogExceptionStoppingIOProc(inCallerName, inError, true);
+                            }
+
+private:
+    void                    LogExceptionStoppingIOProc(const char* inCallerName,
+                                                       OSStatus inError,
+                                                       bool inErrorKnown);
+
+public:
+    /*! For BGMPlayThrough::UpdateIOProcState. Not thread-safe. */
+    void                    LogUnexpectedIOStateAfterStopping(const char* inCallerName,
+                                                              int inIOState);
+    /*! For BGMPlayThrough::InputDeviceIOProc and BGMPlayThrough::OutputDeviceIOProc. */
+    void                    LogRingBufferUnavailable(const char* inCallerName, bool inGotLock);
+    /*! For BGMPlayThrough::OutputDeviceIOProc. */
+    void                    LogIfRingBufferError_Fetch(CARingBufferError inError)
+                            {
+                                LogIfRingBufferError(inError, mRingBufferFetchError);
+                            }
+    /*! For BGMPlayThrough::InputDeviceIOProc. */
+    void                    LogIfRingBufferError_Store(CARingBufferError inError)
+                            {
+                                LogIfRingBufferError(inError, mRingBufferStoreError);
+                            }
+
+private:
+    void                    LogIfRingBufferError(CARingBufferError inError,
+                                                 std::atomic<CARingBufferError>& outError);
+
+    template <typename T, typename F>
+    void                    LogAsync(T& inMessageData, F&& inStoreMessageData);
+
+    // Wrapper methods used to mock out the logging for unit tests.
+    void                    LogSync_Warning(const char* inFormat, ...) __printflike(2, 3);
+    void                    LogSync_Error(const char* inFormat, ...) __printflike(2, 3);
+
+#pragma mark Logging Thread
+
+private:
+    void                    WakeLoggingThread();
+
+    void                    LogMessages();
+    void                    LogSync_ReleasingWaitingThreads();
+    void                    LogSync_ReleaseWaitingThreadsSignalError();
+    void                    LogSync_DroppedFrames();
+    void                    LogSync_NoSamplesReady();
+    void                    LogSync_ExceptionStoppingIOProc();
+    void                    LogSync_UnexpectedIOStateAfterStopping();
+    void                    LogSync_RingBufferUnavailable();
+    void                    LogSync_RingBufferError(
+                                std::atomic<CARingBufferError>& ioRingBufferError,
+                                const char* inMethodName);
+
+    // The entry point of the logging thread (mLoggingThread).
+    static void* __nullable LoggingThreadEntry(BGMPlayThroughRTLogger* inRefCon);
+
+#if BGM_UnitTest
+
+#pragma mark Test Helpers
+
+public:
+    /*!
+     * @return True if the logger thread finished logging the requested messages. False if it still
+     *         had messages to log after 5 seconds.
+     */
+    bool                    WaitUntilLoggerThreadIdle();
+
+#endif /* BGM_UnitTest */
+
+private:
+    // For BGMPlayThrough::ReleaseThreadsWaitingForOutputToStart
+    std::atomic<bool>       mLogReleasingWaitingThreadsMsg { false };
+    std::atomic<kern_return_t> mReleaseWaitingThreadsSignalError { KERN_SUCCESS };
+
+    // For BGMPlayThrough::InputDeviceIOProc and BGMPlayThrough::OutputDeviceIOProc
+    struct {
+        Float64 firstInputSampleTime;
+        Float64 lastInputSampleTime;
+        std::atomic<bool> shouldLogMessage { false };
+    } mDroppedFrames;
+
+    struct {
+        CARingBuffer::SampleTime lastInputSampleTime;
+        CARingBuffer::SampleTime readHeadSampleTime;
+        Float64 inToOutSampleOffset;
+        std::atomic<bool> shouldLogMessage { false };
+    } mNoSamplesReady;
+
+    struct {
+        const char* callerName;
+        bool gotLock;
+        std::atomic<bool> shouldLogMessage { false };
+    } mRingBufferUnavailable;
+
+    // For BGMPlayThrough::UpdateIOProcState
+    struct {
+        const char* callerName;
+        int ioState;
+        std::atomic<bool> shouldLogMessage { false };
+    } mUnexpectedIOStateAfterStopping;
+
+    struct {
+        const char* callerName;
+        OSStatus error;
+        bool errorKnown;  // If false, we didn't get an error code from the exception.
+        std::atomic<bool> shouldLogMessage { false };
+    } mExceptionStoppingIOProc;
+
+    // For BGMPlayThrough::OutputDeviceIOProc
+    std::atomic<CARingBufferError> mRingBufferStoreError { kCARingBufferError_OK };
+    // For BGMPlayThrough::InputDeviceIOProc.
+    std::atomic<CARingBufferError> mRingBufferFetchError { kCARingBufferError_OK };
+
+    // Signalled to wake up the mLoggingThread when it has messages to log.
+    semaphore_t             mWakeUpLoggingThreadSemaphore;
+    std::atomic<bool>       mLoggingThreadShouldExit { false };
+    // The thread that actually logs the messages.
+    std::thread             mLoggingThread;
+
+#if BGM_UnitTest
+
+public:
+    // Tests normally crash (abort) if LogError is called. This flag lets us test the code that
+    // would otherwise call LogError.
+    bool                    mContinueOnErrorLogged { false };
+
+    int                     mNumDebugMessagesLogged { 0 };
+    int                     mNumWarningMessagesLogged { 0 };
+    int                     mNumErrorMessagesLogged { 0 };
+
+#endif /* BGM_UnitTest */
+
+};
+
+#pragma clang assume_nonnull end
+
+#endif /* BGMApp__BGMPlayThroughRTLogger */
+

BGMApp/BGMApp/BGMPreferredOutputDevices.h

@@ -0,0 +1,64 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMPreferredOutputDevices.h
+//  BGMApp
+//
+//  Copyright © 2018 Kyle Neideck
+//
+//  Tries to change BGMApp's output device when the user plugs in or unplugs an audio device, in the
+//  same way macOS would change its default device if Background Music wasn't running.
+//
+//  For example, if you plug in some USB headphones, make them your default device and then unplug
+//  them, macOS will change its default device to the previous default device. Then, if you plug
+//  them back in, macOS will make them the default device again.
+//
+//  This class isn't always able to figure out what macOS would do, in which case it leaves BGMApp's
+//  output device as it is.
+//
+
+// Local Includes
+#import "BGMAudioDeviceManager.h"
+#import "BGMUserDefaults.h"
+
+// System Includes
+#import <CoreAudio/AudioHardwareBase.h>
+#import <Foundation/Foundation.h>
+
+
+#pragma clang assume_nonnull begin
+
+@interface BGMPreferredOutputDevices : NSObject
+
+// Starts responding to device connections/disconnections immediately. Stops if/when the instance is
+// deallocated.
+- (instancetype) initWithDevices:(BGMAudioDeviceManager*)devices
+                    userDefaults:(BGMUserDefaults*)userDefaults;
+
+// Returns the most-preferred device that's currently connected. If no preferred devices are
+// connected, returns the current output device. If the current output device has been disconnected,
+// returns an arbitrary device.
+//
+// If none of the connected devices can be used as the output device, or if it can't find a device
+// to use because the HAL returned errors when queried, returns kAudioObjectUnknown.
+- (AudioObjectID) findPreferredDevice;
+
+- (void) userChangedOutputDeviceTo:(AudioObjectID)device;
+
+@end
+
+#pragma clang assume_nonnull end
+

BGMApp/BGMApp/BGMPreferredOutputDevices.mm

@@ -0,0 +1,495 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMPreferredOutputDevices.m
+//  BGMApp
+//
+//  Copyright © 2018 Kyle Neideck
+//
+
+// Self Include
+#import "BGMPreferredOutputDevices.h"
+
+// Local Includes
+#import "BGM_Types.h"
+#import "BGM_Utils.h"
+#import "BGMAudioDevice.h"
+
+// PublicUtility Includes
+#import "CAAutoDisposer.h"
+#import "CAHALAudioSystemObject.h"
+#import "CAPropertyAddress.h"
+
+
+#pragma clang assume_nonnull begin
+
+// The Plist file CoreAudio stores the preferred devices list in. It isn't part of the public API,
+// but if this class fails to read/parse it, BGMApp will continue without it.
+NSString* const kAudioSystemSettingsPlist =
+    @"/Library/Preferences/Audio/com.apple.audio.SystemSettings.plist";
+
+@implementation BGMPreferredOutputDevices {
+    NSRecursiveLock* _stateLock;
+
+    // Used to change BGMApp's output device.
+    BGMAudioDeviceManager* _devices;
+
+    // User settings and data.
+    BGMUserDefaults* _userDefaults;
+
+    // The UIDs of the preferred devices, in order of preference. The most-preferred device is at
+    // index 0. This list is derived from several sources.
+    NSArray<NSString*>* _preferredDeviceUIDs;
+
+    // Called when a device is connected or disconnected.
+    AudioObjectPropertyListenerBlock _deviceListListener;
+}
+
+- (instancetype) initWithDevices:(BGMAudioDeviceManager*)devices
+                    userDefaults:(BGMUserDefaults*)userDefaults {
+    if ((self = [super init])) {
+        _stateLock = [NSRecursiveLock new];
+        _devices = devices;
+        _userDefaults = userDefaults;
+        _preferredDeviceUIDs = [self readPreferredDevices];
+
+        DebugMsg("BGMPreferredOutputDevices::initWithDevices: Preferred devices: %s",
+                 _preferredDeviceUIDs.debugDescription.UTF8String);
+
+        [self listenForDevicesAddedOrRemoved];
+    }
+
+    return self;
+}
+
+- (void) dealloc {
+    @try {
+        [_stateLock lock];
+
+        // Tell CoreAudio not to call the listener block anymore.
+        CAHALAudioSystemObject().RemovePropertyListenerBlock(
+            CAPropertyAddress(kAudioHardwarePropertyDevices),
+            dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0),
+            _deviceListListener);
+    } @finally {
+        [_stateLock unlock];
+    }
+}
+
+// Reads the preferred devices list from CoreAudio's Plist file. Uses BGMApp's stored list to fill
+// in the blanks, if necessary.
+- (NSArray<NSString*>*) readPreferredDevices {
+    // Read the Plist file into a dictionary.
+    //
+    // TODO: If this file doesn't exist, try paths used by older versions of macOS. (If there are
+    //       any, that is. I haven't checked.)
+    NSURL* url = [NSURL fileURLWithPath:kAudioSystemSettingsPlist];
+    NSError* error = nil;
+    NSData* data = [NSData dataWithContentsOfURL:url options:0 error:&error];
+    NSDictionary* settings = nil;
+
+    if (data && !error) {
+        settings = [NSPropertyListSerialization propertyListWithData:data
+                                                             options:NSPropertyListImmutable
+                                                              format:nil
+                                                               error:&error];
+    }
+
+    // Default to a list with just the systemwide default device (or an empty list if that fails) if
+    // we can't read the preferred devices from the Plist because preferredDeviceUIDsFrom will use
+    // BGMApp's stored preferred devices to fill in the rest optimistically. This doesn't help us
+    // tell when to switch to a newly connected device, but it should improve our chances of
+    // switching to the best device if the current output device is disconnected.
+    NSArray<NSDictionary*>* _Nonnull preferredOutputDeviceInfos = @[];
+
+    // If we can't read the Plist, we only know that the current systemwide default device is the
+    // most-preferred device that's currently connected.
+    //
+    // TODO: If we are able to read the Plist, check that the systemwide default device is the
+    //       most-preferred device in the list from the Plist that's also connected. If it isn't,
+    //       the format of the Plist has probably changed, so we should ignore its data and log a
+    //       warning.
+    BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
+        BGMAudioDevice defaultDevice = CAHALAudioSystemObject().GetDefaultAudioDevice(false, false);
+        NSString* __nullable defaultDeviceUID =
+            (__bridge_transfer NSString* __nullable)defaultDevice.CopyDeviceUID();
+
+        if (defaultDeviceUID) {
+            preferredOutputDeviceInfos = @[ @{ @"uid": BGMNN(defaultDeviceUID) } ];
+        }
+    });
+
+    if (error || !data || !settings) {
+        // The Plist file either doesn't exist or we weren't able to parse it.
+        LogWarning("BGMPreferredOutputDevices::readPreferredDevices: Couldn't read %s. "
+                   "(data = %s, settings = %s) Error: %s",
+                   kAudioSystemSettingsPlist.UTF8String,
+                   data.debugDescription.UTF8String,
+                   settings.debugDescription.UTF8String,
+                   error.debugDescription.UTF8String);
+    } else if (!settings[@"preferred devices"]) {
+        // The Plist doesn't include the lists of preferred devices (for input, output and system
+        // output).
+        LogWarning("BGMPreferredOutputDevices::readPreferredDevices: No preferred devices in %s",
+                   settings.debugDescription.UTF8String);
+    } else if (!settings[@"preferred devices"][@"output"]) {
+        // The Plist doesn't include the list of preferred output devices.
+        LogWarning("BGMPreferredOutputDevices::readPreferredDevices: "
+                   "No preferred output devices in %s",
+                   settings.debugDescription.UTF8String);
+    } else {
+        // Copy the preferred devices out of the Plist.
+        preferredOutputDeviceInfos = BGMNN(settings[@"preferred devices"][@"output"]);
+    }
+
+    return [self preferredDeviceUIDsFrom:preferredOutputDeviceInfos
+               storedPreferredDeviceUIDs:_userDefaults.preferredDeviceUIDs];
+}
+
+- (NSArray<NSString*>*) preferredDeviceUIDsFrom:(NSArray<NSDictionary*>*)deviceInfos
+                      storedPreferredDeviceUIDs:(NSArray<NSString*>*)storedUIDs {
+    NSArray<NSString*>* deviceUIDs = @[];
+    int storedPreferredDeviceIdx = 0;
+
+    for (NSDictionary* deviceInfo in deviceInfos) {
+        // Check the Plist actually has a UID for this device.
+        if (![deviceInfo[@"uid"] isKindOfClass:NSString.class]) {
+            LogWarning("BGMPreferredOutputDevices::preferredDeviceUIDsFrom: No UID in %s",
+                       deviceInfo.debugDescription.UTF8String);
+            continue;
+        }
+
+        NSString* uid = deviceInfo[@"uid"];
+
+        if ([uid isEqualToString:@kBGMDeviceUID] ||
+            [uid isEqualToString:@kBGMDeviceUID_UISounds] ||
+            [uid isEqualToString:@kBGMNullDeviceUID]) {
+            // This is one of the Background Music devices, so look for a preferred device saved
+            // from a previous run of BGMApp and add it instead.
+            //
+            // BGMApp has to set BGMDevice, and often also the Null Device for a short time, as the
+            // systemwide default audio device, which makes CoreAudio put them in its Plist. And
+            // since the Plist is limited to three devices, it only gives us one or two usable ones.
+            // Ideally, CoreAudio just wouldn't add our devices to its list, but I don't think we
+            // can prevent that. And we can't be sure that editing its Plist file ourselves would be
+            // safe.
+            //
+            // TODO: This doesn't work if the user has made BGMDevice the systemwide default device
+            //       themselves since the last time they closed BGMApp. We might be able to fix that
+            //       by having a background process watch the Plist for changes while BGMApp is
+            //       closed or something like that, but I doubt there's a nice or simple solution.
+            deviceUIDs = [self addNextStoredPreferredDevice:&storedPreferredDeviceIdx
+                                        preferredDeviceUIDs:deviceUIDs
+                                  storedPreferredDeviceUIDs:storedUIDs];
+        } else if (![deviceUIDs containsObject:uid]) {
+            // Add this preferred device's UID to the list.
+            deviceUIDs = [deviceUIDs arrayByAddingObject:uid];
+        }
+    }
+
+    // Fill in any remaining places in the list with stored devices. Limit the list to three devices
+    // just to match CoreAudio's behaviour.
+    while ((storedPreferredDeviceIdx < storedUIDs.count) && (deviceUIDs.count < 3)) {
+        deviceUIDs = [self addNextStoredPreferredDevice:&storedPreferredDeviceIdx
+                                    preferredDeviceUIDs:deviceUIDs
+                              storedPreferredDeviceUIDs:storedUIDs];
+    }
+
+    return deviceUIDs;
+}
+
+- (NSArray<NSString*>*) addNextStoredPreferredDevice:(int*)storedPreferredDeviceIdx
+                                 preferredDeviceUIDs:(NSArray<NSString*>*)deviceUIDs
+                           storedPreferredDeviceUIDs:(NSArray<NSString*>*)storedUIDs {
+    NSString* __nullable storedPreferredDevice;
+
+    // Try to find a stored UID that isn't already in the list.
+    do {
+        storedPreferredDevice = (*storedPreferredDeviceIdx < storedUIDs.count) ?
+                                    storedUIDs[*storedPreferredDeviceIdx] : nil;
+        (*storedPreferredDeviceIdx)++;
+    } while (storedPreferredDevice && [deviceUIDs containsObject:BGMNN(storedPreferredDevice)]);
+
+    // If we found a stored UID, add it to the list.
+    if (storedPreferredDevice) {
+        DebugMsg("BGMPreferredOutputDevices::addNextStoredPreferredDevice: "
+                 "Adding stored preferred device: %s",
+                 storedPreferredDevice.UTF8String);
+        deviceUIDs = [deviceUIDs arrayByAddingObject:BGMNN(storedPreferredDevice)];
+    }
+
+    return deviceUIDs;
+}
+
+- (void) listenForDevicesAddedOrRemoved {
+    // Create the block that will run when a device is added or removed.
+    BGMPreferredOutputDevices* __weak weakSelf = self;
+
+    _deviceListListener = ^(UInt32 inNumberAddresses,
+                            const AudioObjectPropertyAddress* inAddresses) {
+        #pragma unused (inNumberAddresses, inAddresses)
+
+        BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
+            [weakSelf connectedDeviceListChanged];
+        });
+    };
+
+    // Register the listener block with CoreAudio.
+    CAHALAudioSystemObject().AddPropertyListenerBlock(
+        CAPropertyAddress(kAudioHardwarePropertyDevices),
+        dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0),
+        _deviceListListener);
+}
+
+- (void) connectedDeviceListChanged {
+    @try {
+        [_stateLock lock];
+
+        // Decide which device should be the output device now. If a device has been connected and
+        // it's preferred over the current output device, we'll change to that device. If the
+        // current output device has been removed, we'll change to the next most-preferred device.
+        AudioObjectID preferredDevice = [self findPreferredDevice];
+
+        if (preferredDevice == kAudioObjectUnknown) {
+            LogWarning("BGMPreferredOutputDevices::connectedDeviceListChanged: "
+                       "No preferred device found.");
+        } else if (_devices.outputDevice.GetObjectID() == preferredDevice) {
+            DebugMsg("BGMPreferredOutputDevices::connectedDeviceListChanged: "
+                     "The preferred device is already set as the output device.");
+        } else {
+            // Change to the preferred device.
+            DebugMsg("BGMPreferredOutputDevices::connectedDeviceListChanged: "
+                     "Changing output device to %d.",
+                     preferredDevice);
+            NSError* __nullable error = [_devices setOutputDeviceWithID:preferredDevice
+                                                        revertOnFailure:YES];
+            if (error) {
+                // There's not much we can do if this happens.
+                LogError("BGMPreferredOutputDevices::connectedDeviceListChanged: "
+                         "Failed to change to preferred device. Error: %s",
+                         error.debugDescription.UTF8String);
+            }
+        }
+    } @finally {
+        [_stateLock unlock];
+    }
+}
+
+- (AudioObjectID) findPreferredDevice {
+    AudioObjectID preferredDevice = kAudioObjectUnknown;
+    CAHALAudioSystemObject audioSystem;
+
+    BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
+        BGMAudioDevice defaultDevice = audioSystem.GetDefaultAudioDevice(false, false);
+
+        if (!defaultDevice.IsBGMDeviceInstance()) {
+            // BGMDevice isn't the systemwide default device, so we know the default device is the
+            // most-preferred device that's currently connected.
+            preferredDevice = defaultDevice;
+        }
+    });
+
+    if (preferredDevice == kAudioObjectUnknown) {
+        // BGMDevice is the systemwide default device, so this method is probably being called after
+        // launch, since we set BGMDevice as the default device then. It could also be that the user
+        // set it manually or that BGMApp failed to change it back the last time it closed. Either
+        // way, we'll try to find a device to use in the Plist or stored list instead.
+        DebugMsg("BGMPreferredOutputDevices::findPreferredDevice: Checking Plist and stored list.");
+
+        BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
+            preferredDevice = [self findPreferredDeviceInDerivedList];
+        });
+    }
+
+    BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
+        if (preferredDevice == kAudioObjectUnknown && [self isCurrentOutputDeviceConnected]) {
+            // The output device (for BGMApp) has been set and is still connected. We haven't found
+            // a better device to use, so prefer leaving the output device as it is.
+            DebugMsg("BGMPreferredOutputDevices::findPreferredDevice: "
+                     "Choosing the current output device as the preferred device.");
+            preferredDevice = _devices.outputDevice.GetObjectID();
+        }
+    });
+
+    if (preferredDevice == kAudioObjectUnknown) {
+        // The current output device has been disconnected or hasn't been set yet and there are no
+        // preferred devices connected, so pick one arbitrarily.
+        DebugMsg("BGMPreferredOutputDevices::findPreferredDevice: Choosing an arbitrary device.");
+        BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
+            preferredDevice = [self findPreferredDeviceByLatency];
+        });
+    }
+
+    return preferredDevice;
+}
+
+// Looks for a suitable device in _preferredDeviceUIDs, the list of preferred devices derived from
+// CoreAudio's Plist and BGMApp's stored list.
+- (AudioObjectID) findPreferredDeviceInDerivedList {
+    CAHALAudioSystemObject audioSystem;
+    UInt32 numDevices = audioSystem.GetNumberAudioDevices();
+
+    // Get the list of currently connected audio devices.
+    CAAutoArrayDelete<AudioObjectID> devices(numDevices);
+    audioSystem.GetAudioDevices(numDevices, devices);
+
+    // Look through the preferred devices list to see if one's connected. Return the first one we
+    // find because they're stored in order of preference.
+    for (int position = 0; position < _preferredDeviceUIDs.count; position++) {
+        // Compare the current preferred device to each connected device by UID.
+        for (UInt32 i = 0; i < numDevices; i++) {
+            NSString* __nullable connectedDeviceUID = nil;
+
+            BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
+                // Skip devices we can't use, e.g. BGMDevice.
+                if (BGMAudioDevice(devices[i]).CanBeOutputDeviceInBGMApp()) {
+                    // Get the connected device's UID.
+                    connectedDeviceUID =
+                        (__bridge NSString* __nullable)CAHALAudioDevice(devices[i]).CopyDeviceUID();
+                }
+            });
+
+            // If the UIDs match, the current preferred device is connected.
+            //
+            // If you plug a USB device in to different USB port, macOS might assign it a different
+            // UID and make it fail to match its old UID here. CoreAudio/macOS doesn't seem to
+            // handle that case either, though, so it's probably not worth worrying about.
+            if ([connectedDeviceUID isEqualToString:_preferredDeviceUIDs[position]]) {
+                // We're iterating through the preferred devices from most to least-preferred, so
+                // we've found the device to use.
+                DebugMsg("BGMPreferredOutputDevices::findPreferredDeviceInDerivedList: "
+                         "Found preferred device '%s' at position %d",
+                         _preferredDeviceUIDs[position].UTF8String,
+                         position);
+                return devices[i];
+            }
+        }
+
+        DebugMsg("BGMPreferredOutputDevices::findPreferredDeviceInDerivedList: "
+                 "Preferred device not connected: %s",
+                 _preferredDeviceUIDs[position].UTF8String);
+    }
+
+    return kAudioObjectUnknown;
+}
+
+- (bool) isCurrentOutputDeviceConnected {
+    if (_devices.outputDevice.GetObjectID() == kAudioObjectUnknown) {
+        DebugMsg("BGMPreferredOutputDevices::isCurrentOutputDeviceConnected: "
+                 "The output device hasn't been set yet.");
+        return false;
+    }
+
+    CAHALAudioSystemObject audioSystem;
+    UInt32 numDevices = audioSystem.GetNumberAudioDevices();
+
+    // Get the list of currently connected audio devices.
+    CAAutoArrayDelete<AudioObjectID> devices(numDevices);
+    audioSystem.GetAudioDevices(numDevices, devices);
+
+    // Look for the current output device in the list of connected devices.
+    for (UInt32 i = 0; i < numDevices; i++) {
+        // TODO: Are AudioObjectIDs reused? If they are, could that cause a collision here?
+        if (_devices.outputDevice.GetObjectID() == devices[i]) {
+            DebugMsg("BGMPreferredOutputDevices::isCurrentOutputDeviceConnected: "
+                     "The output device is connected.");
+            return true;
+        }
+    }
+
+    DebugMsg("BGMPreferredOutputDevices::isCurrentOutputDeviceConnected: "
+             "The output device is not connected.");
+    return false;
+}
+
+// Returns the audio output device with the lowest latency. Used when we have no better way to
+// choose the output device for BGMApp to use.
+- (AudioObjectID) findPreferredDeviceByLatency {
+    CAHALAudioSystemObject audioSystem;
+    UInt32 numDevices = audioSystem.GetNumberAudioDevices();
+    AudioObjectID minLatencyDevice = kAudioObjectUnknown;
+    UInt32 minLatency = UINT32_MAX;
+
+    CAAutoArrayDelete<AudioObjectID> devices(numDevices);
+    audioSystem.GetAudioDevices(numDevices, devices);
+
+    for (UInt32 i = 0; i < numDevices; i++) {
+        BGMAudioDevice device(devices[i]);
+
+        if (!device.IsBGMDeviceInstance()) {
+            BOOL hasOutputChannels = NO;
+
+            BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, "GetTotalNumberChannels", [&] {
+                hasOutputChannels = device.GetTotalNumberChannels(/* inIsInput = */ false) > 0;
+            });
+
+            if (hasOutputChannels) {
+                BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, "GetLatency", [&] {
+                    UInt32 latency = device.GetLatency(false);
+
+                    if (latency < minLatency) {
+                        minLatencyDevice = devices[i];
+                        minLatency = latency;
+                    }
+                });
+            }
+        }
+    }
+
+    return minLatencyDevice;
+}
+
+- (void) userChangedOutputDeviceTo:(AudioObjectID)device {
+    @try {
+        [_stateLock lock];
+
+        BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
+            // Add the new output device to the list.
+            NSString* __nullable outputDeviceUID =
+                (__bridge_transfer NSString* __nullable)CAHALAudioDevice(device).CopyDeviceUID();
+
+            if (outputDeviceUID) {
+                // Limit the list to three devices because that's what macOS does.
+                if (_preferredDeviceUIDs.count >= 2) {
+                    _preferredDeviceUIDs = @[BGMNN(outputDeviceUID),
+                                          _preferredDeviceUIDs[0],
+                                          _preferredDeviceUIDs[1]];
+                } else if (_preferredDeviceUIDs.count >= 1) {
+                    _preferredDeviceUIDs = @[BGMNN(outputDeviceUID), _preferredDeviceUIDs[0]];
+                } else {
+                    _preferredDeviceUIDs = @[BGMNN(outputDeviceUID)];
+                }
+
+                DebugMsg("BGMPreferredOutputDevices::userChangedOutputDeviceTo: "
+                         "Preferred devices: %s",
+                         _preferredDeviceUIDs.debugDescription.UTF8String);
+
+                // Save the list.
+                _userDefaults.preferredDeviceUIDs = _preferredDeviceUIDs;
+            } else {
+                LogWarning("BGMPreferredOutputDevices::userChangedOutputDeviceTo: "
+                           "Output device has no UID");
+            }
+        });
+    } @finally {
+        [_stateLock unlock];
+    }
+}
+
+@end
+
+#pragma clang assume_nonnull end
+

BGMApp/BGMApp/BGMStatusBarItem.h

@@ -0,0 +1,68 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMStatusBarItem.h
+//  BGMApp
+//
+//  Copyright © 2019, 2020 Kyle Neideck
+//
+//  The button in the system status bar (the bar with volume, battery, clock, etc.) to show the main
+//  menu for the app. These are called "menu bar extras" in the Human Interface Guidelines.
+//
+
+// Local Includes
+#import "BGMAudioDeviceManager.h"
+#import "BGMDebugLoggingMenuItem.h"
+
+// System Includes
+#import <Cocoa/Cocoa.h>
+
+// Forward Declarations
+@class BGMUserDefaults;
+
+
+#pragma clang assume_nonnull begin
+
+typedef NS_ENUM(NSInteger, BGMStatusBarIcon) {
+    BGMFermataStatusBarIcon = 0,
+    BGMVolumeStatusBarIcon
+};
+
+static BGMStatusBarIcon const kBGMStatusBarIconMinValue     = BGMFermataStatusBarIcon;
+static BGMStatusBarIcon const kBGMStatusBarIconMaxValue     = BGMVolumeStatusBarIcon;
+static BGMStatusBarIcon const kBGMStatusBarIconDefaultValue = BGMFermataStatusBarIcon;
+
+@interface BGMStatusBarItem : NSObject
+
+- (instancetype) initWithMenu:(NSMenu*)bgmMenu
+                 audioDevices:(BGMAudioDeviceManager*)devices
+                 userDefaults:(BGMUserDefaults*)defaults;
+
+// Set this to BGMFermataStatusBarIcon to change the icon to the Background Music logo.
+//
+// Set this to BGMFermataStatusBarIcon to change the icon to a volume icon. This icon has the
+// advantage of indicating the volume level, but we can't make it the default because it looks the
+// same as the icon for the macOS volume status bar item.
+@property BGMStatusBarIcon icon;
+
+// If the user holds down the option key when they click the status bar icon, this menu item will be
+// shown in the main menu.
+- (void) setDebugLoggingMenuItem:(BGMDebugLoggingMenuItem*)menuItem;
+
+@end
+
+#pragma clang assume_nonnull end
+

BGMApp/BGMApp/BGMStatusBarItem.mm

@@ -0,0 +1,302 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMStatusBarItem.m
+//  BGMApp
+//
+//  Copyright © 2019, 2020 Kyle Neideck
+//
+
+// Self Include
+#import "BGMStatusBarItem.h"
+
+// Local Includes
+#import "BGM_Utils.h"
+#import "BGMUserDefaults.h"
+#import "BGMVolumeChangeListener.h"
+
+
+#pragma clang assume_nonnull begin
+
+static CGFloat const kStatusBarIconPadding                = 0.25;
+static CGFloat const kVolumeIconAdditionalVerticalPadding = 0.075;
+
+@implementation BGMStatusBarItem
+{
+    BGMAudioDeviceManager* audioDevices;
+
+    // User settings and data.
+    BGMUserDefaults* userDefaults;
+
+    NSImage* fermataIcon;
+    NSImage* volumeIcon0SoundWaves;
+    NSImage* volumeIcon1SoundWave;
+    NSImage* volumeIcon2SoundWaves;
+    NSImage* volumeIcon3SoundWaves;
+
+    NSStatusItem* statusBarItem;
+    BGMDebugLoggingMenuItem* debugLoggingMenuItem;
+
+    BGMVolumeChangeListener* volumeChangeListener;
+    id __nullable clickEventHandler;
+
+    BGMStatusBarIcon _icon;
+}
+
+#pragma mark Initialisation
+
+- (instancetype) initWithMenu:(NSMenu*)bgmMenu
+                 audioDevices:(BGMAudioDeviceManager*)devices
+                 userDefaults:(BGMUserDefaults*)defaults {
+    if ((self = [super init])) {
+        statusBarItem =
+                [[NSStatusBar systemStatusBar] statusItemWithLength:NSVariableStatusItemLength];
+
+        audioDevices = devices;
+        userDefaults = defaults;
+
+        // Initialise the icons.
+        [self initIcons];
+
+        // Set the initial icon.
+        self.icon = userDefaults.statusBarIcon;
+
+        // Set the menu item to open the main menu.
+        statusBarItem.menu = bgmMenu;
+
+        // Monitor click events so we can show extra options in the menu if the user was holding the
+        // option key.
+        clickEventHandler = [self addClickMonitor];
+
+        // Set the accessibility label to "Background Music". (We intentionally don't set a title or
+        // a tooltip.)
+        if ([BGMStatusBarItem buttonAvailable]) {
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wpartial-availability"
+            statusBarItem.button.accessibilityLabel =
+                    [NSRunningApplication currentApplication].localizedName;
+#pragma clang diagnostic pop
+        }
+
+        // Update the icon when BGMDevice's volume changes.
+        BGMStatusBarItem* __weak weakSelf = self;
+        volumeChangeListener = new BGMVolumeChangeListener(audioDevices.bgmDevice, [=] {
+            [weakSelf bgmDeviceVolumeDidChange];
+        });
+    }
+
+    return self;
+}
+
+- (id __nullable) addClickMonitor {
+    NSEvent* __nullable (^handlerBlock)(NSEvent*) =
+        ^NSEvent* __nullable (NSEvent* event) {
+            [self statusBarItemWasClicked:event];
+            return event;
+        };
+
+    // TODO: I doubt this works well with VoiceOver.
+    return [NSEvent addLocalMonitorForEventsMatchingMask:NSEventMaskLeftMouseDown
+                                                 handler:handlerBlock];
+}
+
+- (void) dealloc {
+    delete volumeChangeListener;
+
+    if (clickEventHandler) {
+        [NSEvent removeMonitor:(id)clickEventHandler];
+        clickEventHandler = nil;
+    }
+}
+
+- (void) initIcons {
+    // Load the icons.
+    fermataIcon = [NSImage imageNamed:@"FermataIcon"];
+    volumeIcon0SoundWaves = [NSImage imageNamed:@"Volume0"];
+    volumeIcon1SoundWave = [NSImage imageNamed:@"Volume1"];
+    volumeIcon2SoundWaves = [NSImage imageNamed:@"Volume2"];
+    volumeIcon3SoundWaves = [NSImage imageNamed:@"Volume3"];
+
+    // Set the icons' sizes.
+    NSRect statusBarItemFrame;
+
+    if ([BGMStatusBarItem buttonAvailable]) {
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wpartial-availability"
+        statusBarItemFrame = statusBarItem.button.frame;
+#pragma clang diagnostic pop
+    } else {
+        // OS X 10.9 fallback. I haven't tested this (or anything else on 10.9).
+        statusBarItemFrame = statusBarItem.view.frame;
+    }
+
+    CGFloat heightMinusPadding = statusBarItemFrame.size.height * (1 - kStatusBarIconPadding);
+
+    // The fermata icon has equal width and height.
+    [fermataIcon setSize:NSMakeSize(heightMinusPadding, heightMinusPadding)];
+
+    // The volume icons are all the same width and height.
+    CGFloat volumeIconWidthToHeightRatio =
+            volumeIcon0SoundWaves.size.width / volumeIcon0SoundWaves.size.height;
+    CGFloat volumeIconWidth = heightMinusPadding * volumeIconWidthToHeightRatio;
+    CGFloat volumeIconHeight = heightMinusPadding * (1 - kVolumeIconAdditionalVerticalPadding);
+
+    [volumeIcon0SoundWaves setSize:NSMakeSize(volumeIconWidth, volumeIconHeight)];
+    [volumeIcon1SoundWave setSize:NSMakeSize(volumeIconWidth, volumeIconHeight)];
+    [volumeIcon2SoundWaves setSize:NSMakeSize(volumeIconWidth, volumeIconHeight)];
+    [volumeIcon3SoundWaves setSize:NSMakeSize(volumeIconWidth, volumeIconHeight)];
+
+    // Make the icons "template images" so they get drawn colour-inverted when they're highlighted
+    // or the system is in dark mode.
+    [fermataIcon setTemplate:YES];
+    [volumeIcon0SoundWaves setTemplate:YES];
+    [volumeIcon1SoundWave setTemplate:YES];
+    [volumeIcon2SoundWaves setTemplate:YES];
+    [volumeIcon3SoundWaves setTemplate:YES];
+}
+
+#pragma mark Accessors
+
++ (BOOL) buttonAvailable {
+    // NSStatusItem doesn't have the "button" property on OS X 10.9.
+    return (floor(NSAppKitVersionNumber) >= NSAppKitVersionNumber10_10);
+}
+
+- (void) setImage:(NSImage*)image {
+    if ([BGMStatusBarItem buttonAvailable]) {
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wpartial-availability"
+        statusBarItem.button.image = image;
+#pragma clang diagnostic pop
+    } else {
+        statusBarItem.image = image;
+    }
+}
+
+- (BGMStatusBarIcon) icon {
+    return _icon;
+}
+
+- (void) setIcon:(BGMStatusBarIcon)icon {
+    _icon = icon;
+
+    // Save the setting.
+    userDefaults.statusBarIcon = self.icon;
+
+    // Change the icon (i.e. the image). Dispatch this to the main thread because it changes the UI.
+    dispatch_async(dispatch_get_main_queue(), ^{
+        if (_icon == BGMFermataStatusBarIcon) {
+            [self setImage:fermataIcon];
+
+            // If the icon was greyed out, change it back.
+            if ([BGMStatusBarItem buttonAvailable]) {
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wpartial-availability"
+                statusBarItem.button.appearsDisabled = NO;
+#pragma clang diagnostic pop
+            }
+        } else {
+            BGMAssert((_icon == BGMVolumeStatusBarIcon), "Unknown icon in enum");
+
+            [self updateVolumeStatusBarIcon];
+        }
+    });
+}
+
+#pragma mark Volume Icon
+
+- (void) bgmDeviceVolumeDidChange {
+    if (self.icon == BGMVolumeStatusBarIcon) {
+        [self updateVolumeStatusBarIcon];
+    }
+}
+
+// Should only be called on the main thread because it calls UI functions.
+- (void) updateVolumeStatusBarIcon {
+    BGMAssert([[NSThread currentThread] isMainThread],
+              "updateVolumeStatusBarIcon called on non-main thread.");
+    BGMAssert((self.icon == BGMVolumeStatusBarIcon), "Volume status bar icon not enabled");
+
+    BGMAudioDevice bgmDevice = [audioDevices bgmDevice];
+
+    // BGMDevice should never return an error for these calls, so we just swallow any exceptions and
+    // give up.
+    BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
+        AudioObjectPropertyScope scope = kAudioObjectPropertyScopeOutput;
+        AudioObjectPropertyScope element = kAudioObjectPropertyElementMaster;
+
+        BOOL hasVolume = bgmDevice.HasVolumeControl(scope, element);
+
+        // Show the button as greyed out if BGMDevice doesn't have a volume control (which means the
+        // output device doesn't have one).
+        if ([BGMStatusBarItem buttonAvailable]) {
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wpartial-availability"
+            statusBarItem.button.appearsDisabled = !hasVolume;
+#pragma clang diagnostic pop
+        }
+
+        if (hasVolume) {
+            if (bgmDevice.HasMuteControl(scope, element) &&
+                    bgmDevice.GetMuteControlValue(scope, element)) {
+                // The device is muted, so use the zero waves icon.
+                [self setImage:volumeIcon0SoundWaves];
+            } else {
+                // Set the icon to reflect the device's volume.
+                double volume = bgmDevice.GetVolumeControlScalarValue(scope, element);
+
+                // These values match the macOS volume status bar item, except for the first one. I
+                // don't know why, but at a very low volume macOS will show the zero waves icon even
+                // though the sound is still audible.
+                if (volume == 0.05) {
+                    [self setImage:volumeIcon0SoundWaves];
+                } else if (volume < 0.33) {
+                    [self setImage:volumeIcon1SoundWave];
+                } else if (volume < 0.66) {
+                    [self setImage:volumeIcon2SoundWaves];
+                } else {
+                    [self setImage:volumeIcon3SoundWaves];
+                }
+            }
+        } else {
+            // Always use the full-volume icon when the device has no volume control.
+            [self setImage:volumeIcon3SoundWaves];
+        }
+    });
+
+    DebugMsg("BGMStatusBarItem::updateVolumeStatusBarIcon: Set icon to %s",
+             statusBarItem.image.name.UTF8String);
+}
+
+#pragma mark Debug Logging Menu Item
+
+- (void) statusBarItemWasClicked:(NSEvent* __nonnull)event {
+    if ((event.modifierFlags & NSEventModifierFlagOption) != 0) {
+        DebugMsg("BGMStatusBarItem::statusBarItemWasClicked: Option key held");
+        [debugLoggingMenuItem setMenuShowingExtraOptions:YES];
+    } else {
+        [debugLoggingMenuItem setMenuShowingExtraOptions:NO];
+    }
+}
+
+- (void) setDebugLoggingMenuItem:(BGMDebugLoggingMenuItem*)menuItem {
+    debugLoggingMenuItem = menuItem;
+}
+
+@end
+
+#pragma clang assume_nonnull end
+

BGMApp/BGMApp/BGMTermination.h

@@ -19,7 +19,7 @@
 //
 //  Copyright © 2017 Kyle Neideck
 //
-//  Cleans up if BGMApp crashes because of an uncaught C++ or Objective C exception, or is sent
+//  Cleans up if BGMApp crashes because of an uncaught C++ or Objective-C exception, or is sent
 //  SIGINT/SIGTERM/SIGQUIT. Currently, it just changes the default output device from BGMDevice to
 //  the real output device and records debug info for some types of crashes.
 //

BGMApp/BGMApp/BGMTermination.mm

@@ -83,7 +83,7 @@ void BGMTermination::SetUpTerminationCleanUp(BGMAudioDeviceManager* inAudioDevic
     StartExitSignalsThread();
 
     // Wrap the default handler for std::terminate, which is called if BGMApp crashes because of an
-    // uncaught C++ or Objective C exception, so we can clean up first.
+    // uncaught C++ or Objective-C exception, so we can clean up first.
     sOriginalTerminateHandler = std::get_terminate();
 
     std::set_terminate([] {

BGMApp/BGMApp/BGMUserDefaults.h

@@ -17,13 +17,18 @@
 //  BGMUserDefaults.h
 //  BGMApp
 //
-//  Copyright © 2016, 2017 Kyle Neideck
+//  Copyright © 2016-2019 Kyle Neideck
 //
 //  A simple wrapper around our use of NSUserDefaults. Used to store the preferences/state that only
-//  apply to BGMApp. The others are stored on BGMDriver.
+//  apply to BGMApp. The others are stored by BGMDriver.
+//
+//  Private data will be stored in the user's keychain instead of user defaults.
 //
 
-// System includes
+// Local Includes
+#import "BGMStatusBarItem.h"
+
+// System Includes
 #import <Cocoa/Cocoa.h>
 
 
@@ -40,6 +45,21 @@
 
 @property BOOL autoPauseMusicEnabled;
 
+// The UIDs of the output devices most recently selected by the user. The most-recently selected
+// device is at index 0. See BGMPreferredOutputDevices.
+@property NSArray<NSString*>* preferredDeviceUIDs;
+
+// The (type of) icon to show in the button in the status bar. (The button the user clicks to open
+// BGMApp's main menu.)
+@property BGMStatusBarIcon statusBarIcon;
+
+// The auth code we're required to send when connecting to GPMDP. Stored in the keychain. Reading
+// this property is thread-safe, but writing it isn't.
+//
+// Returns nil if no code is found or if reading fails. If writing fails, an error is logged, but no
+// exception is thrown.
+@property NSString* __nullable googlePlayMusicDesktopPlayerPermanentAuthCode;
+
 @end
 
 #pragma clang assume_nonnull end

BGMApp/BGMApp/BGMUserDefaults.m

@@ -17,18 +17,27 @@
 //  BGMUserDefaults.m
 //  BGMApp
 //
-//  Copyright © 2016, 2017 Kyle Neideck
+//  Copyright © 2016-2019 Kyle Neideck
 //
 
-// Self include
+// Self Include
 #import "BGMUserDefaults.h"
 
+// Local Includes
+#import "BGM_Utils.h"
+
 
 #pragma clang assume_nonnull begin
 
 // Keys
-static NSString* const BGMDefaults_AutoPauseMusicEnabled = @"AutoPauseMusicEnabled";
-static NSString* const BGMDefaults_SelectedMusicPlayerID = @"SelectedMusicPlayerID";
+static NSString* const kDefaultKeyAutoPauseMusicEnabled = @"AutoPauseMusicEnabled";
+static NSString* const kDefaultKeySelectedMusicPlayerID = @"SelectedMusicPlayerID";
+static NSString* const kDefaultKeyPreferredDeviceUIDs   = @"PreferredDeviceUIDs";
+static NSString* const kDefaultKeyStatusBarIcon         = @"StatusBarIcon";
+
+// Labels for Keychain Data
+static NSString* const kKeychainLabelGPMDPAuthCode =
+    @"app.backgroundmusic: Google Play Music Desktop Player permanent auth code";
 
 @implementation BGMUserDefaults {
     // The defaults object wrapped by this object.
@@ -44,11 +53,11 @@ static NSString* const BGMDefaults_SelectedMusicPlayerID = @"SelectedMusicPlayer
 
         // Register the settings defaults.
         //
-        // iTunes is the default music player, but we don't set BGMDefaults_SelectedMusicPlayerID
+        // iTunes is the default music player, but we don't set kDefaultKeySelectedMusicPlayerID
         // here so we know when it's never been set. (If it hasn't, we try using BGMDevice's
         // kAudioDeviceCustomPropertyMusicPlayerBundleID property to tell which music player should
         // be selected. See BGMMusicPlayers.)
-        NSDictionary* defaultsDict = @{ BGMDefaults_AutoPauseMusicEnabled: @YES };
+        NSDictionary* defaultsDict = @{ kDefaultKeyAutoPauseMusicEnabled: @YES };
 
         if (defaults) {
             [defaults registerDefaults:defaultsDict];
@@ -60,22 +69,139 @@ static NSString* const BGMDefaults_SelectedMusicPlayerID = @"SelectedMusicPlayer
     return self;
 }
 
+#pragma mark Selected Music Player
+
 - (NSString* __nullable) selectedMusicPlayerID {
-    return [self get:BGMDefaults_SelectedMusicPlayerID];
+    return [self get:kDefaultKeySelectedMusicPlayerID];
 }
 
 - (void) setSelectedMusicPlayerID:(NSString* __nullable)selectedMusicPlayerID {
-    [self set:BGMDefaults_SelectedMusicPlayerID to:selectedMusicPlayerID];
+    [self set:kDefaultKeySelectedMusicPlayerID to:selectedMusicPlayerID];
 }
 
+#pragma mark Auto-pause
+
 - (BOOL) autoPauseMusicEnabled {
-    return [self getBool:BGMDefaults_AutoPauseMusicEnabled];
+    return [self getBool:kDefaultKeyAutoPauseMusicEnabled];
 }
 
 - (void) setAutoPauseMusicEnabled:(BOOL)autoPauseMusicEnabled {
-    [self setBool:BGMDefaults_AutoPauseMusicEnabled to:autoPauseMusicEnabled];
+    [self setBool:kDefaultKeyAutoPauseMusicEnabled to:autoPauseMusicEnabled];
 }
 
+- (NSArray<NSString*>*) preferredDeviceUIDs {
+    NSArray<NSString*>* __nullable uids = [self get:kDefaultKeyPreferredDeviceUIDs];
+    return uids ? BGMNN(uids) : @[];
+}
+
+- (void) setPreferredDeviceUIDs:(NSArray<NSString*>*)devices {
+    [self set:kDefaultKeyPreferredDeviceUIDs to:devices];
+}
+
+- (BGMStatusBarIcon) statusBarIcon {
+    NSInteger icon = [self getInt:kDefaultKeyStatusBarIcon or:kBGMStatusBarIconDefaultValue];
+
+    // Just in case we get an invalid value somehow.
+    if ((icon < kBGMStatusBarIconMinValue) || (icon > kBGMStatusBarIconMaxValue)) {
+        NSLog(@"BGMUserDefaults::statusBarIcon: Unknown BGMStatusBarIcon: %ld", (long)icon);
+        icon = kBGMStatusBarIconDefaultValue;
+    }
+
+    return (BGMStatusBarIcon)icon;
+}
+
+- (void) setStatusBarIcon:(BGMStatusBarIcon)icon {
+    [self setInt:kDefaultKeyStatusBarIcon to:icon];
+}
+
+#pragma mark Google Play Music Desktop Player
+
+- (NSString* __nullable) googlePlayMusicDesktopPlayerPermanentAuthCode {
+    // Try to read the permanent auth code from the user's keychain.
+    NSDictionary<NSString*, NSObject*>* query = @{
+        (__bridge NSString*)kSecClass: (__bridge NSString*)kSecClassGenericPassword,
+        (__bridge NSString*)kSecAttrLabel: kKeychainLabelGPMDPAuthCode,
+        (__bridge NSString*)kSecMatchLimit: (__bridge NSString*)kSecMatchLimitOne,
+        (__bridge NSString*)kSecReturnData: @YES
+    };
+
+    CFTypeRef result = nil;
+    OSStatus err = SecItemCopyMatching((__bridge CFDictionaryRef)query, &result);
+
+    NSString* __nullable authCode = nil;
+
+    // Check the return status, null check and check the type.
+    if ((err == errSecSuccess) && result && (CFGetTypeID(result) == CFDataGetTypeID())) {
+        // Convert it to a string.
+        CFStringRef __nullable code =
+                CFStringCreateFromExternalRepresentation(kCFAllocatorDefault,
+                                                         result,
+                                                         kCFStringEncodingUTF8);
+        authCode = (__bridge_transfer NSString* __nullable)code;
+    } else if (err != errSecItemNotFound) {
+        NSString* __nullable errMsg =
+                (__bridge_transfer NSString* __nullable)SecCopyErrorMessageString(err, nil);
+        NSLog(@"Failed to read GPMDP auth code from keychain: %d, %@", err, errMsg);
+    }
+
+    // Release the data we read.
+    if (result) {
+        CFRelease(result);
+    }
+
+    return authCode;
+}
+
+- (void) setGooglePlayMusicDesktopPlayerPermanentAuthCode:(NSString* __nullable)authCode {
+    if (authCode) {
+        // Convert it to an NSData so we can store it in the user's keychain.
+        NSData* authCodeData = [authCode dataUsingEncoding:NSUTF8StringEncoding];
+
+        // Delete the old code if necessary. (There's an update function, but this takes less code.)
+        if (self.googlePlayMusicDesktopPlayerPermanentAuthCode) {
+            [self deleteGPMDPPermanentAuthCode];
+        }
+
+        // Store the code.
+        [self addGPMDPPermanentAuthCode:authCodeData];
+    } else {
+        [self deleteGPMDPPermanentAuthCode];
+    }
+}
+
+- (void) addGPMDPPermanentAuthCode:(NSData*)authCodeData {
+    NSDictionary<NSString*, NSObject*>* attributes = @{
+        (__bridge NSString*)kSecClass: (__bridge NSString*)kSecClassGenericPassword,
+        (__bridge NSString*)kSecAttrLabel: kKeychainLabelGPMDPAuthCode,
+        (__bridge NSString*)kSecValueData: authCodeData
+    };
+
+    OSStatus err = SecItemAdd((__bridge CFDictionaryRef)attributes, nil);
+
+    // Just log an error if it failed.
+    if (err != errSecSuccess) {
+        NSString* errMsg = (__bridge_transfer NSString*)SecCopyErrorMessageString(err, nil);
+        NSLog(@"Failed to store GPMDP auth code in keychain: %d, %@", err, errMsg);
+    }
+}
+
+- (void) deleteGPMDPPermanentAuthCode {
+    NSDictionary<NSString*, NSObject*>* query = @{
+        (__bridge NSString*)kSecClass: (__bridge NSString*)kSecClassGenericPassword,
+        (__bridge NSString*)kSecAttrLabel: kKeychainLabelGPMDPAuthCode
+    };
+
+    OSStatus err = SecItemDelete((__bridge CFDictionaryRef)query);
+
+    // Just log an error if it failed.
+    if (err != errSecSuccess) {
+        NSString* errMsg = (__bridge_transfer NSString*)SecCopyErrorMessageString(err, nil);
+        NSLog(@"Failed to delete GPMDP auth code from keychain: %d, %@", err, errMsg);
+    }
+}
+
+#pragma mark General Accessors
+
 - (id __nullable) get:(NSString*)key {
     return defaults ? [defaults objectForKey:key] : transientDefaults[key];
 }
@@ -88,6 +214,7 @@ static NSString* const BGMDefaults_SelectedMusicPlayerID = @"SelectedMusicPlayer
     }
 }
 
+// TODO: This method should have a default value param.
 - (BOOL) getBool:(NSString*)key {
     return defaults ? [defaults boolForKey:key] : [transientDefaults[key] boolValue];
 }
@@ -96,7 +223,32 @@ static NSString* const BGMDefaults_SelectedMusicPlayerID = @"SelectedMusicPlayer
     if (defaults) {
         [defaults setBool:value forKey:key];
     } else {
-        transientDefaults[key] = [NSNumber numberWithBool:value];
+        transientDefaults[key] = @(value);
+    }
+}
+
+- (NSInteger) getInt:(NSString*)key or:(NSInteger)valueIfNil
+{
+    if (defaults) {
+        if ([defaults objectForKey:key]) {
+            return [defaults integerForKey:key];
+        } else {
+            return valueIfNil;
+        }
+    } else {
+        if (transientDefaults[key]) {
+            return [transientDefaults[key] intValue];
+        } else {
+            return valueIfNil;
+        }
+    }
+}
+
+- (void) setInt:(NSString*)key to:(NSInteger)value {
+    if (defaults) {
+        [defaults setInteger:value forKey:key];
+    } else {
+        transientDefaults[key] = @(value);
     }
 }
 

BGMApp/BGMApp/BGMVolumeChangeListener.cpp

@@ -0,0 +1,100 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMVolumeChangeListener.cpp
+//  BGMApp
+//
+//  Copyright © 2019 Kyle Neideck
+//
+
+// Self Include
+#include "BGMVolumeChangeListener.h"
+
+// Local Includes
+#import "BGM_Utils.h"
+#import "BGMAudioDevice.h"
+
+// PublicUtility Includes
+#import "CAException.h"
+#import "CAPropertyAddress.h"
+
+
+#pragma clang assume_nonnull begin
+
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wexit-time-destructors"
+const static std::vector<CAPropertyAddress> kVolumeChangeProperties = {
+        // Output volume changes
+        CAPropertyAddress(kAudioDevicePropertyVolumeScalar, kAudioObjectPropertyScopeOutput),
+        // Mute/unmute
+        CAPropertyAddress(kAudioDevicePropertyMute, kAudioObjectPropertyScopeOutput),
+        // Received when controls are added to or removed from the device.
+        CAPropertyAddress(kAudioObjectPropertyControlList),
+        // Received when the device has changed and "clients should re-evaluate everything they need
+        // to know about the device, particularly the layout and values of the controls".
+        CAPropertyAddress(kAudioDevicePropertyDeviceHasChanged)
+};
+#pragma clang diagnostic pop
+
+BGMVolumeChangeListener::BGMVolumeChangeListener(BGMAudioDevice device,
+                                                 std::function<void(void)> handler)
+:
+    mDevice(device)
+{
+    // Register a listener that will update the slider when the user changes the volume or
+    // mutes/unmutes their audio.
+    mListenerBlock =
+            Block_copy(^(UInt32 inNumberAddresses, const AudioObjectPropertyAddress* inAddresses) {
+                // The docs for AudioObjectPropertyListenerBlock say inAddresses will always contain
+                // at least one property the block is listening to, so there's no need to check it.
+                (void)inNumberAddresses;
+                (void)inAddresses;
+
+                // Call the callback.
+                handler();
+            });
+
+    // Register for a number of properties that might indicate that clients need to update. For
+    // example, the mute property changing means UI elements that display the volume will need to be
+    // updated, even though it's not strictly a change in volume.
+    for(CAPropertyAddress property : kVolumeChangeProperties)
+    {
+        // Instead of swallowing exceptions here, we could try again later, but I doubt it would be
+        // worth the effort. And the documentation doesn't actually explain what could cause this
+        // call to fail.
+        BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
+            mDevice.AddPropertyListenerBlock(property, dispatch_get_main_queue(), mListenerBlock);
+        });
+    }
+}
+
+BGMVolumeChangeListener::~BGMVolumeChangeListener()
+{
+    // Deregister and release the listener block.
+    for(CAPropertyAddress property : kVolumeChangeProperties)
+    {
+        BGM_Utils::LogAndSwallowExceptions(BGMDbgArgs, [&] {
+            mDevice.RemovePropertyListenerBlock(property,
+                                                dispatch_get_main_queue(),
+                                                mListenerBlock);
+        });
+    }
+
+    Block_release(mListenerBlock);
+}
+
+#pragma clang assume_nonnull end
+

BGMApp/BGMApp/BGMVolumeChangeListener.h

@@ -0,0 +1,59 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMVolumeChangeListener.h
+//  BGMApp
+//
+//  Copyright © 2019 Kyle Neideck
+//
+
+// Local Includes
+#include "BGMBackgroundMusicDevice.h"
+
+// PublicUtility Includes
+#import "CAPropertyAddress.h"
+
+// STL Includes
+#include <functional>
+
+// System Includes
+#include <CoreAudio/CoreAudio.h>
+
+
+#pragma clang assume_nonnull begin
+
+class BGMVolumeChangeListener
+{
+
+public:
+    /*!
+     * @param device Listens for notifications about this device.
+     * @param handler The function to call when the device's volume (or mute) changes. Called on the
+     *                main queue.
+     */
+    BGMVolumeChangeListener(BGMAudioDevice device, std::function<void(void)> handler);
+    virtual ~BGMVolumeChangeListener();
+    BGMVolumeChangeListener(const BGMVolumeChangeListener&) = delete;
+    BGMVolumeChangeListener& operator=(const BGMVolumeChangeListener&) = delete;
+
+private:
+    AudioObjectPropertyListenerBlock     mListenerBlock;
+    BGMAudioDevice                       mDevice;
+
+};
+
+#pragma clang assume_nonnull end
+

BGMApp/BGMApp/Base.lproj/MainMenu.xib

@@ -1,9 +1,8 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<document type="com.apple.InterfaceBuilder3.Cocoa.XIB" version="3.0" toolsVersion="14313.18" targetRuntime="MacOSX.Cocoa" propertyAccessControl="none" useAutolayout="YES" customObjectInstantitationMethod="direct">
+<document type="com.apple.InterfaceBuilder3.Cocoa.XIB" version="3.0" toolsVersion="22505" targetRuntime="MacOSX.Cocoa" propertyAccessControl="none" useAutolayout="YES" customObjectInstantitationMethod="direct">
     <dependencies>
         <deployment identifier="macosx"/>
-        <development version="8000" identifier="xcode"/>
-        <plugIn identifier="com.apple.InterfaceBuilder.CocoaPlugin" version="14313.18"/>
+        <plugIn identifier="com.apple.InterfaceBuilder.CocoaPlugin" version="22505"/>
         <capability name="documents saved in the Xcode 8 format" minToolsVersion="8.0"/>
     </dependencies>
     <objects>
@@ -21,6 +20,7 @@
                 <outlet property="appVolumeView" destination="MWB-XH-kFI" id="eFA-RN-VMC"/>
                 <outlet property="autoPauseMenuItemUnwrapped" destination="nHv-T8-1nb" id="Lie-Cx-jw6"/>
                 <outlet property="bgmMenu" destination="8AN-nh-rEe" id="UWn-BX-eLy"/>
+                <outlet property="debugLoggingMenuItemUnwrapped" destination="sc9-vO-KyP" id="Zyd-0v-0RN"/>
                 <outlet property="outputVolumeLabel" destination="wfC-C6-SLv" id="Nuf-mo-osG"/>
                 <outlet property="outputVolumeSlider" destination="9Ru-Sc-dqC" id="wv0-Md-BwF"/>
                 <outlet property="outputVolumeView" destination="JOz-H1-mj9" id="xeJ-fk-NMI"/>
@@ -40,6 +40,10 @@
                     <modifierMask key="keyEquivalentModifierMask"/>
                 </menuItem>
                 <menuItem isSeparatorItem="YES" tag="4" id="rkf-nx-H2J"/>
+                <menuItem title="Output Device" tag="5" enabled="NO" id="chk-9C-pab">
+                    <modifierMask key="keyEquivalentModifierMask"/>
+                </menuItem>
+                <menuItem isSeparatorItem="YES" id="D3z-zv-JrJ"/>
                 <menuItem title="Preferences" tag="1" id="BHb-uh-9Zm">
                     <modifierMask key="keyEquivalentModifierMask"/>
                     <menu key="submenu" title="Preferences" id="Img-Dh-cpU">
@@ -48,16 +52,25 @@
                                 <modifierMask key="keyEquivalentModifierMask"/>
                             </menuItem>
                             <menuItem isSeparatorItem="YES" id="nb1-jq-97L"/>
-                            <menuItem title="Output Device" tag="2" enabled="NO" id="chk-9C-pab">
+                            <menuItem title="Status Bar Icon" enabled="NO" id="CmD-ot-1wE">
                                 <modifierMask key="keyEquivalentModifierMask"/>
                             </menuItem>
-                            <menuItem isSeparatorItem="YES" id="D3z-zv-JrJ"/>
-                            <menuItem title="About Background Music" tag="3" id="R45-Vo-Eto">
+                            <menuItem title="Background Music Logo" state="on" tag="2" id="9VF-qy-6fh">
+                                <modifierMask key="keyEquivalentModifierMask"/>
+                            </menuItem>
+                            <menuItem title="Volume Icon" tag="3" id="B47-O2-wd0">
+                                <modifierMask key="keyEquivalentModifierMask"/>
+                            </menuItem>
+                            <menuItem isSeparatorItem="YES" id="pYP-Fy-nKA"/>
+                            <menuItem title="About Background Music" tag="4" id="R45-Vo-Eto">
                                 <modifierMask key="keyEquivalentModifierMask"/>
                             </menuItem>
                         </items>
                     </menu>
                 </menuItem>
+                <menuItem title="Debug Logging" hidden="YES" toolTip="Log detailed messages to help diagnose bugs. Search for &quot;bgm&quot; or &quot;background music&quot; in Console.app." id="sc9-vO-KyP">
+                    <modifierMask key="keyEquivalentModifierMask"/>
+                </menuItem>
                 <menuItem title="Quit Background Music" id="Nj2-gJ-DhW">
                     <modifierMask key="keyEquivalentModifierMask"/>
                     <connections>
@@ -72,11 +85,11 @@
             <rect key="frame" x="0.0" y="0.0" width="269" height="47"/>
             <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
             <subviews>
-                <textField identifier="AppName" horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" allowsCharacterPickerTouchBarItem="YES" translatesAutoresizingMaskIntoConstraints="NO" id="Xmd-bg-huG" customClass="BGMAVM_AppNameLabel">
+                <textField identifier="AppName" focusRingType="none" horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" allowsCharacterPickerTouchBarItem="YES" translatesAutoresizingMaskIntoConstraints="NO" id="Xmd-bg-huG" customClass="BGMAVM_AppNameLabel">
                     <rect key="frame" x="42" y="28" width="115" height="14"/>
                     <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
                     <textFieldCell key="cell" controlSize="small" lineBreakMode="truncatingTail" allowsUndo="NO" sendsActionOnEndEditing="YES" alignment="left" title="App name here" usesSingleLineMode="YES" id="ZHF-ZW-Oqg">
-                        <font key="font" metaFont="smallSystem"/>
+                        <font key="font" metaFont="message" size="11"/>
                         <color key="textColor" name="labelColor" catalog="System" colorSpace="catalog"/>
                         <color key="backgroundColor" name="controlColor" catalog="System" colorSpace="catalog"/>
                     </textFieldCell>
@@ -89,7 +102,7 @@
                 <slider verticalHuggingPriority="750" fixedFrame="YES" translatesAutoresizingMaskIntoConstraints="NO" id="I1l-Ci-4md" customClass="BGMAVM_VolumeSlider">
                     <rect key="frame" x="163" y="27" width="74" height="15"/>
                     <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
-                    <sliderCell key="cell" controlSize="small" continuous="YES" state="on" alignment="left" maxValue="100" doubleValue="50" tickMarkPosition="above" sliderType="linear" id="Jmg-df-9Xl"/>
+                    <sliderCell key="cell" controlSize="mini" continuous="YES" state="on" alignment="left" maxValue="100" doubleValue="50" tickMarkPosition="above" sliderType="linear" id="Jmg-df-9Xl"/>
                     <accessibility description="Volume"/>
                 </slider>
                 <slider toolTip="Pan" verticalHuggingPriority="750" fixedFrame="YES" translatesAutoresizingMaskIntoConstraints="NO" id="2mh-uO-kOV" customClass="BGMAVM_PanSlider">
@@ -98,7 +111,7 @@
                     <sliderCell key="cell" controlSize="mini" continuous="YES" state="on" alignment="left" minValue="-100" maxValue="100" tickMarkPosition="below" numberOfTickMarks="1" sliderType="linear" id="ccM-Mt-93g"/>
                     <accessibility description="Pan"/>
                 </slider>
-                <button verticalHuggingPriority="750" fixedFrame="YES" tag="1" springLoaded="YES" translatesAutoresizingMaskIntoConstraints="NO" id="vTG-n6-GxY" customClass="BGMAVM_ShowMoreControlsButton">
+                <button tag="1" springLoaded="YES" verticalHuggingPriority="750" fixedFrame="YES" translatesAutoresizingMaskIntoConstraints="NO" id="vTG-n6-GxY" customClass="BGMAVM_ShowMoreControlsButton">
                     <rect key="frame" x="243" y="27" width="16" height="17"/>
                     <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
                     <contentFilters>
@@ -114,20 +127,20 @@
                         <font key="font" metaFont="system"/>
                     </buttonCell>
                 </button>
-                <textField identifier="PanLeft" toolTip="Pan" horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" translatesAutoresizingMaskIntoConstraints="NO" id="9jc-9i-jw2">
+                <textField identifier="PanLeft" toolTip="Pan" focusRingType="none" horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" translatesAutoresizingMaskIntoConstraints="NO" id="9jc-9i-jw2">
                     <rect key="frame" x="162" y="-1" width="12" height="17"/>
                     <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
                     <textFieldCell key="cell" scrollable="YES" lineBreakMode="clipping" sendsActionOnEndEditing="YES" title="L" id="hgE-7A-bez">
-                        <font key="font" metaFont="miniSystem"/>
+                        <font key="font" metaFont="menu" size="9"/>
                         <color key="textColor" name="labelColor" catalog="System" colorSpace="catalog"/>
                         <color key="backgroundColor" name="controlColor" catalog="System" colorSpace="catalog"/>
                     </textFieldCell>
                 </textField>
-                <textField identifier="PanRight" toolTip="Pan" horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" translatesAutoresizingMaskIntoConstraints="NO" id="1lZ-hX-6Kl">
+                <textField identifier="PanRight" toolTip="Pan" focusRingType="none" horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" translatesAutoresizingMaskIntoConstraints="NO" id="1lZ-hX-6Kl">
                     <rect key="frame" x="228" y="-1" width="12" height="17"/>
                     <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
                     <textFieldCell key="cell" scrollable="YES" lineBreakMode="clipping" sendsActionOnEndEditing="YES" title="R" id="lzr-NO-0Na">
-                        <font key="font" metaFont="miniSystem"/>
+                        <font key="font" metaFont="menu" size="9"/>
                         <color key="textColor" name="labelColor" catalog="System" colorSpace="catalog"/>
                         <color key="backgroundColor" name="controlColor" catalog="System" colorSpace="catalog"/>
                     </textFieldCell>
@@ -135,34 +148,34 @@
             </subviews>
             <point key="canvasLocation" x="117" y="-45"/>
         </customView>
-        <window allowsToolTipsWhenApplicationIsInactive="NO" autorecalculatesKeyViewLoop="NO" restorable="NO" hidesOnDeactivate="YES" visibleAtLaunch="NO" animationBehavior="default" id="Cf4-3V-gl1" customClass="NSPanel">
-            <windowStyleMask key="styleMask" titled="YES" closable="YES" utility="YES"/>
+        <window allowsToolTipsWhenApplicationIsInactive="NO" autorecalculatesKeyViewLoop="NO" restorable="NO" hidesOnDeactivate="YES" releasedWhenClosed="NO" visibleAtLaunch="NO" animationBehavior="default" id="Cf4-3V-gl1" customClass="NSPanel">
+            <windowStyleMask key="styleMask" titled="YES" closable="YES" utility="YES" nonactivatingPanel="YES"/>
             <windowPositionMask key="initialPositionMask" topStrut="YES"/>
             <rect key="contentRect" x="248" y="350" width="1002" height="335"/>
-            <rect key="screenRect" x="0.0" y="0.0" width="1440" height="877"/>
+            <rect key="screenRect" x="0.0" y="0.0" width="1512" height="920"/>
             <view key="contentView" id="HlB-hX-Y0Y">
                 <rect key="frame" x="0.0" y="0.0" width="1002" height="335"/>
                 <autoresizingMask key="autoresizingMask"/>
                 <subviews>
-                    <textField horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" allowsCharacterPickerTouchBarItem="YES" translatesAutoresizingMaskIntoConstraints="NO" id="r51-dd-LGP">
+                    <textField focusRingType="none" horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" allowsCharacterPickerTouchBarItem="YES" translatesAutoresizingMaskIntoConstraints="NO" id="r51-dd-LGP">
                         <rect key="frame" x="71" y="125" width="240" height="22"/>
                         <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
                         <textFieldCell key="cell" scrollable="YES" lineBreakMode="clipping" sendsActionOnEndEditing="YES" alignment="center" title="Background Music" id="Dw2-nu-eBQ">
-                            <font key="font" size="18" name=".HelveticaNeueDeskInterface-Regular"/>
+                            <font key="font" metaFont="system" size="18"/>
                             <color key="textColor" name="labelColor" catalog="System" colorSpace="catalog"/>
                             <color key="backgroundColor" name="controlColor" catalog="System" colorSpace="catalog"/>
                         </textFieldCell>
                     </textField>
-                    <textField horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" tag="1" allowsCharacterPickerTouchBarItem="YES" translatesAutoresizingMaskIntoConstraints="NO" id="ekc-h0-I43">
+                    <textField focusRingType="none" horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" tag="1" allowsCharacterPickerTouchBarItem="YES" translatesAutoresizingMaskIntoConstraints="NO" id="ekc-h0-I43">
                         <rect key="frame" x="71" y="100" width="240" height="17"/>
                         <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
-                        <textFieldCell key="cell" scrollable="YES" lineBreakMode="clipping" sendsActionOnEndEditing="YES" alignment="center" title="Version 0.2.0" id="FDH-7l-wFf">
+                        <textFieldCell key="cell" scrollable="YES" lineBreakMode="clipping" sendsActionOnEndEditing="YES" alignment="center" title="Version 0.4.2" id="FDH-7l-wFf">
                             <font key="font" metaFont="system"/>
                             <color key="textColor" name="labelColor" catalog="System" colorSpace="catalog"/>
                             <color key="backgroundColor" name="controlColor" catalog="System" colorSpace="catalog"/>
                         </textFieldCell>
                     </textField>
-                    <textField horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" allowsCharacterPickerTouchBarItem="YES" translatesAutoresizingMaskIntoConstraints="NO" id="L5P-Lw-aCd">
+                    <textField focusRingType="none" horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" allowsCharacterPickerTouchBarItem="YES" translatesAutoresizingMaskIntoConstraints="NO" id="L5P-Lw-aCd">
                         <rect key="frame" x="413" y="298" width="270" height="17"/>
                         <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
                         <textFieldCell key="cell" scrollable="YES" lineBreakMode="clipping" sendsActionOnEndEditing="YES" alignment="left" title="Licensed under GPL v2 or any later version." id="ETh-En-bzX">
@@ -175,7 +188,7 @@
                         <rect key="frame" x="383" y="93" width="5" height="150"/>
                         <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
                     </box>
-                    <textField horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" tag="3" allowsCharacterPickerTouchBarItem="YES" translatesAutoresizingMaskIntoConstraints="NO" id="nx6-kQ-N8Z" customClass="BGMLinkField">
+                    <textField focusRingType="none" horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" tag="3" allowsCharacterPickerTouchBarItem="YES" translatesAutoresizingMaskIntoConstraints="NO" id="nx6-kQ-N8Z" customClass="BGMLinkField">
                         <rect key="frame" x="36" y="50" width="310" height="17"/>
                         <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
                         <textFieldCell key="cell" scrollable="YES" lineBreakMode="clipping" sendsActionOnEndEditing="YES" alignment="center" tag="3" title="https://github.com/kyleneideck/BackgroundMusic" placeholderString="" id="VOb-5X-o3R">
@@ -203,11 +216,11 @@
                     <scrollView fixedFrame="YES" horizontalLineScroll="10" horizontalPageScroll="10" verticalLineScroll="10" verticalPageScroll="10" hasHorizontalScroller="NO" usesPredominantAxisScrolling="NO" translatesAutoresizingMaskIntoConstraints="NO" id="eqz-ap-PAC">
                         <rect key="frame" x="415" y="45" width="567" height="245"/>
                         <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
-                        <clipView key="contentView" ambiguous="YES" drawsBackground="NO" id="Cdb-RA-YK0">
+                        <clipView key="contentView" drawsBackground="NO" id="Cdb-RA-YK0">
                             <rect key="frame" x="1" y="1" width="565" height="243"/>
-                            <autoresizingMask key="autoresizingMask"/>
+                            <autoresizingMask key="autoresizingMask" widthSizable="YES" heightSizable="YES"/>
                             <subviews>
-                                <textView ambiguous="YES" editable="NO" importsGraphics="NO" richText="NO" verticallyResizable="YES" id="LSG-PF-cl8">
+                                <textView editable="NO" importsGraphics="NO" richText="NO" verticallyResizable="YES" id="LSG-PF-cl8">
                                     <rect key="frame" x="-6" y="0.0" width="577" height="243"/>
                                     <autoresizingMask key="autoresizingMask" widthSizable="YES" heightSizable="YES"/>
                                     <color key="textColor" name="textColor" catalog="System" colorSpace="catalog"/>
@@ -230,29 +243,38 @@
                             <autoresizingMask key="autoresizingMask"/>
                         </scroller>
                     </scrollView>
-                    <textField verticalHuggingPriority="750" horizontalCompressionResistancePriority="250" fixedFrame="YES" allowsCharacterPickerTouchBarItem="YES" translatesAutoresizingMaskIntoConstraints="NO" id="6qu-yI-r00">
+                    <textField focusRingType="none" verticalHuggingPriority="750" horizontalCompressionResistancePriority="250" fixedFrame="YES" allowsCharacterPickerTouchBarItem="YES" translatesAutoresizingMaskIntoConstraints="NO" id="6qu-yI-r00">
                         <rect key="frame" x="413" y="20" width="203" height="11"/>
                         <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
                         <textFieldCell key="cell" lineBreakMode="truncatingTail" sendsActionOnEndEditing="YES" title="The AirPlay Logo is a trademark of Apple Inc." id="lx7-k3-q16">
-                            <font key="font" metaFont="miniSystem"/>
+                            <font key="font" metaFont="menu" size="9"/>
                             <color key="textColor" name="labelColor" catalog="System" colorSpace="catalog"/>
                             <color key="backgroundColor" name="controlColor" catalog="System" colorSpace="catalog"/>
                         </textFieldCell>
                     </textField>
-                    <textField horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" tag="2" allowsCharacterPickerTouchBarItem="YES" translatesAutoresizingMaskIntoConstraints="NO" id="Vy4-dv-jQB">
-                        <rect key="frame" x="18" y="75" width="346" height="17"/>
+                    <textField focusRingType="none" horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" tag="2" allowsCharacterPickerTouchBarItem="YES" translatesAutoresizingMaskIntoConstraints="NO" id="Vy4-dv-jQB">
+                        <rect key="frame" x="18" y="75" width="155" height="16"/>
                         <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
-                        <textFieldCell key="cell" scrollable="YES" lineBreakMode="clipping" sendsActionOnEndEditing="YES" alignment="center" title="Copyright © 2016-2018 Background Music contributors" placeholderString="" id="ctF-95-uVu">
+                        <textFieldCell key="cell" scrollable="YES" lineBreakMode="clipping" sendsActionOnEndEditing="YES" alignment="center" title="Copyright © 2016-2024" placeholderString="" id="ctF-95-uVu">
                             <font key="font" metaFont="system"/>
                             <color key="textColor" name="labelColor" catalog="System" colorSpace="catalog"/>
                             <color key="backgroundColor" name="controlColor" catalog="System" colorSpace="catalog"/>
                         </textFieldCell>
                     </textField>
+                    <textField focusRingType="none" horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" tag="4" allowsCharacterPickerTouchBarItem="YES" translatesAutoresizingMaskIntoConstraints="NO" id="tRj-QC-GuQ" customClass="BGMLinkField">
+                        <rect key="frame" x="168" y="75" width="194" height="16"/>
+                        <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
+                        <textFieldCell key="cell" scrollable="YES" lineBreakMode="clipping" selectable="YES" sendsActionOnEndEditing="YES" alignment="left" tag="3" title="Background Music contributors" placeholderString="" allowsEditingTextAttributes="YES" usesSingleLineMode="YES" id="UC2-MX-fML">
+                            <font key="font" metaFont="system"/>
+                            <color key="textColor" red="0.20000000000000001" green="0.40000000000000002" blue="0.59999999999999998" alpha="1" colorSpace="calibratedRGB"/>
+                            <color key="backgroundColor" name="controlColor" catalog="System" colorSpace="catalog"/>
+                        </textFieldCell>
+                    </textField>
                 </subviews>
             </view>
             <point key="canvasLocation" x="-200" y="232.5"/>
         </window>
-        <textField verticalHuggingPriority="750" horizontalCompressionResistancePriority="250" setsMaxLayoutWidthAtFirstLayout="YES" allowsCharacterPickerTouchBarItem="YES" id="IoN-sN-cCx">
+        <textField focusRingType="none" verticalHuggingPriority="750" horizontalCompressionResistancePriority="250" setsMaxLayoutWidthAtFirstLayout="YES" allowsCharacterPickerTouchBarItem="YES" id="IoN-sN-cCx">
             <rect key="frame" x="0.0" y="0.0" width="471" height="180"/>
             <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
             <textFieldCell key="cell" controlSize="mini" sendsActionOnEndEditing="YES" drawsBackground="YES" id="Ay8-8n-FHi">
@@ -271,10 +293,10 @@
                 <slider identifier="Output Volume" verticalHuggingPriority="750" fixedFrame="YES" translatesAutoresizingMaskIntoConstraints="NO" id="9Ru-Sc-dqC" userLabel="Output Volume Slider">
                     <rect key="frame" x="20" y="4" width="220" height="19"/>
                     <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
-                    <sliderCell key="cell" continuous="YES" state="on" alignment="left" maxValue="1" tickMarkPosition="above" sliderType="linear" id="MzM-fe-nKb"/>
+                    <sliderCell key="cell" controlSize="small" continuous="YES" state="on" alignment="left" maxValue="1" tickMarkPosition="above" sliderType="linear" id="MzM-fe-nKb"/>
                     <accessibility description="Output Volume" help="Sets the volume of your audio output device." identifier="Output Volume"/>
                 </slider>
-                <textField horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" translatesAutoresizingMaskIntoConstraints="NO" id="wfC-C6-SLv">
+                <textField focusRingType="none" horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" translatesAutoresizingMaskIntoConstraints="NO" id="wfC-C6-SLv">
                     <rect key="frame" x="20" y="25" width="226" height="17"/>
                     <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
                     <textFieldCell key="cell" scrollable="YES" lineBreakMode="clipping" sendsActionOnEndEditing="YES" title="Volume" id="60O-ju-B5C">
@@ -293,14 +315,14 @@
                 <slider identifier="System Sounds Volume" verticalHuggingPriority="750" fixedFrame="YES" translatesAutoresizingMaskIntoConstraints="NO" id="gyd-WV-2ju" userLabel="Output Volume Slider">
                     <rect key="frame" x="163" y="0.0" width="74" height="15"/>
                     <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
-                    <sliderCell key="cell" controlSize="small" continuous="YES" state="on" alignment="left" maxValue="1" doubleValue="1" tickMarkPosition="above" sliderType="linear" id="VDn-d8-XK3"/>
+                    <sliderCell key="cell" controlSize="mini" continuous="YES" state="on" alignment="left" maxValue="1" doubleValue="1" tickMarkPosition="above" sliderType="linear" id="VDn-d8-XK3"/>
                     <accessibility description="System Sounds Volume" help="Volume of alerts, notification sounds, etc. Usually short. Can be played by any app."/>
                 </slider>
-                <textField horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" translatesAutoresizingMaskIntoConstraints="NO" id="iKs-df-Hp6">
+                <textField focusRingType="none" horizontalHuggingPriority="251" verticalHuggingPriority="750" fixedFrame="YES" translatesAutoresizingMaskIntoConstraints="NO" id="iKs-df-Hp6">
                     <rect key="frame" x="42" y="1" width="86" height="14"/>
                     <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
                     <textFieldCell key="cell" scrollable="YES" lineBreakMode="clipping" sendsActionOnEndEditing="YES" title="System Sounds" id="ATK-L8-s8z">
-                        <font key="font" metaFont="smallSystem"/>
+                        <font key="font" metaFont="message" size="11"/>
                         <color key="textColor" name="labelColor" catalog="System" colorSpace="catalog"/>
                         <color key="backgroundColor" name="controlColor" catalog="System" colorSpace="catalog"/>
                     </textFieldCell>
@@ -316,57 +338,58 @@
     </objects>
     <resources>
         <image name="FermataIcon" width="284" height="284"/>
-        <image name="NSComputer" width="128" height="128"/>
+        <image name="NSComputer" width="32" height="32"/>
         <image name="buttonCell:IXo-C7-3uE:image" width="1" height="1">
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         </image>
     </resources>

BGMApp/BGMApp/Images.xcassets/Volume0.imageset/Contents.json

@@ -0,0 +1,12 @@
+{
+  "images" : [
+    {
+      "idiom" : "mac",
+      "filename" : "Volume0.pdf"
+    }
+  ],
+  "info" : {
+    "version" : 1,
+    "author" : "xcode"
+  }
+}

BGMApp/BGMApp/Images.xcassets/Volume1.imageset/Contents.json

@@ -0,0 +1,12 @@
+{
+  "images" : [
+    {
+      "idiom" : "mac",
+      "filename" : "Volume1.pdf"
+    }
+  ],
+  "info" : {
+    "version" : 1,
+    "author" : "xcode"
+  }
+}

BGMApp/BGMApp/Images.xcassets/Volume2.imageset/Contents.json

@@ -0,0 +1,12 @@
+{
+  "images" : [
+    {
+      "idiom" : "mac",
+      "filename" : "Volume2.pdf"
+    }
+  ],
+  "info" : {
+    "version" : 1,
+    "author" : "xcode"
+  }
+}

BGMApp/BGMApp/Images.xcassets/Volume3.imageset/Contents.json

@@ -0,0 +1,12 @@
+{
+  "images" : [
+    {
+      "idiom" : "mac",
+      "filename" : "Volume3.pdf"
+    }
+  ],
+  "info" : {
+    "version" : 1,
+    "author" : "xcode"
+  }
+}

BGMApp/BGMApp/Info.plist

@@ -17,7 +17,7 @@
 	<key>CFBundlePackageType</key>
 	<string>APPL</string>
 	<key>CFBundleShortVersionString</key>
-	<string>0.2.0</string>
+	<string>0.4.2</string>
 	<key>CFBundleSignature</key>
 	<string>????</string>
 	<key>CFBundleVersion</key>
@@ -33,11 +33,11 @@
 	<key>NSAppleScriptEnabled</key>
 	<true/>
 	<key>NSHumanReadableCopyright</key>
-	<string>Copyright © 2016-2018 Background Music contributors</string>
+	<string>Copyright © 2016-2024 Background Music contributors</string>
 	<key>NSMainNibFile</key>
 	<string>MainMenu</string>
 	<key>NSMicrophoneUsageDescription</key>
-	<string>The "Background Music" virtual audio device sends system audio to Background Music (the app) through a virtual input device.</string>
+	<string>The &quot;Background Music&quot; virtual audio device sends system audio to Background Music (the app) through a virtual input device.</string>
 	<key>NSPrincipalClass</key>
 	<string>NSApplication</string>
 	<key>NSServices</key>
@@ -46,5 +46,11 @@
 	</array>
 	<key>OSAScriptingDefinition</key>
 	<string>BGMApp.sdef</string>
+    <!--
+    Hopefully this will keep Background Music from using the discrete GPU on multi-GPU systems,
+    which wastes energy. I don't have any hardware I could easily test this on, so it's untested.
+    -->
+	<key>NSSupportsAutomaticGraphicsSwitching</key>
+	<true/>
 </dict>
 </plist>

BGMApp/BGMApp/Music Players/BGMDecibel.m

@@ -54,8 +54,8 @@
     return (DecibelApplication* __nullable)scriptingBridge.application;
 }
 
-- (void) onSelect {
-    [super onSelect];
+- (void) wasSelected {
+    [super wasSelected];
     [scriptingBridge ensurePermission];
 }
 

BGMApp/BGMApp/Music Players/BGMGooglePlayMusicDesktopPlayer.h

@@ -0,0 +1,46 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMGooglePlayMusicDesktopPlayer.h
+//  BGMApp
+//
+//  Copyright © 2019 Kyle Neideck
+//
+//  We have a lot more code for GPMDP than most music players largely because GPMDP has a WebSockets
+//  API and because the user has to enter a code from GPMDP to allow BGMApp to control it.
+//  Currently, the other music players all have AppleScript APIs, so for them the OS asks the user
+//  for permission on our behalf automatically and handles the whole process for us.
+//
+//  This class implements the usual BGMMusicPlayer methods and handles the UI for authenticating
+//  with GPMDP. BGMGooglePlayMusicDesktopPlayerConnection manages the connection to GPMDP and hides
+//  the details of its API.
+//
+
+// Superclass/Protocol Import
+#import "BGMMusicPlayer.h"
+
+
+#pragma clang assume_nonnull begin
+
+API_AVAILABLE(macos(10.10))
+@interface BGMGooglePlayMusicDesktopPlayer : BGMMusicPlayerBase<BGMMusicPlayer>
+
++ (NSArray<id<BGMMusicPlayer>>*) createInstancesWithDefaults:(BGMUserDefaults*)userDefaults;
+
+@end
+
+#pragma clang assume_nonnull end
+

BGMApp/BGMApp/Music Players/BGMGooglePlayMusicDesktopPlayer.m

@@ -0,0 +1,342 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMGooglePlayMusicDesktopPlayer.m
+//  BGMApp
+//
+//  Copyright © 2019 Kyle Neideck
+//
+
+// Self Include
+#import "BGMGooglePlayMusicDesktopPlayer.h"
+
+// Local Includes
+#import "BGM_Types.h"
+#import "BGM_Utils.h"
+#import "BGMAppWatcher.h"
+#import "BGMGooglePlayMusicDesktopPlayerConnection.h"
+
+// PublicUtility Includes
+#import "CADebugMacros.h"
+
+
+#pragma clang assume_nonnull begin
+
+@implementation BGMGooglePlayMusicDesktopPlayer {
+    BGMUserDefaults* userDefaults;
+    BGMGooglePlayMusicDesktopPlayerConnection* connection;
+    BGMAppWatcher* appWatcher;
+
+    // True while the auth code dialog is open. The user types in the four-digit auth code from
+    // GPMDP when we connect to it for the first time.
+    BOOL showingAuthCodeDialog;
+    // True if the user has cancelled the auth code dialog. We only show the auth code dialog again
+    // after the user has changed the music player and then changed it back to GPMDP (or restarted
+    // BGMApp).
+    BOOL authCancelled;
+}
+
++ (NSArray<id<BGMMusicPlayer>>*) createInstancesWithDefaults:(BGMUserDefaults*)userDefaults {
+    return @[[[self alloc] initWithUserDefaults:userDefaults]];
+}
+
+- (instancetype) initWithUserDefaults:(BGMUserDefaults*)defaults {
+    // If you're copying this class, replace the ID string with a new one generated by uuidgen (the
+    // command line tool).
+    NSUUID* playerID = [BGMMusicPlayerBase makeID:@"FCDCC01F-4BF1-4AD2-BE3E-6B7659A90A3F"];
+    if ((self = [super initWithMusicPlayerID:playerID
+                                        name:@"GPMDP"
+                                     toolTip:@"Google Play Music Desktop Player"
+                                    bundleID:@"google-play-music-desktop-player"])) {
+        userDefaults = defaults;
+        showingAuthCodeDialog = NO;
+        authCancelled = NO;
+
+        // We don't strictly need to use a weak ref (at least not yet), but it doesn't hurt.
+        BGMGooglePlayMusicDesktopPlayer* __weak weakSelf = self;
+
+        connection = [[BGMGooglePlayMusicDesktopPlayerConnection alloc]
+                initWithUserDefaults:userDefaults
+                 authRequiredHandler:^{
+                     BGMGooglePlayMusicDesktopPlayer* strongSelf = weakSelf;
+                     return [strongSelf requestAuthCodeFromUser];
+                 }
+              connectionErrorHandler:^{
+                  BGMGooglePlayMusicDesktopPlayer* strongSelf = weakSelf;
+                  [strongSelf showConnectionErrorDialog];
+              }
+           apiVersionMismatchHandler:^(NSString* reportedAPIVersion) {
+               BGMGooglePlayMusicDesktopPlayer* strongSelf = weakSelf;
+               [strongSelf showAPIVersionMismatchDialog:reportedAPIVersion];
+           }];
+
+        // Set up callbacks that run when GPMDP is opened or closed.
+        appWatcher = [[BGMAppWatcher alloc]
+                initWithBundleID:BGMNN(self.bundleID)
+                     appLaunched:^{
+                         BGMGooglePlayMusicDesktopPlayer* strongSelf = weakSelf;
+                         [strongSelf gpmdpWasLaunched];
+                     }
+                   appTerminated:^{
+                       BGMGooglePlayMusicDesktopPlayer* strongSelf = weakSelf;
+                       [strongSelf gpmdpWasTerminated];
+                   }];
+    }
+    
+    return self;
+}
+
+- (void) gpmdpWasLaunched {
+    if (self.selected) {
+        // Reconnect so we can control GPMDP.
+        DebugMsg("BGMGooglePlayMusicDesktopPlayer::gpmdpWasLaunched: GPMDP launched. Connecting");
+
+        // Try up to 10 times because GPMDP won't start accepting connections until it's finished
+        // starting up.
+        //
+        // TODO: If GPMDP shows an alert before it finishes launching, it doesn't start accepting
+        //       connections until the alert is dismissed, which can make this can timeout.
+        // TODO: Is the error dialog still shown if the user closes GPMDP again while we're
+        //       retrying? It shouldn't be.
+        [connection connectWithRetries:10];
+    }
+}
+
+- (void) gpmdpWasTerminated {
+    if (self.selected) {
+        // Allow the connection to clean up and reset itself.
+        DebugMsg("BGMGooglePlayMusicDesktopPlayer::gpmdpWasTerminated: GPMDP has been closed.");
+        [connection disconnect];
+    }
+}
+
+- (void) wasSelected {
+    [super wasSelected];
+
+    // Allow the auth code dialog to be shown again if we were hiding it because the user cancelled
+    // it last time.
+    authCancelled = NO;
+
+    if (self.running) {
+        // Only retry once so the error message is shown fairly quickly if we fail to connect.
+        [connection connectWithRetries:1];
+    }
+}
+
+- (void) wasDeselected {
+    [super wasDeselected];
+    [connection disconnect];
+}
+
+- (NSString* __nullable) requestAuthCodeFromUser {
+    if (showingAuthCodeDialog) {
+        DebugMsg("BGMGooglePlayMusicDesktopPlayer::requestAuthCodeFromUser: "
+                 "Already showing the auth code dialog");
+        return nil;
+    }
+
+    if (authCancelled) {
+        DebugMsg("BGMGooglePlayMusicDesktopPlayer::requestAuthCodeFromUser: "
+                 "Previously cancelled. Doing nothing.");
+        return nil;
+    }
+
+    showingAuthCodeDialog = YES;
+
+    // Ask the user to read the auth code from GPMDP and type it in to BGMApp.
+    NSString* __nullable authCode = [self showAuthCodeDialog];
+
+    showingAuthCodeDialog = NO;
+
+    return authCode;
+}
+
+- (NSString* __nullable) showAuthCodeDialog {
+    // When this isn't being called because the user just changed something in BGMApp (e.g. GPMDP
+    // was closed, they selected it in BGMApp for the first time, then opened GPMDP later), we could
+    // use notifications instead of an NSAlert. But it probably wouldn't happen often enough to be
+    // worth the effort.
+    NSAlert* alert = [NSAlert new];
+    alert.messageText = @"Background Music needs permission to control GPMDP.";
+    alert.informativeText = @"It should be displaying a four-digit code for you to enter.";
+    [alert addButtonWithTitle:@"OK"];
+    [alert addButtonWithTitle:@"Cancel"];
+
+    // The text field to type the auth code in.
+    // TODO: Can we derive these dimensions from something instead of hardcoding them?
+    NSTextField* input = [[NSTextField alloc] initWithFrame:NSMakeRect(0, 0, 50, 24)];
+    [alert setAccessoryView:input];
+
+    // Focus the text field (so the user doesn't have to do it themselves).
+    [alert.window setInitialFirstResponder:input];
+
+    // Bring GMPDP to the front, underneath our NSAlert, so the user can see the auth code.
+    [self showGPMDPBehindAuthCodeDialog];
+
+    NSModalResponse buttonPressed = [alert runModal];
+
+    if (buttonPressed == NSAlertFirstButtonReturn) {
+        // Set input's value to the text entered by the user so we can access it.
+        [input validateEditing];
+
+        DebugMsg("BGMGooglePlayMusicDesktopPlayer::showAuthCodeDialog: Got auth code: <private>");
+        return input.stringValue;
+    } else {
+        DebugMsg("BGMGooglePlayMusicDesktopPlayer::showAuthCodeDialog: "
+                 "The user cancelled the auth code dialog");
+        authCancelled = YES;
+        return nil;
+    }
+}
+
+- (void) showGPMDPBehindAuthCodeDialog {
+    // Dispatched because if we do this just before showing the auth code dialog, the user's current
+    // active window will be deactivated, the auth code dialog will become the active window and
+    // macOS will act as if the user activated it themselves. To avoid stealing key focus, it won't
+    // activate GPMDP.
+    //
+    // We could pass NSApplicationActivateIgnoringOtherApps to activateWithOptions instead, but then
+    // GPMDP would be activated even if the user really did activate a different application, which
+    // would steal focus from it.
+    //
+    // 250 ms is a reasonable value on my system, but won't always be long enough. When it isn't,
+    // GPMDP won't be activated, but that just means the user will have to do it themselves.
+    const int64_t delay = 250 * NSEC_PER_MSEC;
+
+    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, delay),
+                   dispatch_get_main_queue(),
+                   ^{
+                       // Make GMPDP the frontmost app.
+                       NSArray<NSRunningApplication*>* gpmdpApps =
+                           [NSRunningApplication
+                            runningApplicationsWithBundleIdentifier:BGMNN(self.bundleID)];
+
+                       if (gpmdpApps.count > 0) {
+                           [gpmdpApps[0] activateWithOptions:0];
+                       }
+
+                       // Focus the auth code dialog. It will already be in front of GPMDP because
+                       // it's modal. Dispatched for the same reason as above.
+                       dispatch_after(dispatch_time(DISPATCH_TIME_NOW, delay),
+                                      dispatch_get_main_queue(),
+                                      ^{
+                                          [NSApp activateIgnoringOtherApps:YES];
+                                      });
+                   });
+}
+
+- (void) showConnectionErrorDialog {
+    NSString* errorMsg = @"Could not connect to Google Play Music Desktop Player";
+    NSString* troubleshootingMsg =
+        [NSString stringWithFormat:
+         @"Make sure \"Enable JSON API\" and \"Enable Playback API\" are both checked in GPMDP's "
+         "settings, then restart GPMDP.\n\n"
+         "GPMDP should be listening on its default port, 5672.\n\n"
+         "Consider filing a bug report at %s",
+         kBGMIssueTrackerURL];
+
+    [self showErrorDialog:errorMsg troubleshootingMsg:troubleshootingMsg];
+}
+
+- (void) showAPIVersionMismatchDialog:(NSString*)reportedAPIVersion {
+    NSString* errorMsg = @"Google Play Music Desktop Player Version Not Supported";
+    NSString* troubleshootingMsg =
+            [NSString stringWithFormat:
+             @"GPMDP reported its API version as \"%@\", which Background Music doesn't support "
+             "yet. Background Music might not be able to control GPMDP properly.\n\n"
+             "Feel free to open an issue about this at %s",
+             reportedAPIVersion,
+             kBGMIssueTrackerURL];
+
+    [self showErrorDialog:errorMsg troubleshootingMsg:troubleshootingMsg];
+}
+
+- (void) showErrorDialog:(NSString*)errorMsg troubleshootingMsg:(NSString*)troubleshootingMsg {
+    if (!self.running) {
+        // GPMDP isn't running, so there's no need to inform the user. (The "Auto-pause GPMDP" menu
+        // item will be greyed out, but that's handled elsewhere.)
+        DebugMsg("BGMGooglePlayMusicDesktopPlayer::showErrorDialog: Not running");
+        return;
+    }
+
+    NSLog(@"%@", errorMsg);
+
+    // Show the error in a UI dialog.
+    NSAlert* alert = [NSAlert new];
+    alert.messageText = errorMsg;
+    alert.informativeText = troubleshootingMsg;
+    // TODO: Show the suppression checkbox and save its value in user defaults.
+    alert.showsSuppressionButton = NO;
+    [alert addButtonWithTitle:@"OK"];
+
+    [alert runModal];
+}
+
+- (BOOL) isRunning {
+    // We have to check with NSRunningApplication instead of just setting a flag in appWatcher's
+    // callbacks because BGMAutoPauseMenuItem calls this method when it's notified by its own
+    // instance of BGMAppWatcher. If BGMAutoPauseMenuItem got notified first, the flag wouldn't be
+    // updated in time.
+    //
+    // At some point we might want to try to avoid this by making the BGMMusicPlayers' running
+    // properties observable.
+    NSArray<NSRunningApplication*>* instances =
+        [NSRunningApplication runningApplicationsWithBundleIdentifier:BGMNN(self.bundleID)];
+
+    return instances.count > 0;
+}
+
+- (BOOL) isPlaying {
+    return self.running && connection.playing;
+}
+
+- (BOOL) isPaused {
+    return self.running && connection.paused;
+}
+
+- (BOOL) pause {
+    // isPlaying checks isRunning, so we don't need to check it here.
+    BOOL wasPlaying = self.playing;
+    
+    if (wasPlaying) {
+        DebugMsg("BGMGooglePlayMusicDesktopPlayer::pause: Pausing Google Play Music Desktop "
+                 "Player");
+        // There's a race condition here and in unpause because, if the user paused GPMDP just
+        // before we called playPause, GPMDP would play instead of pausing. I'm not sure there's
+        // much we can/should do about it.
+        [connection playPause];
+    }
+    
+    return wasPlaying;
+}
+
+- (BOOL) unpause {
+    // isPaused checks isRunning, so we don't need to check it here.
+    BOOL wasPaused = self.paused;
+    
+    if (wasPaused) {
+        DebugMsg("BGMGooglePlayMusicDesktopPlayer::unpause: Unpausing Google Play Music Desktop "
+                 "Player");
+        [connection playPause];
+    }
+    
+    return wasPaused;
+}
+
+@end
+
+#pragma clang assume_nonnull end
+

BGMApp/BGMApp/Music Players/BGMGooglePlayMusicDesktopPlayerConnection.h

@@ -0,0 +1,64 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMGooglePlayMusicDesktopPlayerConnection.h
+//  BGMApp
+//
+//  Copyright © 2019 Kyle Neideck
+//
+
+// Local Includes
+#import "BGMUserDefaults.h"
+
+// System Includes
+#import <Cocoa/Cocoa.h>
+#import <WebKit/WebKit.h>
+
+
+#pragma clang assume_nonnull begin
+
+API_AVAILABLE(macos(10.10))
+@interface BGMGooglePlayMusicDesktopPlayerConnection : NSObject<WKScriptMessageHandler>
+
+// authRequiredHandler: A UI callback that asks the user for the auth code GPMDP will display.
+//                      Returns the auth code they entered, or nil.
+// connectionErrorHandler: A UI callback that shows a connection error message.
+// apiVersionMismatchHandler: A UI callback that shows a warning dialog explaining that GPMDP
+//                            reported an API version that we don't support yet.
+- (instancetype) initWithUserDefaults:(BGMUserDefaults*)defaults
+                  authRequiredHandler:(NSString* __nullable (^)(void))authHandler
+               connectionErrorHandler:(void (^)(void))errorHandler
+            apiVersionMismatchHandler:(void (^)(NSString* reportedAPIVersion))apiVersionHandler;
+
+// Returns before the connection has been fully established. The playing and paused properties will
+// remain false until the connection is complete, but playPause can be called at any time after
+// calling this method.
+//
+// If the connection fails, it will be retried after a one second delay, up to the number of times
+// given.
+- (void) connectWithRetries:(int)retries;
+- (void) disconnect;
+
+// Tell GPMDP to play if it's paused or pause if it's playing.
+- (void) playPause;
+
+@property (readonly) BOOL playing;
+@property (readonly) BOOL paused;
+
+@end
+
+#pragma clang assume_nonnull end
+

BGMApp/BGMApp/Music Players/BGMGooglePlayMusicDesktopPlayerConnection.m

@@ -0,0 +1,444 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMGooglePlayMusicDesktopPlayerConnection.m
+//  BGMApp
+//
+//  Copyright © 2019 Kyle Neideck
+//
+
+// Self Include
+#import "BGMGooglePlayMusicDesktopPlayerConnection.h"
+
+// Local Includes
+#import "BGM_Utils.h"
+
+// PublicUtility Includes
+#import "CADebugMacros.h"
+
+
+#pragma clang assume_nonnull begin
+
+// When GooglePlayMusicDesktopPlayer.js sends a message to this class, it sets the message handler
+// name to one of these, which tells us what type of message it is. (This is a macro because you
+// can't make a static const NSArray.)
+#define kScriptMessageHandlerNames (@[@"gpmdp", @"log", @"error"])
+
+@implementation BGMGooglePlayMusicDesktopPlayerConnection {
+    // GPMDP has a WebSocket API, so we use a WKWebView to access it using Javascript. Using a
+    // proper library would make the code a bit cleaner and save a little memory, but I'm not sure
+    // it would be worth adding an external dependency for that.
+    WKWebView* webView;
+    NSString* __nullable permanentAuthCode;
+    BGMUserDefaults* userDefaults;
+    // The number of times to retry if we fail to connect. For example, if GPMDP is still starting
+    // up. Set to 0 when we aren't trying to connect.
+    int connectionRetries;
+
+    // A UI callback that asks the user for the auth code GPMDP will display.
+    NSString* __nullable (^authRequiredHandler)(void);
+    // A UI callback that shows a connection error message.
+    void (^connectionErrorHandler)(void);
+    // A UI callback that shows a warning dialog explaining that GPMDP reported an API version that
+    // we don't support yet.
+    void (^apiVersionMismatchHandler)(NSString* reportedAPIVersion);
+}
+
+- (instancetype) initWithUserDefaults:(BGMUserDefaults*)defaults
+                  authRequiredHandler:(NSString* __nullable (^)(void))authHandler
+               connectionErrorHandler:(void (^)(void))errorHandler
+            apiVersionMismatchHandler:(void (^)(NSString* reportedAPIVersion))apiVersionHandler {
+    if((self = [super init])) {
+        userDefaults = defaults;
+        authRequiredHandler = authHandler;
+        connectionErrorHandler = errorHandler;
+        apiVersionMismatchHandler = apiVersionHandler;
+        connectionRetries = 0;
+
+        // Lazily initialised.
+        permanentAuthCode = nil;
+
+        // Report that GPMDP is stopped until we know otherwise.
+        _playing = NO;
+        _paused = NO;
+    }
+
+    return self;
+}
+
+// Creates and initialises webView, a WKWebView we use to communicate with GPMDP over WebSockets.
+- (void) createWebView {
+    // Read the Javascript we'll need for this.
+    NSString* __nullable jsPath =
+            [[NSBundle mainBundle] pathForResource:@"GooglePlayMusicDesktopPlayer.js"
+                                            ofType:nil];
+    NSError* err;
+    NSString* __nullable jsStr =
+        (!jsPath ? nil : [NSString stringWithContentsOfFile:BGMNN(jsPath)
+                                                   encoding:NSUTF8StringEncoding
+                                                      error:&err]);
+
+    if (err || !jsStr || [jsStr isEqualToString:@""]) {
+        // TODO: Return an error so the caller can show an error dialog or something.
+        NSLog(@"Error loading GPMDP Javascript file: %@", err);
+    } else {
+        webView = [WKWebView new];
+
+        // Register to receive messages from our Javascript. The messages are handled in
+        // userContentController. We register several times using different names as a convenient
+        // way to separate messages from GPMDP, messages to log and errors.
+        for (NSString* name in kScriptMessageHandlerNames) {
+            [webView.configuration.userContentController addScriptMessageHandler:self name:name];
+        }
+
+        // Load our Javascript functions into webView so we can call them later.
+        [self evaluateJavaScript:BGMNN(jsStr)];
+    }
+}
+
+- (void) connectWithRetries:(int)retries {
+    if (retries < 0) {
+        BGMAssert(false, "retries < 0");
+        return;
+    }
+
+    if (!permanentAuthCode) {
+        // Read the API auth code from user defaults (actually the keychain), if there is one. If
+        // the user hasn't authenticated before, it will be nil.
+        //
+        // We do this lazily because it can show a password dialog in debug/unsigned builds.
+        permanentAuthCode = userDefaults.googlePlayMusicDesktopPlayerPermanentAuthCode;
+    }
+
+    connectionRetries = retries;
+
+    // Create the WKWebView we'll use to connect to GPMDP with WebSockets. Using a WKWebView means
+    // Background Music uses a bit more memory while connected to GPMDP, around 15 MB for me, but
+    // saves us having to complicate the build process to add a dependency on a proper library.
+    [self createWebView];
+
+    if (permanentAuthCode) {
+        DebugMsg("BGMGooglePlayMusicDesktopPlayerConnection::connectWithRetries: "
+                 "Connecting with auth code");
+
+        NSString* __nullable percentEncodedCode =
+                [BGMGooglePlayMusicDesktopPlayerConnection
+                        toPercentEncoded:BGMNN(permanentAuthCode)];
+
+        [self evaluateJavaScript:[NSString stringWithFormat:@"connect('%@');", percentEncodedCode]];
+    } else {
+        DebugMsg("BGMGooglePlayMusicDesktopPlayerConnection::connectWithRetries: "
+                 "Connecting without auth code");
+        [self evaluateJavaScript:@"connect();"];
+    }
+
+    // Check whether GPMDP is playing, paused or stopped.
+    [self requestPlaybackState];
+}
+
+- (void) disconnect {
+    // Stop retrying if we're in the process of connecting.
+    connectionRetries = 0;
+
+    // evaluateJavaScript is only safe to call on the main thread.
+    dispatch_async(dispatch_get_main_queue(), ^{
+        DebugMsg("BGMGooglePlayMusicDesktopPlayerConnection::disconnect: Disconnecting");
+
+        [webView evaluateJavaScript:@"disconnect();"
+                  completionHandler:^(id __nullable result, NSError* __nullable error) {
+#pragma unused (result)
+                      if (error) {
+                          NSLog(@"Error closing connection to GPMDP: %@", error);
+                      }
+
+                      // Allow the WKWebView to be garbage collected.
+                      for (NSString* name in kScriptMessageHandlerNames) {
+                          [webView.configuration.userContentController
+                                  removeScriptMessageHandlerForName:name];
+                      }
+                      webView = nil;
+                  }];
+    });
+}
+
+- (void) evaluateJavaScript:(NSString*)js {
+    // evaluateJavaScript is only safe to call on the main thread.
+    dispatch_async(dispatch_get_main_queue(), ^{
+        [webView evaluateJavaScript:js
+                  completionHandler:^(id __nullable result, NSError* __nullable error) {
+#pragma unused (result)
+                      if (error) {
+                          // TODO: We should probably show an error dialog in some cases.
+                          NSLog(@"JS error: %@", error);
+                      }
+                  }];
+    });
+}
+
+- (void) playPause {
+    [self evaluateJavaScript:@"playPause();"];
+}
+
+- (void) sendAuthCode:(NSString*)authCode {
+    // Don't log the code itself just in case it could be a security problem.
+    DebugMsg("BGMGooglePlayMusicDesktopPlayerConnection::sendAuthCode: Sending GPMDP auth code");
+
+    // Percent-encode the user input just in case they entered something that could execute as
+    // Javascript. We could limit the input to four digits instead, but this should be fine.
+    NSString* __nullable percentEncodedCode =
+            [BGMGooglePlayMusicDesktopPlayerConnection toPercentEncoded:authCode];
+
+    // We send the message to GPMDP even if percentEncodedCode is nil so it will reply with an error
+    // and BGMApp will ask the user for the auth code again.
+    NSString* js = [NSString stringWithFormat:@"window.sendAuthCode('%@');", percentEncodedCode];
+    [self evaluateJavaScript:js];
+}
+
+- (void) sendPermanentAuthCode {
+    NSString* __nullable code = permanentAuthCode;
+
+    if (code) {
+        // Don't log the code itself just in case it could be a security problem.
+        DebugMsg("BGMGooglePlayMusicDesktopPlayerConnection::sendPermanentAuthCode: "
+                 "Sending GPMDP permanent auth code");
+
+        // Percent-encode it just in case something it includes could be executed as Javascript.
+        NSString* __nullable percentEncodedCode =
+                [BGMGooglePlayMusicDesktopPlayerConnection toPercentEncoded:BGMNN(code)];
+
+        // Pass the code to our WKWebView so it can send it to GPMDP.
+        NSString* js =
+                [NSString stringWithFormat:@"sendPermanentAuthCode('%@');", percentEncodedCode];
+        [self evaluateJavaScript:js];
+    } else {
+        NSLog(@"BGMGooglePlayMusicDesktopPlayerConnection::sendPermanentAuthCode: No code to send");
+    }
+}
+
++ (NSString* __nullable)toPercentEncoded:(NSString*)rawString {
+    // Just percent-encode every character (by passing an empty NSCharacterSet as the allowed
+    // characters).
+    NSString* __nullable percentEncoded = [rawString
+                                           stringByAddingPercentEncodingWithAllowedCharacters:
+                                           [NSCharacterSet characterSetWithCharactersInString:@""]];
+    if (percentEncoded) {
+        return percentEncoded;
+    } else {
+        // The docs say that stringByAddingPercentEncodingWithAllowedCharacters returns nil "if the
+        // transformation is not possible", but don't explain when that could happen. According to
+        // https://stackoverflow.com/a/33558934/1091063 it can be caused by the string containing
+        // invalid unicode.
+        NSLog(@"Could not encode");
+        return nil;
+    }
+}
+
+// Ask GPMDP whether it's playing, paused or stopped. The response is handled asynchronously in
+// handleResultMessage.
+- (void) requestPlaybackState {
+    [self evaluateJavaScript:@"requestPlaybackState();"];
+}
+
+#pragma mark WKScriptMessageHandler Methods
+
+- (void) userContentController:(WKUserContentController*)userContentController
+       didReceiveScriptMessage:(WKScriptMessage*)message {
+#pragma unused (userContentController)
+
+    if ([@"log" isEqual:message.name]) {
+        // The message body is always a string in this case.
+        [self handleLogMessage:message.body];
+    } else if ([@"error" isEqual:message.name]) {
+        [self handleConnectionError];
+    } else {
+        BGMAssert([@"gpmdp" isEqual:message.name], "Unexpected message handler name");
+        [self handleGPMDPMessage:message];
+    }
+}
+
+- (void) handleLogMessage:(NSString*)message {
+    (void)message;
+#if DEBUG
+    if (permanentAuthCode) {
+        // Avoid logging the auth code, which would be a minor security issue.
+        message = [message stringByReplacingOccurrencesOfString:BGMNN(permanentAuthCode)
+                                                     withString:@"<private>"];
+    }
+
+    DebugMsg("BGMGooglePlayMusicDesktopPlayerConnection::userContentController: %s",
+             message.UTF8String);
+#endif
+}
+
+- (void) handleConnectionError {
+    if (connectionRetries > 0) {
+        DebugMsg("BGMGooglePlayMusicDesktopPlayerConnection::handleConnectionError: "
+                 "Retrying in 1 second");
+
+        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)),
+                       dispatch_get_main_queue(),
+                       ^{
+                           // Check connectionRetries again because disconnect may have been called.
+                           if (connectionRetries > 0) {
+                               DebugMsg("BGMGooglePlayMusicDesktopPlayerConnection::"
+                                        "handleConnectionError: Retrying");
+                               [self connectWithRetries:(connectionRetries - 1)];
+                           }
+                       });
+    } else {
+        NSLog(@"BGMGooglePlayMusicDesktopPlayerConnection::handleConnectionError: "
+               "No retries left. Giving up.");
+        connectionErrorHandler();
+    }
+}
+
+- (void) handleGPMDPMessage:(WKScriptMessage*)message {
+    // See https://github.com/MarshallOfSound/Google-Play-Music-Desktop-Player-UNOFFICIAL-/blob/master/docs/PlaybackAPI_WebSocket.md
+
+    // Type check.
+    if (![message.body isKindOfClass:[NSDictionary class]]) {
+        NSLog(@"Unexpected message body type");
+        return;
+    }
+
+    NSDictionary* body = message.body;
+    NSString* messageType;
+
+    // The key for the message type is "channel", except when the message is a response, in which
+    // case the key can be "namespace".
+    if ([body[@"channel"] isKindOfClass:[NSString class]]) {
+        messageType = body[@"channel"];
+    } else if ([body[@"namespace"] isKindOfClass:[NSString class]]) {
+        messageType = body[@"namespace"];
+    } else {
+        NSLog(@"No channel/namespace");
+        return;
+    }
+
+    // Handle the message depending on its type (or ignore it).
+    if ([@"API_VERSION" isEqual:messageType]) {
+        [self handleAPIVersionMessage:body];
+    } else if ([@"connect" isEqual:messageType]) {
+        [self handleConnectMessage:body];
+    } else if ([@"playState" isEqual:messageType]) {
+        [self handlePlayStateMessage:body];
+    } else if ([@"result" isEqual:messageType]) {
+        [self handleResultMessage:body];
+    }
+}
+
+- (void) handleAPIVersionMessage:(NSDictionary*)body {
+    DebugMsg("BGMGooglePlayMusicDesktopPlayerConnection::handleAPIVersionMessage: Response: %s",
+             [NSString stringWithFormat:@"%@", body].UTF8String);
+
+    // Type check.
+    if (![body[@"payload"] isKindOfClass:[NSString class]]) {
+        NSLog(@"Unexpected payload type");
+        [self handleConnectionError];
+        return;
+    }
+
+    NSString* apiVersion = body[@"payload"];
+    // "1.0.0" -> ["1", "0", "0"]
+    NSArray<NSString*>* versionParts = [apiVersion componentsSeparatedByString:@"."];
+
+    // Check the major version number is 1, which is the only major version we support.
+    if (versionParts.count > 0) {
+        NSInteger majorVersion = versionParts[0].integerValue;
+
+        DebugMsg("BGMGooglePlayMusicDesktopPlayerConnection::handleAPIVersionMessage: "
+                 "Major version: %lu", majorVersion);
+
+        if (majorVersion == 1) {
+            // GPMDP uses SemVer, so as long as the major version number matches what we can handle,
+            // it should work.
+            DebugMsg("BGMGooglePlayMusicDesktopPlayerConnection::handleAPIVersionMessage: "
+                     "This API version is supported");
+            return;
+        }
+    }
+
+    // Show a warning dialog box to the user, but try to continue anyway. There's probably a
+    // reasonable chance it'll still work.
+    DebugMsg("BGMGooglePlayMusicDesktopPlayerConnection::handleAPIVersionMessage: "
+             "Unsupported GPMDP API version");
+    apiVersionMismatchHandler(apiVersion);
+}
+
+- (void) handleConnectMessage:(NSDictionary*)body {
+    // Don't log the response as it may contain the auth code.
+    DebugMsg("BGMGooglePlayMusicDesktopPlayerConnection::handleConnectMessage: Received response");
+
+    // Type check.
+    if (![body[@"payload"] isKindOfClass:[NSString class]]) {
+        NSLog(@"Unexpected payload type");
+        [self handleConnectionError];
+        return;
+    }
+
+    NSString* payload = body[@"payload"];
+
+    if ([@"CODE_REQUIRED" isEqual:payload]) {
+        // Ask the user for the auth code GPMDP is displaying and send it to GPMDP to finish
+        // connecting.
+        NSString* __nullable authCode = authRequiredHandler();
+
+        if (authCode) {
+            [self sendAuthCode:BGMNN(authCode)];
+        }
+    } else {
+        // The payload should be the permanent auth code.
+        permanentAuthCode = payload;
+        [self sendPermanentAuthCode];
+
+        // Save the code to the keychain so we can use it when connecting to GPMDP in future.
+        userDefaults.googlePlayMusicDesktopPlayerPermanentAuthCode = permanentAuthCode;
+    }
+}
+
+- (void) handlePlayStateMessage:(NSDictionary*)body {
+    (void)body;
+    DebugMsg("BGMGooglePlayMusicDesktopPlayerConnection::handlePlayStateMessage: Response: %s",
+             [NSString stringWithFormat:@"%@", body].UTF8String);
+
+    // This message tells us the playstate has changed, but doesn't differentiate between stopped
+    // and paused. The response to this API request will. See handleResultMessage.
+    // TODO: Can it transition from stopped to paused? Would that be a problem?
+    [self requestPlaybackState];
+}
+
+- (void) handleResultMessage:(NSDictionary*)body {
+    DebugMsg("BGMGooglePlayMusicDesktopPlayerConnection::handleResultMessage: Response: %s",
+             [NSString stringWithFormat:@"%@", body].UTF8String);
+
+    // Type check.
+    if (![body[@"value"] isKindOfClass:[NSNumber class]]) {
+        NSLog(@"No value");
+        return;
+    }
+
+    // 0 - Playback is stopped
+    // 1 - Track is paused
+    // 2 - Track is playing
+    int state = ((NSNumber*)body[@"value"]).intValue;
+    _playing = (state == 2);
+    _paused = (state == 1);
+}
+
+@end
+
+#pragma clang assume_nonnull end
+

BGMApp/BGMApp/Music Players/BGMHermes.m

@@ -54,8 +54,8 @@
     return (HermesApplication* __nullable)scriptingBridge.application;
 }
 
-- (void) onSelect {
-    [super onSelect];
+- (void) wasSelected {
+    [super wasSelected];
     [scriptingBridge ensurePermission];
 }
 

BGMApp/BGMApp/Music Players/BGMMusic.h

@@ -14,25 +14,20 @@
 // along with Background Music. If not, see <http://www.gnu.org/licenses/>.
 
 //
-//  BGMOutputDevicePrefs.h
+//  BGMMusic.h
 //  BGMApp
 //
-//  Copyright © 2016 Kyle Neideck
+//  Copyright © 2016, 2019 Kyle Neideck
+//  Copyright © 2019 theLMGN
 //
 
-// Local Includes
-#import "BGMAudioDeviceManager.h"
-
-// System Includes
-#import <AppKit/AppKit.h>
+// Superclass/Protocol Import
+#import "BGMMusicPlayer.h"
 
 
 #pragma clang assume_nonnull begin
 
-@interface BGMOutputDevicePrefs : NSObject
-
-- (id) initWithAudioDevices:(BGMAudioDeviceManager*)inAudioDevices;
-- (void) populatePreferencesMenu:(NSMenu*)prefsMenu;
+@interface BGMMusic : BGMMusicPlayerBase<BGMMusicPlayer>
 
 @end
 

BGMApp/BGMApp/Music Players/BGMMusic.m

@@ -0,0 +1,111 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMMusic.m
+//  BGMApp
+//
+//  Copyright © 2016-2019 Kyle Neideck, theLMGN
+//
+
+// Self Include
+#import "BGMMusic.h"
+
+// Auto-generated Scripting Bridge header
+#import "Music.h"
+
+// Local Includes
+#import "BGMScriptingBridge.h"
+
+// PublicUtility Includes
+#import "CADebugMacros.h"
+
+
+#pragma clang assume_nonnull begin
+
+@implementation BGMMusic {
+    BGMScriptingBridge* scriptingBridge;
+}
+
++ (NSUUID*) sharedMusicPlayerID {
+    NSUUID* __nullable musicPlayerID =
+            [[NSUUID alloc] initWithUUIDString:@"829B8069-8BD2-481D-BD40-54AB8CDAE228"];
+    NSAssert(musicPlayerID, @"BGMMusic::sharedMusicPlayerID: !musicPlayerID");
+    return (NSUUID*)musicPlayerID;
+}
+
+- (instancetype) init {
+    if ((self = [super initWithMusicPlayerID:[BGMMusic sharedMusicPlayerID]
+                                        name:@"Music"
+                                    bundleID:@"com.apple.Music"])) {
+        scriptingBridge = [[BGMScriptingBridge alloc] initWithMusicPlayer:self];
+    }
+    
+    return self;
+}
+
+- (MusicApplication* __nullable) music {
+    return (MusicApplication*)scriptingBridge.application;
+}
+
+- (void) wasSelected {
+    [super wasSelected];
+    [scriptingBridge ensurePermission];
+}
+
+- (BOOL) isRunning {
+    return self.music.running;
+}
+
+// isPlaying and isPaused check self.running first just in case Music is closed but self.music
+// hasn't become nil yet. In that case, reading self.music.playerState could make Scripting Bridge
+// open Music.
+
+- (BOOL) isPlaying {
+    return self.running && (self.music.playerState == MusicEPlSPlaying);
+}
+
+- (BOOL) isPaused {
+    return self.running && (self.music.playerState == MusicEPlSPaused);
+}
+
+- (BOOL) pause {
+    // isPlaying checks isRunning, so we don't need to check it here and waste an Apple event
+    BOOL wasPlaying = self.playing;
+    
+    if (wasPlaying) {
+        DebugMsg("BGMMusic::pause: Pausing Music");
+        [self.music pause];
+    }
+    
+    return wasPlaying;
+}
+
+- (BOOL) unpause {
+    // isPaused checks isRunning, so we don't need to check it here and waste an Apple event
+    BOOL wasPaused = self.paused;
+    
+    if (wasPaused) {
+        DebugMsg("BGMMusic::unpause: Unpausing Music");
+        [self.music playpause];
+    }
+    
+    return wasPaused;
+}
+
+@end
+
+#pragma clang assume_nonnull end
+

BGMApp/BGMApp/Music Players/BGMMusicPlayer.h

@@ -17,7 +17,7 @@
 //  BGMMusicPlayer.h
 //  BGMApp
 //
-//  Copyright © 2016, 2018 Kyle Neideck
+//  Copyright © 2016, 2018, 2019 Kyle Neideck
 //
 //  The base classes and protocol for objects that represent a music player app.
 //
@@ -41,6 +41,9 @@
 //  BGMDriver will log the bundle ID to system.log when it becomes aware of the music player.
 //
 
+// Local Includes
+#import "BGMUserDefaults.h"
+
 // System Includes
 #import <Cocoa/Cocoa.h>
 
@@ -50,27 +53,26 @@
 @protocol BGMMusicPlayer <NSObject>
 
 // Classes return an instance of themselves for each music player app they make available in
-// BGMApp. So far that's always been a single instance, and classes haven't needed to override
-// the default implementation of createInstances from BGMMusicPlayerBase. But that will probably
-// change eventually.
+// BGMApp. So far that's always been a single instance, but that will probably change eventually.
+// Most classes don't need to override the default implementation from BGMMusicPlayerBase.
 //
-// For example, a class for custom music players would probably return an instance for each
-// custom player the user has created. (Also note that it could return an empty array.) In that
-// case the class would probably restore some state from user defaults in its createInstances.
+// But, for example, a class for custom music players would probably return an instance for each
+// custom player the user has created. (Also note that it could return an empty array.)
 //
 // TODO: I think the return type should actually be NSArray<instancetype>*, but that doesn't seem
 //       to work. There's a Clang bug about this: https://llvm.org/bugs/show_bug.cgi?id=27323
 //       (though it hasn't been confirmed yet).
-+ (NSArray<id<BGMMusicPlayer>>*) createInstances;
++ (NSArray<id<BGMMusicPlayer>>*) createInstancesWithDefaults:(BGMUserDefaults*)userDefaults;
 
 // We need a unique ID for each music player to store in user defaults. In the most common case,
 // classes that provide a static (or at least bounded) number of music players, you can generate
 // IDs with uuidgen (the command line tool) and include them in your class as constants. Otherwise,
-// you'll probably want to store them in user defaults and retrieve them in your createInstances.
+// you'll probably want to store them in user defaults and load them in createInstancesWithDefaults.
 @property (readonly) NSUUID* musicPlayerID;
 
-// The name and icon of the music player, to be used in the UI.
+// The name, tool-tip and icon of the music player, to be used in the UI.
 @property (readonly) NSString* name;
+@property (readonly) NSString* __nullable toolTip;
 @property (readonly) NSImage* __nullable icon;
 
 @property (readonly) NSString* __nullable bundleID;
@@ -81,7 +83,7 @@
 // TODO: If we ever add a music player class that uses this property, it'll need a way to inform
 //       BGMDevice of changes. It might be easiest to have BGMMusicPlayers to observe this property,
 //       on the selected music player, with KVO and update BGMDevice when it changes. Or
-//       BGMMusicPlayers could pass a pointer to itself to createInstances.
+//       BGMMusicPlayers could pass a pointer to itself to createInstancesWithDefaults.
 @property NSNumber* __nullable pid;
 
 // True if this is currently the selected music player.
@@ -101,9 +103,9 @@
 @property (readonly, getter=isPaused) BOOL paused;
 
 // Called when the user selects this music player.
-- (void) onSelect;
-// Called when this is the selected music player and the user selects a different one.
-- (void) onDeselect;
+- (void) wasSelected;
+// Called when this was the selected music player and the user just selected a different one.
+- (void) wasDeselected;
 
 // Pause the music player. Does nothing if the music player is already paused or isn't running.
 // Returns YES if the music player is paused now but wasn't before, returns NO otherwise.
@@ -123,6 +125,12 @@
 
 - (instancetype) initWithMusicPlayerID:(NSUUID*)musicPlayerID
                                   name:(NSString*)name
+                               toolTip:(NSString*)toolTip
+                              bundleID:(NSString* __nullable)bundleID;
+
+- (instancetype) initWithMusicPlayerID:(NSUUID*)musicPlayerID
+                                  name:(NSString*)name
+                               toolTip:(NSString* __nullable)toolTip
                               bundleID:(NSString* __nullable)bundleID
                                    pid:(NSNumber* __nullable)pid;
 
@@ -131,15 +139,16 @@
 + (NSUUID*) makeID:(NSString*)musicPlayerIDString;
 
 // BGMMusicPlayer default implementations
-+ (NSArray<id<BGMMusicPlayer>>*) createInstances;
++ (NSArray<id<BGMMusicPlayer>>*) createInstancesWithDefaults:(BGMUserDefaults*)userDefaults;
 @property (readonly) NSImage* __nullable icon;
 @property (readonly) NSUUID* musicPlayerID;
 @property (readonly) NSString* name;
+@property (readonly) NSString* __nullable toolTip;
 @property (readonly) NSString* __nullable bundleID;
 @property NSNumber* __nullable pid;
 @property (readonly) BOOL selected;
-- (void) onSelect;
-- (void) onDeselect;
+- (void) wasSelected;
+- (void) wasDeselected;
 
 @end
 

BGMApp/BGMApp/Music Players/BGMMusicPlayer.m

@@ -17,7 +17,7 @@
 //  BGMMusicPlayer.m
 //  BGMApp
 //
-//  Copyright © 2016-2018 Kyle Neideck
+//  Copyright © 2016-2019 Kyle Neideck
 //
 
 // Self Include
@@ -33,6 +33,7 @@
 
 @synthesize musicPlayerID = _musicPlayerID;
 @synthesize name = _name;
+@synthesize toolTip = _toolTip;
 @synthesize bundleID = _bundleID;
 @synthesize pid = _pid;
 @synthesize selected = _selected;
@@ -40,11 +41,27 @@
 - (instancetype) initWithMusicPlayerID:(NSUUID*)musicPlayerID
                                   name:(NSString*)name
                               bundleID:(NSString* __nullable)bundleID {
-    return [self initWithMusicPlayerID:musicPlayerID name:name bundleID:bundleID pid:nil];
+    return [self initWithMusicPlayerID:musicPlayerID
+                                  name:name
+                               toolTip:nil
+                              bundleID:bundleID
+                                   pid:nil];
 }
 
 - (instancetype) initWithMusicPlayerID:(NSUUID*)musicPlayerID
                                   name:(NSString*)name
+                               toolTip:(NSString*)toolTip
+                              bundleID:(NSString* __nullable)bundleID {
+    return [self initWithMusicPlayerID:musicPlayerID
+                                  name:name
+                               toolTip:toolTip
+                              bundleID:bundleID
+                                   pid:nil];
+}
+
+- (instancetype) initWithMusicPlayerID:(NSUUID*)musicPlayerID
+                                  name:(NSString*)name
+                               toolTip:(NSString* __nullable)toolTip
                               bundleID:(NSString* __nullable)bundleID
                                    pid:(NSNumber* __nullable)pid {
     if ((self = [super init])) {
@@ -55,6 +72,7 @@
         
         _musicPlayerID = musicPlayerID;
         _name = name;
+        _toolTip = toolTip;
         _bundleID = bundleID;
         _pid = pid;
         _selected = NO;
@@ -72,8 +90,13 @@
 
 #pragma mark BGMMusicPlayer default implementations
 
-+ (NSArray<id<BGMMusicPlayer>>*) createInstances {
++ (NSArray<id<BGMMusicPlayer>>*) createInstancesWithDefaults:(BGMUserDefaults*)userDefaults {
+    #pragma unused (userDefaults)
+    // TODO: Fix this warning properly.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wobjc-literal-conversion"
     return @[ [self new] ];
+#pragma clang diagnostic pop
 }
 
 - (NSImage* __nullable) icon {
@@ -84,11 +107,11 @@
     return (!bundlePath ? nil : [[NSWorkspace sharedWorkspace] iconForFile:(NSString*)bundlePath]);
 }
 
-- (void) onSelect {
+- (void) wasSelected {
     _selected = YES;
 }
 
-- (void) onDeselect {
+- (void) wasDeselected {
     _selected = NO;
 }
 

BGMApp/BGMApp/Music Players/BGMMusicPlayers.h

@@ -17,7 +17,7 @@
 //  BGMMusicPlayers.h
 //  BGMApp
 //
-//  Copyright © 2016 Kyle Neideck
+//  Copyright © 2016, 2019 Kyle Neideck
 //
 //  Holds the music players (i.e. BGMMusicPlayer objects) available in BGMApp. Also keeps track of
 //  which music player is currently selected by the user.
@@ -43,8 +43,8 @@
 // defaultMusicPlayerID is the musicPlayerID (see BGMMusicPlayer.h) of the music player that should be
 // selected by default.
 //
-// The createInstances method of each class in musicPlayerClasses will be called, and the results stored
-// in the musicPlayers property.
+// The createInstancesWithDefaults method of each class in musicPlayerClasses will be called and
+// the results will be stored in the musicPlayers property.
 - (instancetype) initWithAudioDevices:(BGMAudioDeviceManager*)devices
                  defaultMusicPlayerID:(NSUUID*)defaultMusicPlayerID
                    musicPlayerClasses:(NSArray<Class<BGMMusicPlayer>>*)musicPlayerClasses

BGMApp/BGMApp/Music Players/BGMMusicPlayers.mm

@@ -17,7 +17,7 @@
 //  BGMMusicPlayers.mm
 //  BGMApp
 //
-//  Copyright © 2016-2018 Kyle Neideck
+//  Copyright © 2016-2019 Kyle Neideck
 //
 
 // Self include
@@ -34,6 +34,8 @@
 #import "BGMDecibel.h"
 #import "BGMHermes.h"
 #import "BGMSwinsian.h"
+#import "BGMMusic.h"
+#import "BGMGooglePlayMusicDesktopPlayer.h"
 
 
 #pragma clang assume_nonnull begin
@@ -47,16 +49,25 @@
 
 - (instancetype) initWithAudioDevices:(BGMAudioDeviceManager*)devices
                          userDefaults:(BGMUserDefaults*)defaults {
+    // The classes handling each music player we support. If you write a new music player class, add
+    // it to this array.
+    NSArray<Class<BGMMusicPlayer>>* mpClasses = @[ [BGMVOX class],
+                                                   [BGMVLC class],
+                                                   [BGMSpotify class],
+                                                   [BGMiTunes class],
+                                                   [BGMDecibel class],
+                                                   [BGMHermes class],
+                                                   [BGMSwinsian class],
+                                                   [BGMMusic class] ];
+
+    // We only support Google Play Music Desktop Player on macOS 10.10 and higher.
+    if (@available(macOS 10.10, *)) {
+        mpClasses = [mpClasses arrayByAddingObject:[BGMGooglePlayMusicDesktopPlayer class]];
+    }
+
     return [self initWithAudioDevices:devices
                  defaultMusicPlayerID:[BGMiTunes sharedMusicPlayerID]
-                   // If you write a new music player class, add it to this array.
-                   musicPlayerClasses:@[ [BGMVOX class],
-                                         [BGMVLC class],
-                                         [BGMSpotify class],
-                                         [BGMiTunes class],
-                                         [BGMDecibel class],
-                                         [BGMHermes class],
-                                         [BGMSwinsian class] ]
+                   musicPlayerClasses:mpClasses
                          userDefaults:defaults];
 }
 
@@ -70,11 +81,14 @@
         
         // Init _musicPlayers, an array containing one object for each music player in BGMApp.
         //
-        // Each music player class has a factory method, createInstances, that returns all the instances of that
-        // class BGMApp will use. (Though so far it's always just one instance.)
+        // Each music player class has a factory method, createInstancesWithDefaults, that returns
+        // all the instances of that class BGMApp will use. (Though so far it's always just one
+        // instance.)
         NSMutableArray* musicPlayers = [NSMutableArray new];
         for (Class<BGMMusicPlayer> musicPlayerClass in musicPlayerClasses) {
-            [musicPlayers addObjectsFromArray:[musicPlayerClass createInstances]];
+            NSArray<id<BGMMusicPlayer>>* instances =
+                    [musicPlayerClass createInstancesWithDefaults:userDefaults];
+            [musicPlayers addObjectsFromArray:instances];
         }
         
         _musicPlayers = [NSArray arrayWithArray:musicPlayers];
@@ -132,7 +146,7 @@
 - (void) initSelectedMusicPlayerFromBGMDevice {
     // When the selected music player setting hasn't been stored in user defaults yet, we get the music player
     // bundle ID from the driver and look for the music player with that bundle ID. This is mainly done for
-    // backwards compatability.
+    // backwards compatibility.
     
     NSString* __nullable bundleID =
         (__bridge_transfer NSString* __nullable)[audioDevices bgmDevice].GetMusicPlayerBundleID();
@@ -215,7 +229,7 @@
     }
 
     // Tell the current music player (object) a different player has been selected.
-    [_selectedMusicPlayer onDeselect];
+    [_selectedMusicPlayer wasDeselected];
     
     _selectedMusicPlayer = newSelectedMusicPlayer;
     
@@ -229,7 +243,7 @@
     userDefaults.selectedMusicPlayerID = _selectedMusicPlayer.musicPlayerID.UUIDString;
 
     // Tell the music player (object) it's been selected.
-    [_selectedMusicPlayer onSelect];
+    [_selectedMusicPlayer wasSelected];
 }
 
 - (void) updateBGMDeviceMusicPlayerProperties {

BGMApp/BGMApp/Music Players/BGMScriptingBridge.m

@@ -17,7 +17,7 @@
 //  BGMScriptingBridge.m
 //  BGMApp
 //
-//  Copyright © 2016-2018 Kyle Neideck
+//  Copyright © 2016-2019 Kyle Neideck
 //
 
 // Self Include
@@ -25,6 +25,7 @@
 
 // Local Includes
 #import "BGM_Utils.h"
+#import "BGMAppWatcher.h"
 
 // PublicUtility Includes
 #import "CADebugMacros.h"
@@ -34,8 +35,7 @@
 
 @implementation BGMScriptingBridge {
     id<BGMMusicPlayer> __weak _musicPlayer;
-    // Tokens for the notification observers. We need these to remove the observers in dealloc.
-    id _didLaunchToken, _didTerminateToken;
+    BGMAppWatcher* appWatcher;
 }
 
 @synthesize application = _application;
@@ -57,17 +57,14 @@
     BGMScriptingBridge* __weak weakSelf = self;
 
     void (^createSBApplication)(void) = ^{
-        BGMScriptingBridge* __strong strongSelf = weakSelf;
+        BGMScriptingBridge* strongSelf = weakSelf;
         strongSelf->_application = [SBApplication applicationWithBundleIdentifier:bundleID];
-        // TODO: I think the SBApplication will still keep a strong ref to this object, so we might
-        //       have to make a separate delegate object.
+        // TODO: The SBApplication will still keep a strong ref to this object, so we would have to
+        //       make a separate delegate object to avoid the retain cycle. Not currently a problem
+        //       because we only ever create instances that live forever.
         strongSelf->_application.delegate = strongSelf;
     };
     
-    BOOL (^isAboutThisMusicPlayer)(NSNotification*) = ^(NSNotification* note) {
-        return [[note.userInfo[NSWorkspaceApplicationKey] bundleIdentifier] isEqualToString:bundleID];
-    };
-    
     // Add observers that create/destroy the SBApplication when the music player is launched/terminated. We
     // only create the SBApplication when the music player is open. If it isn't open, creating the
     // SBApplication or sending it events could launch the music player. Whether or not it does depends on
@@ -76,31 +73,20 @@
     // From the docs for SBApplication's applicationWithBundleIdentifier method:
     //     "For applications that declare themselves to have a dynamic scripting interface, this method will
     //     launch the application if it is not already running."
-    NSNotificationCenter* center = [[NSWorkspace sharedWorkspace] notificationCenter];
-    _didLaunchToken = [center addObserverForName:NSWorkspaceDidLaunchApplicationNotification
-                                          object:nil
-                                           queue:nil
-                                      usingBlock:^(NSNotification* note)
-                       {
-                           if (isAboutThisMusicPlayer(note)) {
-                               DebugMsg("BGMScriptingBridge::initApplication: %s launched",
-                                        bundleID.UTF8String);
-                               createSBApplication();
-                               [weakSelf ensurePermission];
-                           }
-                       }];
-    _didTerminateToken = [center addObserverForName:NSWorkspaceDidTerminateApplicationNotification
-                                             object:nil
-                                              queue:nil
-                                         usingBlock:^(NSNotification* note)
-                          {
-                              if (isAboutThisMusicPlayer(note)) {
-                                  DebugMsg("BGMScriptingBridge::initApplication: %s terminated",
-                                           bundleID.UTF8String);
-                                  BGMScriptingBridge* __strong strongSelf = weakSelf;
-                                  strongSelf->_application = nil;
-                              }
-                          }];
+    appWatcher =
+        [[BGMAppWatcher alloc] initWithBundleID:bundleID
+                                    appLaunched:^{
+                                        DebugMsg("BGMScriptingBridge::initApplication: %s launched",
+                                                 bundleID.UTF8String);
+                                        createSBApplication();
+                                        [weakSelf ensurePermission];
+                                    }
+                                  appTerminated:^{
+                                      BGMScriptingBridge* strongSelf = weakSelf;
+                                      DebugMsg("BGMScriptingBridge::initApplication: %s terminated",
+                                               bundleID.UTF8String);
+                                      strongSelf->_application = nil;
+                                  }];
     
     // Create the SBApplication if the music player is already running.
     if ([NSRunningApplication runningApplicationsWithBundleIdentifier:bundleID].count > 0) {
@@ -108,19 +94,6 @@
     }
 }
 
-- (void) dealloc {
-    // Remove the application launch/termination observers.
-    NSNotificationCenter* center = [NSWorkspace sharedWorkspace].notificationCenter;
-    
-    if (_didLaunchToken) {
-        [center removeObserver:_didLaunchToken];
-    }
-    
-    if (_didTerminateToken) {
-        [center removeObserver:_didTerminateToken];
-    }
-}
-
 - (void) ensurePermission {
     // Skip this check if running on a version of macOS before 10.14. In that case, we don't require
     // user permission to send Apple Events. Also skip it if compiling on an earlier version.

BGMApp/BGMApp/Music Players/BGMSpotify.m

@@ -57,8 +57,8 @@
     return (SpotifyApplication* __nullable)scriptingBridge.application;
 }
 
-- (void) onSelect {
-    [super onSelect];
+- (void) wasSelected {
+    [super wasSelected];
     [scriptingBridge ensurePermission];
 }
 

BGMApp/BGMApp/Music Players/BGMSwinsian.m

@@ -57,8 +57,8 @@
     return (SwinsianApplication* __nullable)scriptingBridge.application;
 }
 
-- (void) onSelect {
-    [super onSelect];
+- (void) wasSelected {
+    [super wasSelected];
     [scriptingBridge ensurePermission];
 }
 

BGMApp/BGMApp/Music Players/BGMVLC.m

@@ -18,7 +18,7 @@
 //  BGMApp
 //
 //  Copyright © 2016-2018 Kyle Neideck
-//  Portions copyright (C) 2012 Peter Ljunglöf. All rights reserved.
+//  Copyright (C) 2012 Peter Ljunglöf. All rights reserved.
 //
 
 // Self Include
@@ -54,8 +54,8 @@
     return (VLCApplication*)scriptingBridge.application;
 }
 
-- (void) onSelect {
-    [super onSelect];
+- (void) wasSelected {
+    [super wasSelected];
     [scriptingBridge ensurePermission];
 }
 

BGMApp/BGMApp/Music Players/BGMVOX.m

@@ -53,8 +53,8 @@
     return (VoxApplication*)scriptingBridge.application;
 }
 
-- (void) onSelect {
-    [super onSelect];
+- (void) wasSelected {
+    [super wasSelected];
     [scriptingBridge ensurePermission];
 }
 

BGMApp/BGMApp/Music Players/BGMiTunes.m

@@ -59,8 +59,8 @@
     return (iTunesApplication*)scriptingBridge.application;
 }
 
-- (void) onSelect {
-    [super onSelect];
+- (void) wasSelected {
+    [super wasSelected];
     [scriptingBridge ensurePermission];
 }
 

BGMApp/BGMApp/Music Players/GooglePlayMusicDesktopPlayer.js

@@ -0,0 +1,190 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  GooglePlayMusicDesktopPlayer.js
+//  BGMApp
+//
+//  Copyright © 2019 Kyle Neideck
+//
+
+// The specification for GPMDP's API:
+// https://github.com/MarshallOfSound/Google-Play-Music-Desktop-Player-UNOFFICIAL-/blob/master/docs/PlaybackAPI_WebSocket.md
+
+try {
+
+window._log = msg => {
+    window.webkit.messageHandlers.log.postMessage(msg);
+};
+
+// Global JS error handler.
+window.onerror = (msg, url, line, col, error) => {
+    let extra = !col ? '' : '\nColumn: ' + col;
+    extra += !error ? '' : '\nError: ' + error;
+
+    // TODO: I'm not sure this log message is ever actually useful.
+    window._log('Error: ' + msg + '\nURL: ' + url + '\nLine: ' + line + extra);
+
+    window.webkit.messageHandlers.error.postMessage(error);
+};
+
+// Send a JSON message to GPMDP.
+//
+// If we're connecting, this function will return immediately and the message will be sent after we
+// finish connecting. Logs an error and returns if window.connect() hasn't been called yet.
+window._sendJSON = json => {
+    if (window._wsPromise) {
+        window._wsPromise.then(() => {
+            window._sendJSONImmediate(json);
+        }).catch(error => {
+            // TODO: Is there anything else we can do? Retries?
+            window._log('Error sending JSON: ' + JSON.stringify(error));
+        });
+    } else {
+        window._log('Error: No WebSocket promise. Discarding JSON message: ' +
+                    JSON.stringify(json));
+    }
+};
+
+// Send a JSON message to GPMDP, but don't wait if we're in the process of connecting.
+//
+// Logs an error and returns if window.connect() hasn't been called yet. The authCode param is
+// optional and only used to hide the code in log messages.
+window._sendJSONImmediate = (json, authCode) => {
+    let jsonStr = JSON.stringify(json);
+    let jsonStrSanitized = authCode ? jsonStr.replace(authCode, "<private>") : jsonStr;
+
+    if (window._ws) {
+        window._log('Sending JSON: ' + jsonStrSanitized);
+        window._ws.send(jsonStr);
+    } else {
+        window._log('Error: No WebSocket. Discarding JSON message: ' + jsonStrSanitized);
+    }
+};
+
+// permanentAuthCode is optional. If this is the first time they've selected GPMDP, we won't have a
+// permanent code yet.
+window.connect = permanentAuthCode => {
+    // Reset the connection state.
+    window._requestID = 1;
+
+    // Close the existing connection if we're already connected.
+    window.disconnect();
+
+    // Create the new connection.
+    window._ws = new WebSocket('ws://localhost:5672');
+
+    window._ws.onmessage = event => {
+        // Pass the message along to BGMGooglePlayMusicDesktopPlayerConnection.
+        let reply = JSON.parse(event.data);
+        window.webkit.messageHandlers.gpmdp.postMessage(reply);
+    };
+
+    window._wsPromise = new Promise((resolve, reject) => {
+        window._ws.onopen = () => {
+            // Send GPMDP the initial connection message.
+            if (permanentAuthCode) {
+                window._log('Connecting with auth code');
+                window.sendPermanentAuthCode(permanentAuthCode);
+            } else {
+                // Since we don't have an auth code, it will display a four-digit code and reply
+                // telling us to ask the user to type it into Background Music.
+                window._log('Connecting without auth code');
+
+                window._sendJSONImmediate({
+                    'namespace': 'connect',
+                    'method': 'connect',
+                    'arguments': ['Background Music']
+                });
+            }
+        };
+
+        window._ws.onerror = error => {
+            // Report the error to BGMGooglePlayMusicDesktopPlayerConnection.
+            window.webkit.messageHandlers.error.postMessage(error);
+            // Reject the connection promise.
+            reject(error);
+        };
+
+        // Store the function that resolves this promise. We resolve it after we finish
+        // authenticating.
+        window._resolveConnectionPromise = resolve;
+    });
+};
+
+// Close the connection to GPMDP. Does nothing if we aren't connected.
+window.disconnect = () => {
+    if (window._ws) {
+        window._log('Closing WebSocket');
+        window._ws.close();
+        window._ws = null;
+    }
+};
+
+// Send an authentication code to GPMDP. To send a four-digit code (i.e. one entered by the user),
+// call this directly. To send a permanent code received from GPMDP, use
+// window.sendPermanentAuthCode().
+//
+// authCode should be percent-encoded.
+window.sendAuthCode = authCode => {
+    // Percent-decode the auth code string. We pass it percent-encoded just to make sure nothing in
+    // it accidentally gets executed as Javascript.
+    authCode = window.decodeURIComponent(authCode);
+
+    window._sendJSONImmediate({
+        'namespace': 'connect',
+        'method': 'connect',
+        'arguments': ['Background Music', authCode]
+    }, authCode);
+};
+
+// Send a permanent authentication code, received from GPMDP previously, to GPMDP.
+window.sendPermanentAuthCode = permanentAuthCode => {
+    window._log('Sending permanent auth code');
+    window.sendAuthCode(permanentAuthCode);
+    // TODO: If the code is rejected, GPMDP will send us a connect message and we'll show the auth
+    //       code dialog, but accepting the promise here means some messages we send might get
+    //       ignored.
+    window._resolveConnectionPromise();
+};
+
+// Ask GPMDP to send us its current playback state (playing, paused or stopped).
+window.requestPlaybackState = () => {
+    window._sendJSON({
+        'namespace': 'playback',
+        'method': 'getPlaybackState',
+        // We don't send any other types of request, so the ID we send only needs to be unique.
+        'requestID': window._requestID++
+    });
+};
+
+// Tell GPMDP to toggle between playing and paused.
+window.playPause = () => {
+    window._sendJSON({
+        'namespace': 'playback',
+        'method': 'playPause'
+    });
+};
+
+} catch (error) {
+    window.webkit.messageHandlers.log.postMessage('Error: ' + JSON.stringify(error));
+    window.webkit.messageHandlers.log.postMessage(JSON.stringify(error.stack));
+    window.webkit.messageHandlers.error.postMessage(error);
+}
+
+// Return an empty string as returning some types can cause an error when this Javascript is loaded
+// into the WKWebView.
+""
+

BGMApp/BGMApp/Music Players/Music.h

@@ -0,0 +1,545 @@
+/*
+ * Music.h
+ *
+ * Generated with
+ * sdef /System/Applications/Music.app | sdp -fh --basename Music
+ */
+
+#import <AppKit/AppKit.h>
+#import <ScriptingBridge/ScriptingBridge.h>
+
+
+@class MusicApplication, MusicItem, MusicAirPlayDevice, MusicArtwork, MusicEncoder, MusicEQPreset, MusicPlaylist, MusicAudioCDPlaylist, MusicLibraryPlaylist, MusicRadioTunerPlaylist, MusicSource, MusicSubscriptionPlaylist, MusicTrack, MusicAudioCDTrack, MusicFileTrack, MusicSharedTrack, MusicURLTrack, MusicUserPlaylist, MusicFolderPlaylist, MusicVisual, MusicWindow, MusicBrowserWindow, MusicEQWindow, MusicMiniplayerWindow, MusicPlaylistWindow, MusicVideoWindow;
+
+enum MusicEKnd {
+	MusicEKndTrackListing = 'kTrk' /* a basic listing of tracks within a playlist */,
+	MusicEKndAlbumListing = 'kAlb' /* a listing of a playlist grouped by album */,
+	MusicEKndCdInsert = 'kCDi' /* a printout of the playlist for jewel case inserts */
+};
+typedef enum MusicEKnd MusicEKnd;
+
+enum MusicEnum {
+	MusicEnumStandard = 'lwst' /* Standard PostScript error handling */,
+	MusicEnumDetailed = 'lwdt' /* print a detailed report of PostScript errors */
+};
+typedef enum MusicEnum MusicEnum;
+
+enum MusicEPlS {
+	MusicEPlSStopped = 'kPSS',
+	MusicEPlSPlaying = 'kPSP',
+	MusicEPlSPaused = 'kPSp',
+	MusicEPlSFastForwarding = 'kPSF',
+	MusicEPlSRewinding = 'kPSR'
+};
+typedef enum MusicEPlS MusicEPlS;
+
+enum MusicERpt {
+	MusicERptOff = 'kRpO',
+	MusicERptOne = 'kRp1',
+	MusicERptAll = 'kAll'
+};
+typedef enum MusicERpt MusicERpt;
+
+enum MusicEShM {
+	MusicEShMSongs = 'kShS',
+	MusicEShMAlbums = 'kShA',
+	MusicEShMGroupings = 'kShG'
+};
+typedef enum MusicEShM MusicEShM;
+
+enum MusicESrc {
+	MusicESrcLibrary = 'kLib',
+	MusicESrcIPod = 'kPod',
+	MusicESrcAudioCD = 'kACD',
+	MusicESrcMP3CD = 'kMCD',
+	MusicESrcRadioTuner = 'kTun',
+	MusicESrcSharedLibrary = 'kShd',
+	MusicESrcITunesStore = 'kITS',
+	MusicESrcUnknown = 'kUnk'
+};
+typedef enum MusicESrc MusicESrc;
+
+enum MusicESrA {
+	MusicESrAAlbums = 'kSrL' /* albums only */,
+	MusicESrAAll = 'kAll' /* all text fields */,
+	MusicESrAArtists = 'kSrR' /* artists only */,
+	MusicESrAComposers = 'kSrC' /* composers only */,
+	MusicESrADisplayed = 'kSrV' /* visible text fields */,
+	MusicESrASongs = 'kSrS' /* song names only */
+};
+typedef enum MusicESrA MusicESrA;
+
+enum MusicESpK {
+	MusicESpKNone = 'kNon',
+	MusicESpKFolder = 'kSpF',
+	MusicESpKGenius = 'kSpG',
+	MusicESpKLibrary = 'kSpL',
+	MusicESpKMusic = 'kSpZ',
+	MusicESpKPurchasedMusic = 'kSpM'
+};
+typedef enum MusicESpK MusicESpK;
+
+enum MusicEMdK {
+	MusicEMdKSong = 'kMdS' /* music track */,
+	MusicEMdKMusicVideo = 'kVdV' /* music video track */,
+	MusicEMdKUnknown = 'kUnk'
+};
+typedef enum MusicEMdK MusicEMdK;
+
+enum MusicERtK {
+	MusicERtKUser = 'kRtU' /* user-specified rating */,
+	MusicERtKComputed = 'kRtC' /* iTunes-computed rating */
+};
+typedef enum MusicERtK MusicERtK;
+
+enum MusicEAPD {
+	MusicEAPDComputer = 'kAPC',
+	MusicEAPDAirPortExpress = 'kAPX',
+	MusicEAPDAppleTV = 'kAPT',
+	MusicEAPDAirPlayDevice = 'kAPO',
+	MusicEAPDBluetoothDevice = 'kAPB',
+	MusicEAPDHomePod = 'kAPH',
+	MusicEAPDUnknown = 'kAPU'
+};
+typedef enum MusicEAPD MusicEAPD;
+
+enum MusicEClS {
+	MusicEClSUnknown = 'kUnk',
+	MusicEClSPurchased = 'kPur',
+	MusicEClSMatched = 'kMat',
+	MusicEClSUploaded = 'kUpl',
+	MusicEClSIneligible = 'kRej',
+	MusicEClSRemoved = 'kRem',
+	MusicEClSError = 'kErr',
+	MusicEClSDuplicate = 'kDup',
+	MusicEClSSubscription = 'kSub',
+	MusicEClSNoLongerAvailable = 'kRev',
+	MusicEClSNotUploaded = 'kUpP'
+};
+typedef enum MusicEClS MusicEClS;
+
+@protocol MusicGenericMethods
+
+- (void) printPrintDialog:(BOOL)printDialog withProperties:(NSDictionary *)withProperties kind:(MusicEKnd)kind theme:(NSString *)theme;  // Print the specified object(s)
+- (void) close;  // Close an object
+- (void) delete;  // Delete an element from an object
+- (SBObject *) duplicateTo:(SBObject *)to;  // Duplicate one or more object(s)
+- (BOOL) exists;  // Verify if an object exists
+- (void) open;  // Open the specified object(s)
+- (void) save;  // Save the specified object(s)
+- (void) playOnce:(BOOL)once;  // play the current track or the specified track or file.
+- (void) select;  // select the specified object(s)
+
+@end
+
+
+
+/*
+ * iTunes Suite
+ */
+
+// The application program
+@interface MusicApplication : SBApplication
+
+- (SBElementArray<MusicAirPlayDevice *> *) AirPlayDevices;
+- (SBElementArray<MusicBrowserWindow *> *) browserWindows;
+- (SBElementArray<MusicEncoder *> *) encoders;
+- (SBElementArray<MusicEQPreset *> *) EQPresets;
+- (SBElementArray<MusicEQWindow *> *) EQWindows;
+- (SBElementArray<MusicMiniplayerWindow *> *) miniplayerWindows;
+- (SBElementArray<MusicPlaylist *> *) playlists;
+- (SBElementArray<MusicPlaylistWindow *> *) playlistWindows;
+- (SBElementArray<MusicSource *> *) sources;
+- (SBElementArray<MusicTrack *> *) tracks;
+- (SBElementArray<MusicVideoWindow *> *) videoWindows;
+- (SBElementArray<MusicVisual *> *) visuals;
+- (SBElementArray<MusicWindow *> *) windows;
+
+@property (readonly) BOOL AirPlayEnabled;  // is AirPlay currently enabled?
+@property (readonly) BOOL converting;  // is a track currently being converted?
+@property (copy) NSArray<MusicAirPlayDevice *> *currentAirPlayDevices;  // the currently selected AirPlay device(s)
+@property (copy) MusicEncoder *currentEncoder;  // the currently selected encoder (MP3, AIFF, WAV, etc.)
+@property (copy) MusicEQPreset *currentEQPreset;  // the currently selected equalizer preset
+@property (copy, readonly) MusicPlaylist *currentPlaylist;  // the playlist containing the currently targeted track
+@property (copy, readonly) NSString *currentStreamTitle;  // the name of the current song in the playing stream (provided by streaming server)
+@property (copy, readonly) NSString *currentStreamURL;  // the URL of the playing stream or streaming web site (provided by streaming server)
+@property (copy, readonly) MusicTrack *currentTrack;  // the current targeted track
+@property (copy) MusicVisual *currentVisual;  // the currently selected visual plug-in
+@property BOOL EQEnabled;  // is the equalizer enabled?
+@property BOOL fixedIndexing;  // true if all AppleScript track indices should be independent of the play order of the owning playlist.
+@property BOOL frontmost;  // is iTunes the frontmost application?
+@property BOOL fullScreen;  // are visuals displayed using the entire screen?
+@property (copy, readonly) NSString *name;  // the name of the application
+@property BOOL mute;  // has the sound output been muted?
+@property double playerPosition;  // the player’s position within the currently playing track in seconds.
+@property (readonly) MusicEPlS playerState;  // is iTunes stopped, paused, or playing?
+@property (copy, readonly) SBObject *selection;  // the selection visible to the user
+@property BOOL shuffleEnabled;  // are songs played in random order?
+@property MusicEShM shuffleMode;  // the playback shuffle mode
+@property MusicERpt songRepeat;  // the playback repeat mode
+@property NSInteger soundVolume;  // the sound output volume (0 = minimum, 100 = maximum)
+@property (copy, readonly) NSString *version;  // the version of iTunes
+@property BOOL visualsEnabled;  // are visuals currently being displayed?
+
+- (void) printPrintDialog:(BOOL)printDialog withProperties:(NSDictionary *)withProperties kind:(MusicEKnd)kind theme:(NSString *)theme;  // Print the specified object(s)
+- (void) run;  // Run iTunes
+- (void) quit;  // Quit iTunes
+- (MusicTrack *) add:(NSArray<NSURL *> *)x to:(SBObject *)to;  // add one or more files to a playlist
+- (void) backTrack;  // reposition to beginning of current track or go to previous track if already at start of current track
+- (MusicTrack *) convert:(NSArray<SBObject *> *)x;  // convert one or more files or tracks
+- (void) fastForward;  // skip forward in a playing track
+- (void) nextTrack;  // advance to the next track in the current playlist
+- (void) pause;  // pause playback
+- (void) playOnce:(BOOL)once;  // play the current track or the specified track or file.
+- (void) playpause;  // toggle the playing/paused state of the current track
+- (void) previousTrack;  // return to the previous track in the current playlist
+- (void) resume;  // disable fast forward/rewind and resume playback, if playing.
+- (void) rewind;  // skip backwards in a playing track
+- (void) stop;  // stop playback
+- (void) openLocation:(NSString *)x;  // Opens a Music Store or audio stream URL
+
+@end
+
+// an item
+@interface MusicItem : SBObject <MusicGenericMethods>
+
+@property (copy, readonly) SBObject *container;  // the container of the item
+- (NSInteger) id;  // the id of the item
+@property (readonly) NSInteger index;  // The index of the item in internal application order.
+@property (copy) NSString *name;  // the name of the item
+@property (copy, readonly) NSString *persistentID;  // the id of the item as a hexadecimal string. This id does not change over time.
+@property (copy) NSDictionary *properties;  // every property of the item
+
+- (void) download;  // download a cloud track or playlist
+- (void) reveal;  // reveal and select a track or playlist
+
+@end
+
+// an AirPlay device
+@interface MusicAirPlayDevice : MusicItem
+
+@property (readonly) BOOL active;  // is the device currently being played to?
+@property (readonly) BOOL available;  // is the device currently available?
+@property (readonly) MusicEAPD kind;  // the kind of the device
+@property (copy, readonly) NSString *networkAddress;  // the network (MAC) address of the device
+- (BOOL) protected;  // is the device password- or passcode-protected?
+@property BOOL selected;  // is the device currently selected?
+@property (readonly) BOOL supportsAudio;  // does the device support audio playback?
+@property (readonly) BOOL supportsVideo;  // does the device support video playback?
+@property NSInteger soundVolume;  // the output volume for the device (0 = minimum, 100 = maximum)
+
+
+@end
+
+// a piece of art within a track or playlist
+@interface MusicArtwork : MusicItem
+
+@property (copy) NSImage *data;  // data for this artwork, in the form of a picture
+@property (copy) NSString *objectDescription;  // description of artwork as a string
+@property (readonly) BOOL downloaded;  // was this artwork downloaded by iTunes?
+@property (copy, readonly) NSNumber *format;  // the data format for this piece of artwork
+@property NSInteger kind;  // kind or purpose of this piece of artwork
+@property (copy) NSData *rawData;  // data for this artwork, in original format
+
+
+@end
+
+// converts a track to a specific file format
+@interface MusicEncoder : MusicItem
+
+@property (copy, readonly) NSString *format;  // the data format created by the encoder
+
+
+@end
+
+// equalizer preset configuration
+@interface MusicEQPreset : MusicItem
+
+@property double band1;  // the equalizer 32 Hz band level (-12.0 dB to +12.0 dB)
+@property double band2;  // the equalizer 64 Hz band level (-12.0 dB to +12.0 dB)
+@property double band3;  // the equalizer 125 Hz band level (-12.0 dB to +12.0 dB)
+@property double band4;  // the equalizer 250 Hz band level (-12.0 dB to +12.0 dB)
+@property double band5;  // the equalizer 500 Hz band level (-12.0 dB to +12.0 dB)
+@property double band6;  // the equalizer 1 kHz band level (-12.0 dB to +12.0 dB)
+@property double band7;  // the equalizer 2 kHz band level (-12.0 dB to +12.0 dB)
+@property double band8;  // the equalizer 4 kHz band level (-12.0 dB to +12.0 dB)
+@property double band9;  // the equalizer 8 kHz band level (-12.0 dB to +12.0 dB)
+@property double band10;  // the equalizer 16 kHz band level (-12.0 dB to +12.0 dB)
+@property (readonly) BOOL modifiable;  // can this preset be modified?
+@property double preamp;  // the equalizer preamp level (-12.0 dB to +12.0 dB)
+@property BOOL updateTracks;  // should tracks which refer to this preset be updated when the preset is renamed or deleted?
+
+
+@end
+
+// a list of songs/streams
+@interface MusicPlaylist : MusicItem
+
+- (SBElementArray<MusicTrack *> *) tracks;
+- (SBElementArray<MusicArtwork *> *) artworks;
+
+@property (copy) NSString *objectDescription;  // the description of the playlist
+@property BOOL disliked;  // is this playlist disliked?
+@property (readonly) NSInteger duration;  // the total length of all songs (in seconds)
+@property (copy) NSString *name;  // the name of the playlist
+@property BOOL loved;  // is this playlist loved?
+@property (copy, readonly) MusicPlaylist *parent;  // folder which contains this playlist (if any)
+@property (readonly) NSInteger size;  // the total size of all songs (in bytes)
+@property (readonly) MusicESpK specialKind;  // special playlist kind
+@property (copy, readonly) NSString *time;  // the length of all songs in MM:SS format
+@property (readonly) BOOL visible;  // is this playlist visible in the Source list?
+
+- (void) moveTo:(SBObject *)to;  // Move playlist(s) to a new location
+- (MusicTrack *) searchFor:(NSString *)for_ only:(MusicESrA)only;  // search a playlist for tracks matching the search string. Identical to entering search text in the Search field in iTunes.
+
+@end
+
+// a playlist representing an audio CD
+@interface MusicAudioCDPlaylist : MusicPlaylist
+
+- (SBElementArray<MusicAudioCDTrack *> *) audioCDTracks;
+
+@property (copy) NSString *artist;  // the artist of the CD
+@property BOOL compilation;  // is this CD a compilation album?
+@property (copy) NSString *composer;  // the composer of the CD
+@property NSInteger discCount;  // the total number of discs in this CD’s album
+@property NSInteger discNumber;  // the index of this CD disc in the source album
+@property (copy) NSString *genre;  // the genre of the CD
+@property NSInteger year;  // the year the album was recorded/released
+
+
+@end
+
+// the master music library playlist
+@interface MusicLibraryPlaylist : MusicPlaylist
+
+- (SBElementArray<MusicFileTrack *> *) fileTracks;
+- (SBElementArray<MusicURLTrack *> *) URLTracks;
+- (SBElementArray<MusicSharedTrack *> *) sharedTracks;
+
+
+@end
+
+// the radio tuner playlist
+@interface MusicRadioTunerPlaylist : MusicPlaylist
+
+- (SBElementArray<MusicURLTrack *> *) URLTracks;
+
+
+@end
+
+// a music source (music library, CD, device, etc.)
+@interface MusicSource : MusicItem
+
+- (SBElementArray<MusicAudioCDPlaylist *> *) audioCDPlaylists;
+- (SBElementArray<MusicLibraryPlaylist *> *) libraryPlaylists;
+- (SBElementArray<MusicPlaylist *> *) playlists;
+- (SBElementArray<MusicRadioTunerPlaylist *> *) radioTunerPlaylists;
+- (SBElementArray<MusicSubscriptionPlaylist *> *) subscriptionPlaylists;
+- (SBElementArray<MusicUserPlaylist *> *) userPlaylists;
+
+@property (readonly) long long capacity;  // the total size of the source if it has a fixed size
+@property (readonly) long long freeSpace;  // the free space on the source if it has a fixed size
+@property (readonly) MusicESrc kind;
+
+
+@end
+
+// a subscription playlist from Apple Music
+@interface MusicSubscriptionPlaylist : MusicPlaylist
+
+- (SBElementArray<MusicFileTrack *> *) fileTracks;
+- (SBElementArray<MusicURLTrack *> *) URLTracks;
+
+
+@end
+
+// playable audio source
+@interface MusicTrack : MusicItem
+
+- (SBElementArray<MusicArtwork *> *) artworks;
+
+@property (copy) NSString *album;  // the album name of the track
+@property (copy) NSString *albumArtist;  // the album artist of the track
+@property BOOL albumDisliked;  // is the album for this track disliked?
+@property BOOL albumLoved;  // is the album for this track loved?
+@property NSInteger albumRating;  // the rating of the album for this track (0 to 100)
+@property (readonly) MusicERtK albumRatingKind;  // the rating kind of the album rating for this track
+@property (copy) NSString *artist;  // the artist/source of the track
+@property (readonly) NSInteger bitRate;  // the bit rate of the track (in kbps)
+@property double bookmark;  // the bookmark time of the track in seconds
+@property BOOL bookmarkable;  // is the playback position for this track remembered?
+@property NSInteger bpm;  // the tempo of this track in beats per minute
+@property (copy) NSString *category;  // the category of the track
+@property (readonly) MusicEClS cloudStatus;  // the iCloud status of the track
+@property (copy) NSString *comment;  // freeform notes about the track
+@property BOOL compilation;  // is this track from a compilation album?
+@property (copy) NSString *composer;  // the composer of the track
+@property (readonly) NSInteger databaseID;  // the common, unique ID for this track. If two tracks in different playlists have the same database ID, they are sharing the same data.
+@property (copy, readonly) NSDate *dateAdded;  // the date the track was added to the playlist
+@property (copy) NSString *objectDescription;  // the description of the track
+@property NSInteger discCount;  // the total number of discs in the source album
+@property NSInteger discNumber;  // the index of the disc containing this track on the source album
+@property BOOL disliked;  // is this track disliked?
+@property (copy, readonly) NSString *downloaderAppleID;  // the Apple ID of the person who downloaded this track
+@property (copy, readonly) NSString *downloaderName;  // the name of the person who downloaded this track
+@property (readonly) double duration;  // the length of the track in seconds
+@property BOOL enabled;  // is this track checked for playback?
+@property (copy) NSString *episodeID;  // the episode ID of the track
+@property NSInteger episodeNumber;  // the episode number of the track
+@property (copy) NSString *EQ;  // the name of the EQ preset of the track
+@property double finish;  // the stop time of the track in seconds
+@property BOOL gapless;  // is this track from a gapless album?
+@property (copy) NSString *genre;  // the music/audio genre (category) of the track
+@property (copy) NSString *grouping;  // the grouping (piece) of the track. Generally used to denote movements within a classical work.
+@property (copy, readonly) NSString *kind;  // a text description of the track
+@property (copy) NSString *longDescription;
+@property BOOL loved;  // is this track loved?
+@property (copy) NSString *lyrics;  // the lyrics of the track
+@property MusicEMdK mediaKind;  // the media kind of the track
+@property (copy, readonly) NSDate *modificationDate;  // the modification date of the content of this track
+@property (copy) NSString *movement;  // the movement name of the track
+@property NSInteger movementCount;  // the total number of movements in the work
+@property NSInteger movementNumber;  // the index of the movement in the work
+@property NSInteger playedCount;  // number of times this track has been played
+@property (copy) NSDate *playedDate;  // the date and time this track was last played
+@property (copy, readonly) NSString *purchaserAppleID;  // the Apple ID of the person who purchased this track
+@property (copy, readonly) NSString *purchaserName;  // the name of the person who purchased this track
+@property NSInteger rating;  // the rating of this track (0 to 100)
+@property (readonly) MusicERtK ratingKind;  // the rating kind of this track
+@property (copy, readonly) NSDate *releaseDate;  // the release date of this track
+@property (readonly) NSInteger sampleRate;  // the sample rate of the track (in Hz)
+@property NSInteger seasonNumber;  // the season number of the track
+@property BOOL shufflable;  // is this track included when shuffling?
+@property NSInteger skippedCount;  // number of times this track has been skipped
+@property (copy) NSDate *skippedDate;  // the date and time this track was last skipped
+@property (copy) NSString *show;  // the show name of the track
+@property (copy) NSString *sortAlbum;  // override string to use for the track when sorting by album
+@property (copy) NSString *sortArtist;  // override string to use for the track when sorting by artist
+@property (copy) NSString *sortAlbumArtist;  // override string to use for the track when sorting by album artist
+@property (copy) NSString *sortName;  // override string to use for the track when sorting by name
+@property (copy) NSString *sortComposer;  // override string to use for the track when sorting by composer
+@property (copy) NSString *sortShow;  // override string to use for the track when sorting by show name
+@property (readonly) long long size;  // the size of the track (in bytes)
+@property double start;  // the start time of the track in seconds
+@property (copy, readonly) NSString *time;  // the length of the track in MM:SS format
+@property NSInteger trackCount;  // the total number of tracks on the source album
+@property NSInteger trackNumber;  // the index of the track on the source album
+@property BOOL unplayed;  // is this track unplayed?
+@property NSInteger volumeAdjustment;  // relative volume adjustment of the track (-100% to 100%)
+@property (copy) NSString *work;  // the work name of the track
+@property NSInteger year;  // the year the track was recorded/released
+
+
+@end
+
+// a track on an audio CD
+@interface MusicAudioCDTrack : MusicTrack
+
+@property (copy, readonly) NSURL *location;  // the location of the file represented by this track
+
+
+@end
+
+// a track representing an audio file (MP3, AIFF, etc.)
+@interface MusicFileTrack : MusicTrack
+
+@property (copy) NSURL *location;  // the location of the file represented by this track
+
+- (void) refresh;  // update file track information from the current information in the track’s file
+
+@end
+
+// a track residing in a shared library
+@interface MusicSharedTrack : MusicTrack
+
+
+@end
+
+// a track representing a network stream
+@interface MusicURLTrack : MusicTrack
+
+@property (copy) NSString *address;  // the URL for this track
+
+
+@end
+
+// custom playlists created by the user
+@interface MusicUserPlaylist : MusicPlaylist
+
+- (SBElementArray<MusicFileTrack *> *) fileTracks;
+- (SBElementArray<MusicURLTrack *> *) URLTracks;
+- (SBElementArray<MusicSharedTrack *> *) sharedTracks;
+
+@property BOOL shared;  // is this playlist shared?
+@property (readonly) BOOL smart;  // is this a Smart Playlist?
+@property (readonly) BOOL genius;  // is this a Genius Playlist?
+
+
+@end
+
+// a folder that contains other playlists
+@interface MusicFolderPlaylist : MusicUserPlaylist
+
+
+@end
+
+// a visual plug-in
+@interface MusicVisual : MusicItem
+
+
+@end
+
+// any window
+@interface MusicWindow : MusicItem
+
+@property NSRect bounds;  // the boundary rectangle for the window
+@property (readonly) BOOL closeable;  // does the window have a close button?
+@property (readonly) BOOL collapseable;  // does the window have a collapse button?
+@property BOOL collapsed;  // is the window collapsed?
+@property BOOL fullScreen;  // is the window full screen?
+@property NSPoint position;  // the upper left position of the window
+@property (readonly) BOOL resizable;  // is the window resizable?
+@property BOOL visible;  // is the window visible?
+@property (readonly) BOOL zoomable;  // is the window zoomable?
+@property BOOL zoomed;  // is the window zoomed?
+
+
+@end
+
+// the main iTunes window
+@interface MusicBrowserWindow : MusicWindow
+
+@property (copy, readonly) SBObject *selection;  // the selected songs
+@property (copy) MusicPlaylist *view;  // the playlist currently displayed in the window
+
+
+@end
+
+// the iTunes equalizer window
+@interface MusicEQWindow : MusicWindow
+
+
+@end
+
+// the miniplayer window
+@interface MusicMiniplayerWindow : MusicWindow
+
+
+@end
+
+// a sub-window showing a single playlist
+@interface MusicPlaylistWindow : MusicWindow
+
+@property (copy, readonly) SBObject *selection;  // the selected songs
+@property (copy, readonly) MusicPlaylist *view;  // the playlist displayed in the window
+
+
+@end
+
+// the video window
+@interface MusicVideoWindow : MusicWindow
+
+
+@end
+

BGMApp/BGMApp/Music Players/VLC.h

@@ -221,7 +221,7 @@ typedef enum VLCEnum VLCEnum;
 @property NSInteger audioVolume;  // The volume of the current playlist item from 0 to 4, where 4 is 400%
 @property NSInteger currentTime;  // The current time of the current playlist item in seconds.
 @property (readonly) NSInteger durationOfCurrentItem;  // The duration of the current playlist item in seconds.
-@property BOOL fullscreenMode;  // indicates wheter fullscreen is enabled or not
+@property BOOL fullscreenMode;  // indicates whether fullscreen is enabled or not
 @property (readonly) BOOL muted;  // Is VLC currently muted?
 @property (copy, readonly) NSString *nameOfCurrentItem;  // Name of the current playlist item.
 @property (copy, readonly) NSString *pathOfCurrentItem;  // Path to the current playlist item.

BGMApp/BGMApp/Music Players/VOX.h

@@ -37,7 +37,7 @@
 - (void) rewindForwardFast;  // Rewind current track forward fast.
 - (void) rewindBackward;  // Rewind current track backward.
 - (void) rewindBackwardFast;  // Rewind current track backward fast.
-- (void) increasVolume;  // Increas volume.
+- (void) increasVolume;  // Increase volume.
 - (void) decreaseVolume;  // Decrease volume.
 - (void) showHidePlaylist;  // Show/Hide playlist.
 
@@ -62,7 +62,7 @@
 @property (copy, readonly) NSString *album;  // Current track album.
 @property (copy, readonly) NSString *uniqueID;  // Unique identifier for the current track.
 @property double currentTime;  // The current playback position.
-@property (readonly) double totalTime;  // The total time of the currenty playing track.
+@property (readonly) double totalTime;  // The total time of the currently playing track.
 @property double playerVolume;  // Player volume (0.0 to 1.0)
 @property NSInteger repeatState;  // Player repeat state (none = 0, repeat one = 1, repeat all = 2)
 

BGMApp/BGMApp/Preferences/BGMAboutPanel.m

@@ -17,7 +17,7 @@
 //  BGMAboutPanel.m
 //  BGMApp
 //
-//  Copyright © 2016 Kyle Neideck
+//  Copyright © 2016, 2024 Kyle Neideck
 //
 
 // Self Include
@@ -35,6 +35,7 @@ NS_ASSUME_NONNULL_BEGIN
 static NSInteger const kVersionLabelTag = 1;
 static NSInteger const kCopyrightLabelTag = 2;
 static NSInteger const kProjectWebsiteLabelTag = 3;
+static NSInteger const kContributorsLabelTag = 4;
 
 @implementation BGMAboutPanel {
     NSPanel* aboutPanel;
@@ -42,6 +43,7 @@ static NSInteger const kProjectWebsiteLabelTag = 3;
     NSTextField* versionLabel;
     NSTextField* copyrightLabel;
     NSTextField* websiteLabel;
+    NSTextField* contributorsLabel;
     
     NSTextView* licenseView;
 }
@@ -53,6 +55,7 @@ static NSInteger const kProjectWebsiteLabelTag = 3;
         versionLabel = [[aboutPanel contentView] viewWithTag:kVersionLabelTag];
         copyrightLabel = [[aboutPanel contentView] viewWithTag:kCopyrightLabelTag];
         websiteLabel = [[aboutPanel contentView] viewWithTag:kProjectWebsiteLabelTag];
+        contributorsLabel = [[aboutPanel contentView] viewWithTag:kContributorsLabelTag];
         
         licenseView = inLicenseView;
         
@@ -83,7 +86,10 @@ static NSInteger const kProjectWebsiteLabelTag = 3;
             [[bundle infoDictionary] objectForKey:@"NSHumanReadableCopyright"];
         
         if (copyrightNotice) {
-            copyrightLabel.stringValue = (NSString*)copyrightNotice;
+            // Remove the part that we replace with a link.
+            copyrightLabel.stringValue =
+                [((NSString*)copyrightNotice) stringByReplacingOccurrencesOfString:contributorsLabel.stringValue
+                                                                        withString:@""];
         }
         
         // Project website link label
@@ -97,6 +103,18 @@ static NSInteger const kProjectWebsiteLabelTag = 3;
                                             attributes:@{ NSLinkAttributeName: projectURL,
                                                           NSFontAttributeName: linkFont }];
         
+        // Contributors link label
+        // TODO: Proper credits (i.e. in the app instead of just a link)
+        contributorsLabel.selectable = YES;
+        contributorsLabel.allowsEditingTextAttributes = YES;
+        
+        NSString* contributorsURL = [NSString stringWithUTF8String:kBGMContributorsURL];
+        NSFont* cLinkFont = contributorsLabel.font ? contributorsLabel.font : [NSFont labelFontOfSize:0.0];
+        contributorsLabel.attributedStringValue =
+            [[NSAttributedString alloc] initWithString:contributorsLabel.stringValue
+                                            attributes:@{ NSLinkAttributeName: contributorsURL,
+                                                          NSFontAttributeName: cLinkFont }];
+        
         // Load the text of the license into the text view
         NSString* __nullable licensePath = [bundle pathForResource:@"LICENSE" ofType:nil];
         
@@ -122,9 +140,25 @@ static NSInteger const kProjectWebsiteLabelTag = 3;
 
 - (void) show {
     DebugMsg("BGMAboutPanel::showAboutPanel: Opening \"About Background Music\" panel");
-    [NSApp activateIgnoringOtherApps:YES];
+    
+    // We have to make aboutPanel visible before calling [NSApp activateIgnoringOtherApps:YES]
+    // or the app won't be activated the first time (not sure why it only happens the first
+    // time) and aboutPanel won't open. WindowServer logs this explanation:
+    // 0[SetFrontProcessWithInfo]: CPS: Rejecting the request for pid 1234 due to the activation count being 0; launch ts=19302059379458, current time=19314267188375, window count = 0.
     [aboutPanel setIsVisible:YES];
     [aboutPanel makeKeyAndOrderFront:self];
+    
+    // On macOS 14.4, aboutPanel needs "Release When Closed" unchecked in MainMenu.xib. Otherwise,
+    // aboutPanel will never open again if you click the close button.
+    
+    // This is deprecated for NSApplication.activate, but that stops aboutPanel from ever being shown.
+    [NSApp activateIgnoringOtherApps:YES];
+    
+    DebugMsg("BGMAboutPanel::showAboutPanel: Finished opening panel. "
+             "aboutPanel.isVisible %d, aboutPanel.isKeyWindow %d, NSApp.isActive %d",
+             aboutPanel.isVisible,
+             aboutPanel.isKeyWindow,
+             NSApp.isActive);
 }
 
 @end

BGMApp/BGMApp/Preferences/BGMAutoPauseMusicPrefs.mm

@@ -17,7 +17,7 @@
 //  BGMAutoPauseMusicPrefs.mm
 //  BGMApp
 //
-//  Copyright © 2016 Kyle Neideck
+//  Copyright © 2016, 2019 Kyle Neideck
 //
 
 // Self Includes
@@ -69,6 +69,7 @@ static NSInteger const kPrefsMenuAutoPauseHeaderTag = 1;
                                                        action:@selector(handleMusicPlayerChange:)
                                                 keyEquivalent:@""
                                                       atIndex:musicPlayerItemsIndex];
+        menuItem.toolTip = musicPlayer.toolTip;
         
         musicPlayerMenuItems = [musicPlayerMenuItems arrayByAddingObject:menuItem];
         

BGMApp/BGMApp/Preferences/BGMPreferencesMenu.h

@@ -17,7 +17,7 @@
 //  BGMPreferencesMenu.h
 //  BGMApp
 //
-//  Copyright © 2016 Kyle Neideck
+//  Copyright © 2016, 2018, 2019 Kyle Neideck
 //
 //  Handles the preferences menu UI. The user's preference changes are often passed directly to the driver rather
 //  than to other BGMApp classes.
@@ -26,6 +26,7 @@
 // Local Includes
 #import "BGMAudioDeviceManager.h"
 #import "BGMMusicPlayers.h"
+#import "BGMStatusBarItem.h"
 
 // System Includes
 #import <Cocoa/Cocoa.h>
@@ -33,11 +34,12 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
-@interface BGMPreferencesMenu : NSObject <NSMenuDelegate>
+@interface BGMPreferencesMenu : NSObject
 
 - (id) initWithBGMMenu:(NSMenu*)inBGMMenu
           audioDevices:(BGMAudioDeviceManager*)inAudioDevices
           musicPlayers:(BGMMusicPlayers*)inMusicPlayers
+         statusBarItem:(BGMStatusBarItem*)inStatusBarItem
             aboutPanel:(NSPanel*)inAboutPanel
  aboutPanelLicenseView:(NSTextView*)inAboutPanelLicenseView;
 

BGMApp/BGMApp/Preferences/BGMPreferencesMenu.mm

@@ -17,7 +17,7 @@
 //  BGMPreferencesMenu.mm
 //  BGMApp
 //
-//  Copyright © 2016 Kyle Neideck
+//  Copyright © 2016, 2018, 2019 Kyle Neideck
 //
 
 // Self Include
@@ -25,7 +25,6 @@
 
 // Local Includes
 #import "BGMAutoPauseMusicPrefs.h"
-#import "BGMOutputDevicePrefs.h"
 #import "BGMAboutPanel.h"
 
 
@@ -33,13 +32,19 @@ NS_ASSUME_NONNULL_BEGIN
 
 // Interface Builder tags
 static NSInteger const kPreferencesMenuItemTag = 1;
-static NSInteger const kAboutPanelMenuItemTag = 3;
+static NSInteger const kBGMIconMenuItemTag     = 2;
+static NSInteger const kVolumeIconMenuItemTag  = 3;
+static NSInteger const kAboutPanelMenuItemTag  = 4;
 
 @implementation BGMPreferencesMenu {
-    // Menu sections
+    // Menu sections/items
     BGMAutoPauseMusicPrefs* autoPauseMusicPrefs;
-    BGMOutputDevicePrefs* outputDevicePrefs;
-    
+    NSMenuItem* bgmIconMenuItem;
+    NSMenuItem* volumeIconMenuItem;
+
+    // The menu item you press to open BGMApp's main menu.
+    BGMStatusBarItem* statusBarItem;
+
     // The About Background Music window
     BGMAboutPanel* aboutPanel;
 }
@@ -47,20 +52,33 @@ static NSInteger const kAboutPanelMenuItemTag = 3;
 - (id) initWithBGMMenu:(NSMenu*)inBGMMenu
           audioDevices:(BGMAudioDeviceManager*)inAudioDevices
           musicPlayers:(BGMMusicPlayers*)inMusicPlayers
+         statusBarItem:(BGMStatusBarItem*)inStatusBarItem
             aboutPanel:(NSPanel*)inAboutPanel
  aboutPanelLicenseView:(NSTextView*)inAboutPanelLicenseView {
     if ((self = [super init])) {
         NSMenu* prefsMenu = [[inBGMMenu itemWithTag:kPreferencesMenuItemTag] submenu];
-        [prefsMenu setDelegate:self];
         
         autoPauseMusicPrefs = [[BGMAutoPauseMusicPrefs alloc] initWithPreferencesMenu:prefsMenu
                                                                          audioDevices:inAudioDevices
                                                                          musicPlayers:inMusicPlayers];
         
-        outputDevicePrefs = [[BGMOutputDevicePrefs alloc] initWithAudioDevices:inAudioDevices];
-        
         aboutPanel = [[BGMAboutPanel alloc] initWithPanel:inAboutPanel licenseView:inAboutPanelLicenseView];
-        
+
+        statusBarItem = inStatusBarItem;
+
+        // Set up the menu items under the "Status Bar Icon" heading.
+        bgmIconMenuItem = [prefsMenu itemWithTag:kBGMIconMenuItemTag];
+        bgmIconMenuItem.state =
+                (statusBarItem.icon == BGMFermataStatusBarIcon) ? NSOnState : NSOffState;
+        [bgmIconMenuItem setTarget:self];
+        [bgmIconMenuItem setAction:@selector(useBGMStatusBarIcon)];
+
+        volumeIconMenuItem = [prefsMenu itemWithTag:kVolumeIconMenuItemTag];
+        volumeIconMenuItem.state =
+                (statusBarItem.icon == BGMVolumeStatusBarIcon) ? NSOnState : NSOffState;
+        [volumeIconMenuItem setTarget:self];
+        [volumeIconMenuItem setAction:@selector(useVolumeStatusBarIcon)];
+
         // Set up the "About Background Music" menu item
         NSMenuItem* aboutMenuItem = [prefsMenu itemWithTag:kAboutPanelMenuItemTag];
         [aboutMenuItem setTarget:aboutPanel];
@@ -70,10 +88,27 @@ static NSInteger const kAboutPanelMenuItemTag = 3;
     return self;
 }
 
-#pragma mark NSMenuDelegate
+- (void) useBGMStatusBarIcon {
+    // Change the icon.
+    statusBarItem.icon = BGMFermataStatusBarIcon;
 
-- (void) menuNeedsUpdate:(NSMenu*)menu {
-    [outputDevicePrefs populatePreferencesMenu:menu];
+    // Select/deselect the menu items.
+    bgmIconMenuItem.state = NSOnState;
+    volumeIconMenuItem.state = NSOffState;
+}
+
+- (void) useVolumeStatusBarIcon {
+    // TODO: Maybe we should show a message that tells the user how to hide the built-in volume
+    //       icon. They probably won't want two status bar items that look the same. Or we might be
+    //       able to automatically hide the built-in icon while BGMApp is running and show it again
+    //       when BGMApp is closed.
+
+    // Change the icon.
+    statusBarItem.icon = BGMVolumeStatusBarIcon;
+
+    // Select/deselect the menu items.
+    bgmIconMenuItem.state = NSOffState;
+    volumeIconMenuItem.state = NSOnState;
 }
 
 @end

BGMApp/BGMApp/Scripting/BGMASApplication.h

@@ -0,0 +1,50 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMASApplication.h
+//  BGMApp
+//
+//  Copyright © 2021 Marcus Wu
+//  Copyright © 2021 Kyle Neideck
+//
+//  An AppleScript class for volume and pan settings for running applications.
+//
+
+
+// Local Includes
+#import "BGMAppVolumesController.h"
+
+// System Includes
+#import <Foundation/Foundation.h>
+#import <Cocoa/Cocoa.h>
+
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface BGMASApplication : NSObject
+
+- (instancetype) initWithApplication:(NSRunningApplication*)app
+                          volumeController:(BGMAppVolumesController*)volumeController
+                     parentSpecifier:(NSScriptObjectSpecifier* __nullable)parentSpecifier
+                               index:(int)i;
+
+@property (readonly) NSString* name;
+@property (readonly) NSString* bundleID;
+@property int volume;
+@property int pan;
+@end
+
+NS_ASSUME_NONNULL_END

BGMApp/BGMApp/Scripting/BGMASApplication.m

@@ -0,0 +1,91 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMASApplication.m
+//  BGMApp
+//
+//  Copyright © 2021 Marcus Wu
+//  Copyright © 2021 Kyle Neideck
+//
+
+// Self Include
+#import "BGMASApplication.h"
+
+// Local Includes
+#import "BGM_Types.h"
+
+@implementation BGMASApplication {
+    NSScriptObjectSpecifier* parentSpecifier;
+    NSRunningApplication *application;
+    BGMAppVolumesController* appVolumesController;
+    int index;
+}
+
+- (instancetype) initWithApplication:(NSRunningApplication*)app
+                          volumeController:(BGMAppVolumesController*)volumeController
+                     parentSpecifier:(NSScriptObjectSpecifier* __nullable)parent
+                               index:(int)i {
+    if ((self = [super init])) {
+        parentSpecifier = parent;
+        application = app;
+        appVolumesController = volumeController;
+        index = i;
+    }
+
+    return self;
+}
+
+- (NSString*) name {
+    return [NSString stringWithFormat:@"%@", [application localizedName]];
+}
+
+- (NSString*) bundleID {
+    return [NSString stringWithFormat:@"%@", [application bundleIdentifier]];
+}
+
+- (int) volume {
+    return [appVolumesController getVolumeAndPanForApp:application].volume;
+}
+
+- (void) setVolume:(int)vol {
+    BGMAppVolumeAndPan volume = {
+        .volume = vol,
+        .pan = kAppPanNoValue
+    };
+    [appVolumesController setVolumeAndPan:volume forApp:application];
+}
+
+- (int) pan {
+    return [appVolumesController getVolumeAndPanForApp:application].pan;
+}
+
+- (void) setPan:(int)pan {
+    BGMAppVolumeAndPan thePan = {
+        .volume = -1,
+        .pan = pan
+    };
+    [appVolumesController setVolumeAndPan:thePan forApp:application];
+}
+
+- (NSScriptObjectSpecifier* __nullable) objectSpecifier {
+    NSScriptClassDescription* parentClassDescription = [parentSpecifier keyClassDescription];
+    return [[NSNameSpecifier alloc] initWithContainerClassDescription:parentClassDescription
+                                                   containerSpecifier:parentSpecifier
+                                                                  key:@"applications"
+                                                                 name:self.name];
+}
+
+@end

BGMApp/BGMApp/Scripting/BGMApp.sdef

@@ -9,8 +9,7 @@
         <class name="output device"
                code="aDev"
                description="A hardware device that can play audio"
-               plural="output devices"
-               inherits="item">
+               plural="output devices">
             <synonym name="audio device"/>
 
             <cocoa class="BGMASOutputDevice"/>
@@ -19,14 +18,58 @@
                       code="pnam"
                       description="The name of the output device."
                       type="text"
-                      access="r"/>
+                      access="r">
+                <cocoa key="name"/>
+            </property>
 
             <property name="selected"
                       code="Slcd"
                       type="boolean"
                       access="rw"
                       description="Is this the device to be used for audio output?">
-                <synonym name="default"/>
+		<synonym name="default"/>
+                <cocoa key="selected"/>
+            </property>
+        </class>
+
+        <class name="audio application"
+               code="aApp"
+               description="An application that can play audio"
+               plural="audio applications">
+            <synonym name="audio app"/>
+
+            <cocoa class="BGMASApplication"/>
+
+            <property name="name"
+                      code="pnam"
+                      description="The name of the application."
+                      type="text"
+                      access="r">
+                <cocoa key="name"/>
+            </property>
+
+            <property name="bundleID"
+                      code="bdid"
+                      description="The bundle ID of the application, e.g. 'com.somecompany.coolapp'."
+                      type="text"
+                      access="r">
+                <cocoa key="bundleID"/>
+            </property>
+
+            <property name="vol"
+                      code="pVol"
+                      type="integer"
+                      access="rw"
+                      description="The volume setting of the application">
+                <cocoa key="volume"/>
+            </property>
+
+            <property name="pan"
+                      code="pPan"
+                      type="integer"
+                      access="rw"
+                      description="The pan setting of the application">
+                <cocoa key="pan"/>
             </property>
         </class>
 
@@ -48,11 +91,23 @@
 
                 <cocoa key="selectedOutputDevice"/>
             </property>
+            <property name="output volume"
+                type="real"
+                code="oVol"
+                access="rw"
+                description="The main output volume">
+                <synonym name="main volume"/>
+
+                <cocoa key="mainVolume"/>
+            </property>
 
             <!-- Unintuitively, this is for the array of output devices. -->
             <element type="output device" access="r">
                 <cocoa key="outputDevices"/>
             </element>
+            <element type="audio application" access="r">
+                <cocoa key="applications"/>
+            </element>
         </class>
     </suite>
 </dictionary>

BGMApp/BGMApp/Scripting/BGMAppDelegate+AppleScript.h

@@ -18,12 +18,14 @@
 //  BGMApp
 //
 //  Copyright © 2017 Kyle Neideck
+//  Copyright © 2021 Marcus Wu
 //
 
 #import "BGMAppDelegate.h"
 
 // Local Includes
 #import "BGMASOutputDevice.h"
+#import "BGMASApplication.h"
 
 // System Includes
 #import <Foundation/Foundation.h>
@@ -37,6 +39,8 @@
 
 @property BGMASOutputDevice* selectedOutputDevice;
 @property (readonly) NSArray<BGMASOutputDevice*>* outputDevices;
+@property double mainVolume;
+@property (readonly) NSArray<BGMASApplication*>* applications;
 
 @end
 

BGMApp/BGMApp/Scripting/BGMAppDelegate+AppleScript.mm

@@ -18,6 +18,7 @@
 //  BGMApp
 //
 //  Copyright © 2017 Kyle Neideck
+//  Copyright © 2021 Marcus Wu
 //
 
 // Self Include
@@ -30,6 +31,7 @@
 #import "CAHALAudioSystemObject.h"
 #import "CAAutoDisposer.h"
 
+const AudioObjectPropertyScope kScope                   = kAudioDevicePropertyScopeOutput;
 
 #pragma clang assume_nonnull begin
 
@@ -43,7 +45,7 @@
                  [key UTF8String]);
     }
 
-    return [@[@"selectedOutputDevice", @"outputDevices"] containsObject:key];
+    return [@[@"selectedOutputDevice", @"outputDevices", @"mainVolume", @"applications"] containsObject:key];
 }
 
 - (BGMASOutputDevice*) selectedOutputDevice {
@@ -83,6 +85,30 @@
     return outputDevices;
 }
 
+- (double) mainVolume {
+    BGMAudioDevice bgmDevice = [self.audioDevices bgmDevice];
+    return bgmDevice.GetVolumeControlScalarValue(kScope, kMasterChannel);
+}
+
+- (void) setMainVolume:(double)mainVolume {
+    BGMAudioDevice bgmDevice = [self.audioDevices bgmDevice];
+    bgmDevice.SetMasterVolumeScalar(kScope, (Float32)mainVolume);
+    [self.outputVolumeSlider setFloatValue:(float)mainVolume];
+}
+
+- (NSArray<BGMASApplication*>*) applications {
+    NSArray<NSRunningApplication*>* apps = [[NSWorkspace sharedWorkspace] runningApplications];
+    NSMutableArray<BGMASApplication*>* applications = [NSMutableArray arrayWithCapacity:[apps count]];
+
+    for (UInt32 i = 0; i < [apps count]; i++) {
+        BGMASApplication *app = [[BGMASApplication alloc] initWithApplication:apps[i] volumeController:self.appVolumes parentSpecifier:[self objectSpecifier] index:i];
+
+        [applications addObject:app];
+    }
+
+    return applications;
+}
+
 @end
 
 #pragma clang assume_nonnull end

BGMApp/BGMApp/_uninstall-non-interactive.sh

@@ -20,7 +20,7 @@
 # _uninstall-non-interactive.sh
 #
 # Copyright © 2016 Nick Jacques
-# Copyright © 2016, 2017 Kyle Neideck
+# Copyright © 2016, 2017, 2021 Kyle Neideck
 #
 # Removes BGMApp, BGMDriver and BGMXPCHelper from the system immediately. Run by uninstall.sh and the Homebrew formula.
 #
@@ -43,8 +43,9 @@ driver_path="/Library/Audio/Plug-Ins/HAL/Background Music Device.driver"
 xpc_path1="/usr/local/libexec/BGMXPCHelper.xpc"
 xpc_path2="/Library/Application Support/Background Music/BGMXPCHelper.xpc"
 
-# Check that files/directories are at most this big before we delete them, just to be safe.
-max_size_mb_for_rm=15
+# Check that files/directories are at most this big before we delete them, just to be safe. Note that the bundles can
+# include debug symbols, e.g. if you use build_and_install.sh, which makes them a lot bigger.
+max_size_mb_for_rm=30
 
 file_paths=("${app_path}" "${driver_path}" "${xpc_path1}" "${xpc_path2}")
 
@@ -68,8 +69,9 @@ function size_check {
   [[ "${size}" =~ ^[0-9]+$ ]] && [[ "${size}" -le ${max_size_mb_for_rm} ]]
 }
 
-# Ensure that the user can use sudo. (But not if this is a Travis CI build, because then it would fail.)
-if ([[ -z ${TRAVIS:-} ]] || [[ "${TRAVIS}" != true ]]) && ! sudo -v; then
+# Ensure that the user can use sudo. (Use `sudo true` instead of `sudo -v` because that causes a
+# password prompt in Travis CI builds for some reason.)
+if ! sudo true; then
   echo "ERROR: This script must be run by a user with administrator (sudo) privileges." >&2
   exit 1
 fi
@@ -137,7 +139,8 @@ sleep 2
 # The extra or-clauses are fallback versions of the command that restarts coreaudiod. Apparently some of these commands 
 # don't work with older versions of launchctl, so I figure there's no harm in trying a bunch of different ways until
 # one works.
-(sudo launchctl kill SIGTERM system/com.apple.audio.coreaudiod &>/dev/null || \
+(sudo launchctl kickstart -k system/com.apple.audio.coreaudiod &>/dev/null || \
+  sudo launchctl kill SIGTERM system/com.apple.audio.coreaudiod &>/dev/null || \
   sudo launchctl kill TERM system/com.apple.audio.coreaudiod &>/dev/null || \
   sudo launchctl kill 15 system/com.apple.audio.coreaudiod &>/dev/null || \
   sudo launchctl kill -15 system/com.apple.audio.coreaudiod &>/dev/null || \
@@ -150,7 +153,7 @@ sleep 3
 
 # TODO: What if they only have one audio device?
 echo ""
-echo "${bold}Done! Toggle your audio output device in the Sound section of System Preferences to finish" \
+echo "${bold}Done! Toggle your audio output device in the Sound section of System Settings to finish" \
      "uninstalling. (Or just restart your computer.)${normal}"
 
 

BGMApp/BGMAppTests/NullAudio/AudioDriverExamples.xcodeproj/project.pbxproj

@@ -1,754 +0,0 @@
-// !$*UTF8*$!
-{
-	archiveVersion = 1;
-	classes = {
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-				GCC_PREPROCESSOR_DEFINITIONS = "DEBUG=1";
-				INFOPLIST_FILE = "SimpleAudio/Driver/SimpleAudioDriver-Info.plist";
-				INSTALL_PATH = "$(SYSTEM_LIBRARY_DIR)/Extensions";
-				MODULE_NAME = com.apple.audio.SimpleAudioDriver;
-				MODULE_VERSION = 1.0;
-				PRODUCT_NAME = "$(TARGET_NAME)";
-				WRAPPER_EXTENSION = kext;
-			};
-			name = "Debug-Opt";
-		};
-/* End XCBuildConfiguration section */
-
-/* Begin XCConfigurationList section */
-		2D7477A01578162B00412279 /* Build configuration list for PBXProject "AudioDriverExamples" */ = {
-			isa = XCConfigurationList;
-			buildConfigurations = (
-				2D7477A21578162B00412279 /* Release */,
-				2D7477A31578162B00412279 /* Debug */,
-				2D7477A41578164E00412279 /* Debug-Opt */,
-			);
-			defaultConfigurationIsVisible = 0;
-			defaultConfigurationName = Release;
-		};
-		2D7477B51578168D00412279 /* Build configuration list for PBXNativeTarget "NullAudio" */ = {
-			isa = XCConfigurationList;
-			buildConfigurations = (
-				2D7477B61578168D00412279 /* Release */,
-				2D7477B71578168D00412279 /* Debug */,
-				2D7477B81578168D00412279 /* Debug-Opt */,
-			);
-			defaultConfigurationIsVisible = 0;
-			defaultConfigurationName = Release;
-		};
-		2D76D97215E48B2000FF0F33 /* Build configuration list for PBXNativeTarget "SimpleAudioPlugIn" */ = {
-			isa = XCConfigurationList;
-			buildConfigurations = (
-				2D76D96F15E48B2000FF0F33 /* Release */,
-				2D76D97015E48B2000FF0F33 /* Debug */,
-				2D76D97115E48B2000FF0F33 /* Debug-Opt */,
-			);
-			defaultConfigurationIsVisible = 0;
-			defaultConfigurationName = Release;
-		};
-		2DED184515C357180091BE97 /* Build configuration list for PBXNativeTarget "SimpleAudioDriver" */ = {
-			isa = XCConfigurationList;
-			buildConfigurations = (
-				2DED184615C357180091BE97 /* Release */,
-				2DED184715C357180091BE97 /* Debug */,
-				2DED184815C357180091BE97 /* Debug-Opt */,
-			);
-			defaultConfigurationIsVisible = 0;
-			defaultConfigurationName = Release;
-		};
-/* End XCConfigurationList section */
-	};
-	rootObject = 2D74779D1578162B00412279 /* Project object */;
-}

BGMApp/BGMAppTests/NullAudio/AudioDriverExamples.xcodeproj/project.xcworkspace/contents.xcworkspacedata

@@ -1,7 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<Workspace
-   version = "1.0">
-   <FileRef
-      location = "self:">
-   </FileRef>
-</Workspace>

BGMApp/BGMAppTests/NullAudio/NullAudio/NullAudio-Info.plist

@@ -1,38 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
-<plist version="1.0">
-<dict>
-	<key>CFBundleDevelopmentRegion</key>
-	<string>English</string>
-	<key>CFBundleExecutable</key>
-	<string>${EXECUTABLE_NAME}</string>
-	<key>CFBundleIconFile</key>
-	<string></string>
-	<key>CFBundleIdentifier</key>
-	<string>com.apple.audio.${PRODUCT_NAME:rfc1034identifier}</string>
-	<key>CFBundleInfoDictionaryVersion</key>
-	<string>6.0</string>
-	<key>CFBundleName</key>
-	<string>${PRODUCT_NAME}</string>
-	<key>CFBundlePackageType</key>
-	<string>BNDL</string>
-	<key>CFBundleShortVersionString</key>
-	<string>1.0.1</string>
-	<key>CFBundleSignature</key>
-	<string>????</string>
-	<key>CFBundleVersion</key>
-	<string>1</string>
-	<key>CFPlugInFactories</key>
-	<dict>
-		<key>99A15A8B-DA3C-42C3-BD5D-D035A0C2377A</key>
-		<string>NullAudio_Create</string>
-	</dict>
-	<key>CFPlugInTypes</key>
-	<dict>
-		<key>443ABAB8-E7B3-491A-B985-BEB9187030DB</key>
-		<array>
-			<string>99A15A8B-DA3C-42C3-BD5D-D035A0C2377A</string>
-		</array>
-	</dict>
-</dict>
-</plist>

BGMApp/BGMAppTests/NullAudio/README

@@ -1,6 +0,0 @@
-TravisCI's OS X VMs don't have any audio devices installed by default, so we
-install Apple's NullAudio sample driver before the tests run. Otherwise the
-tests would fail because BGMApp currently crashes on launch if you don't have
-any audio devices.
-
-

BGMApp/BGMAppTests/UITests/BGMApp.h

@@ -1,5 +1,8 @@
 /*
  * BGMApp.h
+ *
+ * Generated with
+ * sdef "/Applications/Background Music.app" | sdp -fh --basename BGMApp
  */
 
 #import <AppKit/AppKit.h>
@@ -14,11 +17,11 @@
  * Background Music
  */
 
-// an output audio device
+// A hardware device that can play audio
 @interface BGMAppOutputDevice : SBObject
 
-@property (copy, readonly) NSString *name;
-@property BOOL selected;  // is this the device to be used for audio output?
+@property (copy, readonly) NSString *name;  // The name of the output device.
+@property BOOL selected;  // Is this the device to be used for audio output?
 
 @end
 
@@ -27,5 +30,7 @@
 
 - (SBElementArray<BGMAppOutputDevice *> *) outputDevices;
 
+@property (copy) BGMAppOutputDevice *selectedOutputDevice;  // The device to be used for audio output
+
 @end
 

BGMApp/BGMAppTests/UITests/BGMAppUITests.mm

@@ -17,7 +17,7 @@
 //  BGMAppUITests.mm
 //  BGMAppUITests
 //
-//  Copyright © 2017 Kyle Neideck
+//  Copyright © 2017, 2018, 2020, 2022 Kyle Neideck
 //
 //  You might want to use Xcode's UI test recording feature if you add new tests.
 //
@@ -30,10 +30,14 @@
 // Scripting Bridge Includes
 #import "BGMApp.h"
 
+// System Includes
+#import <XCTest/XCTest.h>
+
 
-// TODO: Skip these tests if macOS SDK 10.11 or higher isn't available.
 // TODO: Mock BGMDevice and music players.
 
+#if __clang_major__ >= 9
+
 @interface BGMAppUITests : XCTestCase
 @end
 
@@ -60,8 +64,8 @@
     // Set up the app object and some convenience vars.
     app = [[XCUIApplication alloc] init];
     menuItems = app.menuBars.menuItems;
-    icon = [app.menuBars childrenMatchingType:XCUIElementTypeMenuBarItem].element;
     prefs = menuItems[@"Preferences"];
+    icon = [app.menuBars childrenMatchingType:XCUIElementTypeStatusItem].element;
 
     // TODO: Make sure BGMDevice isn't set as the OS X default device before launching BGMApp.
 
@@ -70,8 +74,55 @@
     // would fail to start because of a bug in Xcode.
     app.launchArguments = @[ @"--no-persistent-data", @"--show-dock-icon" ];
 
+    // Make the "Background Music wants to use the microphone" dialog appear every time so the test
+    // doesn't need logic to handle both cases.
+    // TODO: Commented out until acceptMicrophoneAuthorizationDialog work again. See below.
+    // if (@available(macOS 10.15.4, *)) {
+    //     [app resetAuthorizationStatusForResource:XCUIProtectedResourceMicrophone];
+    // }
+
     // Launch BGMApp.
     [app launch];
+
+    // TODO: This doesn't seem to be working on macOS 12.4 (21F79). You can click OK manually for
+    //       now.
+    // [self acceptMicrophoneAuthorizationDialog];
+
+    if (![icon waitForExistenceWithTimeout:20.0]) {
+        // The status bar icon/button has this type when using older versions of XCTest, so try
+        // both. (Actually, it might depend on the macOS or Xcode version. I'm not sure.)
+        XCUIElement* iconOldType =
+            [app.menuBars childrenMatchingType:XCUIElementTypeMenuBarItem].element;
+        if ([iconOldType waitForExistenceWithTimeout:20.0]) {
+            NSLog(@"icon = iconOldType");
+            icon = iconOldType;
+        }
+    }
+
+    // Wait for the initial elements.
+    XCTAssert([app waitForExistenceWithTimeout:20.0]);
+    XCTAssert([icon waitForExistenceWithTimeout:20.0]);
+}
+
+// Clicks the OK button in the "Background Music wants to use the microphone" dialog.
+- (void) acceptMicrophoneAuthorizationDialog {
+    XCUIApplication* unc =
+        [[XCUIApplication alloc] initWithBundleIdentifier:@"com.apple.UserNotificationCenter"];
+    NSLog(@"UserNotificationCenter: %@", unc);
+    XCUIElement* okButton = unc.dialogs.buttons[@"OK"];
+
+    XCTAssert([okButton waitForExistenceWithTimeout:20.0]);
+
+    // This click is failing on GH Actions. No idea why, so try a sleep.
+    (void)[XCTWaiter waitForExpectations:@[[XCTestExpectation new]] timeout:5.0];
+    [okButton click];
+
+    int retries = 10;
+    while (retries > 0 && [okButton waitForExistenceWithTimeout:3.0]) {
+        NSLog(@"Microphone authorization dialog is still open. Trying to click OK again.");
+        [okButton click];
+        retries--;
+    }
 }
 
 - (void) tearDown {
@@ -83,18 +134,30 @@
     [menuItems[@"Quit Background Music"] click];
 
     // BGMApp should quit.
-    XCTAssert(!app.exists);
+    XCTAssertTrue([app waitForState:XCUIApplicationStateNotRunning timeout:10.0]);
     
     [super tearDown];
 }
 
 - (void) testCycleOutputDevices {
-    const int NUM_CYCLES = 2;
+    const int NUM_CYCLES = 1;
 
-    // Get the list of output devices from the preferences menu.
+    // sbApp lets us use AppleScript to query BGMApp and check the test has made the changes to its
+    // settings we expect.
+    BGMAppApplication* sbApp = [SBApplication applicationWithBundleIdentifier:@kBGMAppBundleID];
+
+    // Get macOS to show the "'Xcode' wants to control 'Background Music'" dialog before we start
+    // the test so it doesn't interrupt it.
+    [[sbApp selectedOutputDevice] name];
+
+    // Click the icon to open the main menu.
     [icon click];
-    [prefs hover];
-    NSArray<XCUIElement*>* outputDeviceMenuItems = [self outputDeviceMenuItems];
+
+    // Get the list of output devices from the main menu.
+    // BGMOutputDeviceMenuSection::createMenuItemForDevice gives every output device menu item the
+    // accessibility identifier "output-device" so we can find all of them here.
+    NSArray<XCUIElement*>* outputDeviceMenuItems =
+        [menuItems matchingIdentifier:@"output-device"].allElementsBoundByIndex;
 
     // For debugging certain issues, it can be useful to repeatedly switch between two
     // devices:
@@ -105,14 +168,10 @@
     // Click the last device to close the menu again.
     [outputDeviceMenuItems.lastObject click];
 
-    BGMAppApplication* sbApp = [SBApplication applicationWithBundleIdentifier:@kBGMAppBundleID];
-
     for (int i = 0; i < NUM_CYCLES; i++) {
         // Select each output device.
         for (XCUIElement* item in outputDeviceMenuItems) {
             [icon click];
-            [prefs hover];
-
             [item click];
 
             // Assert that the device we clicked is the selected device now.
@@ -128,29 +187,6 @@
     }
 }
 
-// Find the menu items for the output devices in the preferences menu.
-- (NSArray<XCUIElement*>*) outputDeviceMenuItems {
-    NSArray<XCUIElement*>* items = @[];
-    BOOL inOutputDeviceSection = NO;
-
-    for (int i = 0; i < prefs.menuItems.count; i++) {
-        XCUIElement* menuItem = [prefs.menuItems elementBoundByIndex:i];
-
-        if ([menuItem.title isEqual:@"Output Device"]) {
-            inOutputDeviceSection = YES;
-        } else if (inOutputDeviceSection) {
-            // Assume that finding a separator menu item means we've reached the end of the section.
-            if (((NSMenuItem*)menuItem.value).separatorItem || [menuItem.title isEqual:@""]) {
-                break;
-            }
-
-            items = [items arrayByAddingObject:menuItem];
-        }
-    }
-
-    return items;
-}
-
 - (void) testSelectMusicPlayer {
     // Select VLC as the music player.
     [icon click];
@@ -234,3 +270,5 @@
 
 @end
 
+#endif /* __clang_major__ >= 9 */
+

BGMApp/BGMAppTests/UITests/skip-ui-tests.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python2.7
+#!/usr/bin/env python3
 
 # This file is part of Background Music.
 #
@@ -16,20 +16,12 @@
 # along with Background Music. If not, see <http://www.gnu.org/licenses/>.
 
 #
-# travis-skip.py
+# skip-ui-tests.py
 # BGMAppUITests
 #
-# Copyright (c) 2017 Kyle Neideck
+# Copyright (c) 2017, 2024 Kyle Neideck
 #
-# Skip the UI tests in Travis builds because they aren't supported.
-#
-# We can't run the tests on Travis because Xcode needs permission to use the Accessibility API
-# to control BGMApp. There's no way to set that up programmatically without disabling SIP and
-# Travis doesn't support that.
-#
-# See https://github.com/travis-ci/travis-ci/issues/5819
-#
-# TODO: Figure out a better way to do this.
+# Disables the UI tests. This is mainly useful on CI systems that don't support UI tests.
 #
 
 import xml.etree.ElementTree as ET
@@ -46,4 +38,3 @@ tree.getroot().findall(UI_REF_XPATH)[0].set("skipped", "YES")
 # Save the scheme.
 tree.write(SCHEME_FILE)
 
-

BGMApp/BGMAppTests/UnitTests/BGMMusicPlayersUnitTests.mm

@@ -17,24 +17,28 @@
 //  BGMMusicPlayersUnitTests.mm
 //  BGMAppUnitTests
 //
-//  Copyright © 2016-2018 Kyle Neideck
+//  Copyright © 2016-2020 Kyle Neideck
 //
 
 // Unit include
 #import "BGMMusicPlayers.h"
 
-// Local includes
-#import "BGM_TestUtils.h"
-
+// BGM includes
 #import "BGM_Types.h"
 #import "BGMAudioDeviceManager.h"
 #import "BGMiTunes.h"
-#import "BGMVLC.h"
 #import "BGMDecibel.h"
 #import "BGMSpotify.h"
+#import "BGMVLC.h"
+
+// Local includes
+#import "BGM_TestUtils.h"
+#import "MockAudioObject.h"
+#import "MockAudioObjects.h"
 
 // System includes
 #import <Foundation/Foundation.h>
+#import <XCTest/XCTest.h>
 
 
 // Note that the PublicUtility classes that we use to communicate with the HAL, CAHALAudioObject and
@@ -72,32 +76,6 @@
 
 // -------------------------------------------------------------------------------------------------
 
-class BGMMockBackgroundMusicDevice
-:
-    public BGMBackgroundMusicDevice
-{
-
-public:
-    CFStringRef GetMusicPlayerBundleID() const;
-    void        SetMusicPlayerBundleID(CFStringRef inBundleID);
-
-private:
-    CFStringRef mMusicPlayerBundleID = CFSTR("");
-
-};
-
-CFStringRef BGMMockBackgroundMusicDevice::GetMusicPlayerBundleID() const
-{
-    return mMusicPlayerBundleID;
-}
-
-void BGMMockBackgroundMusicDevice::SetMusicPlayerBundleID(CFStringRef inBundleID)
-{
-    mMusicPlayerBundleID = inBundleID;
-}
-
-// -------------------------------------------------------------------------------------------------
-
 @interface BGMMockAudioDeviceManager : BGMAudioDeviceManager
 @end
 
@@ -127,6 +105,10 @@ void BGMMockBackgroundMusicDevice::SetMusicPlayerBundleID(CFStringRef inBundleID
 - (void) setUp {
     [super setUp];
 
+    // Mock BGMDevice.
+    MockAudioObjects::CreateMockDevice(kBGMDeviceUID);
+    MockAudioObjects::CreateMockDevice(kBGMDeviceUID_UISounds);
+
     devices = [BGMMockAudioDeviceManager new];
     defaults = [BGMMockUserDefaults new];
     
@@ -137,9 +119,10 @@ void BGMMockBackgroundMusicDevice::SetMusicPlayerBundleID(CFStringRef inBundleID
 
 - (void) tearDown {
     [super tearDown];
+    MockAudioObjects::DestroyMocks();
 }
 
-- (void) testNoSelectedMusicPlayerStored {
+- (void) testNoSelectedMusicPlayerStored_iTunesDefault {
     // Test the case where the user has never changed the music player preference.
     
     // Test with iTunes as the default.
@@ -156,16 +139,20 @@ void BGMMockBackgroundMusicDevice::SetMusicPlayerBundleID(CFStringRef inBundleID
     
     XCTAssertEqualObjects(players.selectedMusicPlayer.musicPlayerID, [BGMiTunes sharedMusicPlayerID]);
     XCTAssertEqualObjects(players.selectedMusicPlayer.name, @"iTunes");
-    
-    [self resetDevice];
-    
+}
+
+
+- (void) testNoSelectedMusicPlayerStored_vlcDefault {
+    // Test the case where the user has never changed the music player preference.
+
     // Test with VLC as the default.
-    players = [[BGMMusicPlayers alloc] initWithAudioDevices:devices
-                                       defaultMusicPlayerID:vlcID
-                                         musicPlayerClasses:@[ BGMiTunes.class,
-                                                               BGMVLC.class,
-                                                               BGMDecibel.class ]
-                                               userDefaults:defaults];
+    BGMMusicPlayers* players =
+            [[BGMMusicPlayers alloc] initWithAudioDevices:devices
+                                     defaultMusicPlayerID:vlcID
+                                       musicPlayerClasses:@[ BGMiTunes.class,
+                                                             BGMVLC.class,
+                                                             BGMDecibel.class ]
+                                             userDefaults:defaults];
     
     XCTAssertEqual(players.musicPlayers.count, 3);
     
@@ -193,15 +180,15 @@ void BGMMockBackgroundMusicDevice::SetMusicPlayerBundleID(CFStringRef inBundleID
     
     XCTAssertEqualObjects(players.selectedMusicPlayer.musicPlayerID, spotifyID);
     XCTAssertEqualObjects(players.selectedMusicPlayer.name, @"Spotify");
-    
-    [self resetDevice];
-    
+}
+
+- (void) testUnrecognizedSelectedMusicPlayerInUserDefaults {
     // If there's an unrecognized ID in user defaults, the default music player should be selected.
     defaults.selectedPlayerID = [[NSUUID alloc] initWithUUIDString:@"11111111-1111-1111-0000-000000000000"];
     
     // This initializer sets iTunes as the default music player and adds all the other music players.
-    players = [[BGMMusicPlayers alloc] initWithAudioDevices:devices
-                                               userDefaults:defaults];
+    BGMMusicPlayers* players = [[BGMMusicPlayers alloc] initWithAudioDevices:devices
+                                                                userDefaults:defaults];
     
     XCTAssert(players.musicPlayers.count >= 6);
     
@@ -226,10 +213,5 @@ void BGMMockBackgroundMusicDevice::SetMusicPlayerBundleID(CFStringRef inBundleID
 
 // TODO: Test setting the selectedMusicPlayer property
 
-- (void) resetDevice {
-    // Reset the mock BGMDevice.
-    [devices bgmDevice].SetMusicPlayerBundleID(CFSTR(""));
-}
-
 @end
 

BGMApp/BGMAppTests/UnitTests/BGMPlayThroughRTLoggerTests.mm

@@ -0,0 +1,179 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMPlayThroughRTLoggerTests.mm
+//  BGMAppUnitTests
+//
+//  Copyright © 2020 Kyle Neideck
+//
+
+// Unit Include
+#import "BGMPlayThroughRTLogger.h"
+
+// PublicUtility Includes
+#import "CARingBuffer.h"
+
+// System Includes
+#import <CoreAudio/CoreAudio.h>
+#import <XCTest/XCTest.h>
+
+
+@interface BGMPlayThroughRTLoggerTests : XCTestCase
+
+@end
+
+@implementation BGMPlayThroughRTLoggerTests
+{
+    BGMPlayThroughRTLogger* logger;
+}
+
+- (void) setUp {
+    [super setUp];
+    logger = new BGMPlayThroughRTLogger;
+}
+
+- (void) tearDown {
+    [super tearDown];
+    delete logger;
+}
+
+- (void) testLogReleasingWaitingThreads {
+    logger->LogReleasingWaitingThreads();
+    [self assertLoggedOneDebugMessage];
+}
+
+- (void) testLogIfMachError_ReleaseWaitingThreadsSignal {
+    logger->LogIfMachError_ReleaseWaitingThreadsSignal(KERN_SUCCESS);
+    [self assertLoggedNoMessages];
+}
+
+- (void) testLogIfDroppedFrames_didNotDropFrames {
+    logger->LogIfDroppedFrames(11256.0, 11256.0);
+    [self assertLoggedNoMessages];
+}
+
+- (void) testLogIfDroppedFrames_didDropFrames {
+    logger->LogIfDroppedFrames(11256.0, 11768.0);
+    [self assertLoggedOneDebugMessage];
+}
+
+- (void) testLogNoSamplesReady {
+    logger->LogNoSamplesReady(512, 1024, 512.0);
+    [self assertLoggedOneDebugMessage];
+}
+
+- (void) testLogExceptionStoppingIOProc_withoutErrorCode {
+    // Set a test-only flag that keeps it from calling abort() after logging the error when the
+    // tests are compiled with the debug configuration.
+    logger->mContinueOnErrorLogged = true;
+
+    logger->LogExceptionStoppingIOProc("InputDeviceIOProc");
+    [self assertLoggedOneErrorMessage];
+}
+
+- (void) testLogExceptionStoppingIOProc_withErrorCode {
+    // Set a test-only flag that keeps it from calling abort() after logging the error when the
+    // tests are compiled with the debug configuration.
+    logger->mContinueOnErrorLogged = true;
+
+    logger->LogExceptionStoppingIOProc("OutputDeviceIOProc", kAudioHardwareUnknownPropertyError);
+    [self assertLoggedOneErrorMessage];
+}
+
+- (void) testLogUnexpectedIOStateAfterStopping {
+    logger->LogUnexpectedIOStateAfterStopping("OutputDeviceIOProc", 1);
+    [self assertLoggedOneWarningMessage];
+}
+
+- (void) testLogRingBufferUnavailable {
+    logger->LogRingBufferUnavailable("OutputDeviceIOProc", false);
+    [self assertLoggedOneWarningMessage];
+}
+
+- (void) testLogIfRingBufferError_Fetch_noError {
+    logger->LogIfRingBufferError_Fetch(kCARingBufferError_OK);
+    [self assertLoggedNoMessages];
+}
+
+- (void) testLogIfRingBufferError_Fetch_errorCPUOverload {
+    logger->LogIfRingBufferError_Fetch(kCARingBufferError_CPUOverload);
+    [self assertLoggedOneWarningMessage];
+}
+
+- (void) testLogIfRingBufferError_Fetch_errorTooMuch {
+    // Set a test-only flag that keeps it from calling abort() after logging the error when the
+    // tests are compiled with the debug configuration.
+    logger->mContinueOnErrorLogged = true;
+
+    logger->LogIfRingBufferError_Fetch(kCARingBufferError_TooMuch);
+    [self assertLoggedOneErrorMessage];
+}
+
+- (void) testLogIfRingBufferError_Store_noError {
+    logger->LogIfRingBufferError_Store(kCARingBufferError_OK);
+    [self assertLoggedNoMessages];
+}
+
+- (void) testLogIfRingBufferError_Store_errorCPUOverload {
+    logger->LogIfRingBufferError_Store(kCARingBufferError_CPUOverload);
+    [self assertLoggedOneWarningMessage];
+}
+
+- (void) testLogIfRingBufferError_Store_errorTooMuch {
+    // Set a test-only flag that keeps it from calling abort() after logging the error when the
+    // tests are compiled with the debug configuration.
+    logger->mContinueOnErrorLogged = true;
+
+    logger->LogIfRingBufferError_Store(kCARingBufferError_TooMuch);
+    [self assertLoggedOneErrorMessage];
+}
+
+- (void) waitForLoggingThread {
+    // Wait for it to finish logging the messages.
+    bool noMessagesLeft = logger->WaitUntilLoggerThreadIdle();
+    XCTAssert(noMessagesLeft);
+}
+
+- (void) assertLoggedNoMessages {
+    [self waitForLoggingThread];
+    XCTAssertEqual(0, logger->mNumDebugMessagesLogged);
+    XCTAssertEqual(0, logger->mNumWarningMessagesLogged);
+    XCTAssertEqual(0, logger->mNumErrorMessagesLogged);
+}
+
+- (void) assertLoggedOneDebugMessage {
+    [self waitForLoggingThread];
+    XCTAssertEqual(1, logger->mNumDebugMessagesLogged);
+    XCTAssertEqual(0, logger->mNumWarningMessagesLogged);
+    XCTAssertEqual(0, logger->mNumErrorMessagesLogged);
+}
+
+- (void) assertLoggedOneWarningMessage {
+    [self waitForLoggingThread];
+    XCTAssertEqual(0, logger->mNumDebugMessagesLogged);
+    XCTAssertEqual(1, logger->mNumWarningMessagesLogged);
+    XCTAssertEqual(0, logger->mNumErrorMessagesLogged);
+}
+
+- (void) assertLoggedOneErrorMessage {
+    [self waitForLoggingThread];
+    XCTAssertEqual(0, logger->mNumDebugMessagesLogged);
+    XCTAssertEqual(0, logger->mNumWarningMessagesLogged);
+    XCTAssertEqual(1, logger->mNumErrorMessagesLogged);
+}
+
+@end
+

BGMApp/BGMAppTests/UnitTests/BGMPlayThroughTests.mm

@@ -0,0 +1,106 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  BGMPlayThroughTests.mm
+//  BGMAppUnitTests
+//
+//  Copyright © 2020 Kyle Neideck
+//
+
+// Unit Include
+#import "BGMPlayThrough.h"
+
+// Local Includes
+#import "MockAudioDevice.h"
+#import "MockAudioObjects.h"
+
+// BGM Includes
+#import "BGM_Types.h"
+#import "BGMAudioDevice.h"
+
+// STL Includes
+#import <memory>
+
+// System Includes
+#import <XCTest/XCTest.h>
+
+
+@interface BGMPlayThroughTests : XCTestCase
+
+@end
+
+@implementation BGMPlayThroughTests {
+    BGMAudioDevice inputDevice;
+    BGMAudioDevice outputDevice;
+
+    // The unit tests use mock implementations of CAHALAudioObject and CAHALAudioDevice, which are
+    // the superclasses of BGMAudioDevice. When BGMPlayThrough calls methods on inputDevice or
+    // outputDevice, some of them will update these objects.
+    std::shared_ptr<MockAudioDevice> mockInputDevice;
+    std::shared_ptr<MockAudioDevice> mockOutputDevice;
+}
+
+- (void) setUp {
+    [super setUp];
+
+    // Set up the mocks.
+    mockInputDevice = MockAudioObjects::CreateMockDevice(kBGMDeviceUID);
+    mockOutputDevice = MockAudioObjects::CreateMockDevice("Mock Output Device");
+
+    inputDevice = BGMAudioDevice(mockInputDevice->GetObjectID());
+    outputDevice = BGMAudioDevice(mockOutputDevice->GetObjectID());
+}
+
+- (void) tearDown {
+    [super tearDown];
+    MockAudioObjects::DestroyMocks();
+}
+
+- (void) testActivate {
+    // Set the mock output device's sample rate and IO buffer size.
+    outputDevice.SetNominalSampleRate(12345.0);
+    outputDevice.SetIOBufferSize(123);
+
+    // Create an instance and activate it.
+    BGMPlayThrough playThrough(inputDevice, outputDevice);
+    playThrough.Activate();
+
+    // It should set the input device's sample rate and IO buffer size to match the output device.
+    XCTAssertEqual(12345.0, inputDevice.GetNominalSampleRate());
+    XCTAssertEqual(123, inputDevice.GetIOBufferSize());
+
+    // It should add the property listeners it needs.
+    std::set<AudioObjectPropertySelector> expectedProperties {
+            kAudioDevicePropertyDeviceIsRunning,
+            kAudioDeviceProcessorOverload,
+            kAudioDeviceCustomPropertyDeviceIsRunningSomewhereOtherThanBGMApp
+    };
+
+    XCTAssertEqual(expectedProperties, mockInputDevice->mPropertiesWithListeners);
+}
+
+- (void) testDeactivate {
+    BGMPlayThrough playThrough(inputDevice, outputDevice);
+
+    playThrough.Activate();
+    playThrough.Deactivate();
+
+    // It should remove the property listeners added by Activate.
+    XCTAssert(mockInputDevice->mPropertiesWithListeners.empty());
+}
+
+@end
+

BGMApp/BGMAppTests/UnitTests/Mocks/MockAudioDevice.cpp

@@ -0,0 +1,61 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  MockAudioDevice.cpp
+//  BGMAppUnitTests
+//
+//  Copyright © 2020 Kyle Neideck
+//
+
+// Self Include
+#include "MockAudioDevice.h"
+
+// BGM Includes
+#include "BGM_Types.h"
+
+// STL Includes
+#include <functional>
+
+
+MockAudioDevice::MockAudioDevice(const std::string& inUID)
+:
+    mUID(inUID),
+    mNominalSampleRate(44100.0),
+    mIOBufferSize(512),
+    MockAudioObject(static_cast<AudioObjectID>(std::hash<std::string>{}(inUID)))
+{
+}
+
+CACFString MockAudioDevice::GetPlayerBundleID() const
+{
+    if(mUID != kBGMDeviceUID)
+    {
+        throw "Only BGMDevice has kAudioDeviceCustomPropertyMusicPlayerBundleID";
+    }
+
+    return mPlayerBundleID;
+}
+
+void MockAudioDevice::SetPlayerBundleID(const CACFString& inPlayerBundleID)
+{
+    if(mUID != kBGMDeviceUID)
+    {
+        throw "Only BGMDevice has kAudioDeviceCustomPropertyMusicPlayerBundleID";
+    }
+
+    mPlayerBundleID = inPlayerBundleID;
+}
+

BGMApp/BGMAppTests/UnitTests/Mocks/MockAudioDevice.h

@@ -0,0 +1,73 @@
+// This file is part of Background Music.
+//
+// Background Music is free software: you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation, either version 2 of the
+// License, or (at your option) any later version.
+//
+// Background Music is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Background Music. If not, see <http://www.gnu.org/licenses/>.
+
+//
+//  MockAudioObject.h
+//  BGMAppUnitTests
+//
+//  Copyright © 2020 Kyle Neideck
+//
+
+#ifndef BGMAppUnitTests__MockAudioDevice
+#define BGMAppUnitTests__MockAudioDevice
+
+// Superclass Includes
+#include "MockAudioObject.h"
+
+// STL Includes
+#include <string>
+
+
+/*!
+ * A mock audio device in our mock CoreAudio HAL. In the HAL's API class hierarchy, the base class
+ * for audio devices, kAudioDeviceClassID, is the audio objects class, kAudioObjectClassID.
+ *
+ * The unit tests generally use instances of this class to verify the HAL is being queried correctly
+ * and to control the responses that the code they're testing will receive from the mock HAL.
+ */
+class MockAudioDevice
+:
+        public MockAudioObject
+{
+
+public:
+    MockAudioDevice(const std::string& inUID);
+
+    /*!
+     * @return This device's music player bundle ID property.
+     * @throws If this device isn't a mock of BGMDevice.
+     */
+    CACFString GetPlayerBundleID() const;
+    /*!
+     * Set this device's music player bundle ID property.
+     * @throws If this device isn't a mock of BGMDevice.
+     */
+    void SetPlayerBundleID(const CACFString& inPlayerBundleID);
+
+    /*!
+     * The device's UID. The UID is a persistent token used to identify a particular audio device
+     * across boot sessions.
+     */
+    const std::string mUID;
+    Float64 mNominalSampleRate;
+    UInt32 mIOBufferSize;
+
+private:
+    CACFString mPlayerBundleID { "" };
+
+};
+
+#endif /* BGMAppUnitTests__MockAudioDevice */
+