Fabric: Introducing UIManagerCommitHook, unified way to alter commits

Summary:
The new UIManagerCommitHook can be used to implement commit-atering features without modifying the core.
E.g. State Reconciation seems can be implemented as a coomit hook but first we will use it for a new feature called Timeline (see the coming diffs).

Changelog: [Internal] Fabric-specific internal change.

Reviewed By: sammy-SC

Differential Revision: D25221312

fbshipit-source-id: dbe41b475bc8b36e1780d81447ab43b32758bdff

ReactCommon/react/renderer/uimanager/UIManager.cpp

@@ -10,6 +10,7 @@
 #include <react/renderer/core/ShadowNodeFragment.h>
 #include <react/renderer/debug/SystraceSection.h>
 #include <react/renderer/graphics/Geometry.h>
+#include <react/renderer/uimanager/UIManagerCommitHook.h>
 
 #include <glog/logging.h>
 
@@ -330,13 +331,40 @@ ShadowTreeRegistry const &UIManager::getShadowTreeRegistry() const {
   return shadowTreeRegistry_;
 }
 
+void UIManager::registerCommitHook(
+    UIManagerCommitHook const &commitHook) const {
+  std::unique_lock<better::shared_mutex> lock(commitHookMutex_);
+  assert(
+      std::find(commitHooks_.begin(), commitHooks_.end(), &commitHook) ==
+      commitHooks_.end());
+  commitHook.commitHookWasRegistered(*this);
+  commitHooks_.push_back(&commitHook);
+}
+
+void UIManager::unregisterCommitHook(
+    UIManagerCommitHook const &commitHook) const {
+  std::unique_lock<better::shared_mutex> lock(commitHookMutex_);
+  auto iterator =
+      std::find(commitHooks_.begin(), commitHooks_.end(), &commitHook);
+  assert(iterator != commitHooks_.end());
+  commitHooks_.erase(iterator);
+  commitHook.commitHookWasUnregistered(*this);
+}
+
 #pragma mark - ShadowTreeDelegate
 
 RootShadowNode::Unshared UIManager::shadowTreeWillCommit(
     ShadowTree const &shadowTree,
     RootShadowNode::Shared const &oldRootShadowNode,
     RootShadowNode::Unshared const &newRootShadowNode) const {
+  std::shared_lock<better::shared_mutex> lock(commitHookMutex_);
+
   auto resultRootShadowNode = newRootShadowNode;
+  for (auto const *commitHook : commitHooks_) {
+    resultRootShadowNode = commitHook->shadowTreeWillCommit(
+        shadowTree, oldRootShadowNode, resultRootShadowNode);
+  }
+
   return resultRootShadowNode;
 }
 

ReactCommon/react/renderer/uimanager/UIManager.h

@@ -25,6 +25,7 @@ namespace facebook {
 namespace react {
 
 class UIManagerBinding;
+class UIManagerCommitHook;
 
 class UIManager final : public ShadowTreeDelegate {
  public:
@@ -67,6 +68,12 @@ class UIManager final : public ShadowTreeDelegate {
       std::function<void(UIManagerBinding const &uiManagerBinding)> callback)
       const;
 
+  /*
+   * Registers and unregisters a commit hook.
+   */
+  void registerCommitHook(UIManagerCommitHook const &commitHook) const;
+  void unregisterCommitHook(UIManagerCommitHook const &commitHook) const;
+
 #pragma mark - ShadowTreeDelegate
 
   void shadowTreeDidFinishTransaction(
@@ -167,6 +174,9 @@ class UIManager final : public ShadowTreeDelegate {
   // determine whether a commit should be cancelled. Only to be used
   // inside UIManagerBinding.
   std::atomic_uint_fast8_t completeRootEventCounter_{0};
+
+  mutable better::shared_mutex commitHookMutex_;
+  mutable std::vector<UIManagerCommitHook const *> commitHooks_;
 };
 
 } // namespace react

ReactCommon/react/renderer/uimanager/UIManagerCommitHook.h

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include <react/renderer/components/root/RootShadowNode.h>
+
+namespace facebook {
+namespace react {
+
+class ShadowTree;
+class UIManager;
+
+/*
+ * Implementing a commit hook allows to observe and alter Shadow Tree commits.
+ */
+class UIManagerCommitHook {
+ public:
+  /*
+   * Called right after the commit hook is registered or unregistered.
+   */
+  virtual void commitHookWasRegistered(UIManager const &uiManager) const
+      noexcept = 0;
+  virtual void commitHookWasUnregistered(UIManager const &uiManager) const
+      noexcept = 0;
+
+  /*
+   * Called right before a `ShadowTree` commits a new tree.
+   * The semantic of the method corresponds to a method of the same name
+   * from `ShadowTreeDelegate`.
+   */
+  virtual RootShadowNode::Unshared shadowTreeWillCommit(
+      ShadowTree const &shadowTree,
+      RootShadowNode::Shared const &oldRootShadowNode,
+      RootShadowNode::Unshared const &newRootShadowNode) const noexcept = 0;
+
+  virtual ~UIManagerCommitHook() noexcept = default;
+};
+
+} // namespace react
+} // namespace facebook