Add ReactFragment for Android

Summary:
React Native on Android has currently been focused and targeted at using an Activity for its main form of instantiation.
While this has probably worked for most companies and developers, you lose some of the modularity of a more cohesive application when working in a "brown-field" project that is currently native. This hurts more companies that are looking to adopt React Native and slowly implement it in a fully native application.
A lot of developers follow Android's guidelines of using Fragments in their projects, even if it is a debated subject in the Android community, and this addition will allow others to embrace React Native more freely. (I even assume it could help with managing navigation state in applications that contain a decent amount of Native code and would be appreciated in those projects. Such as sharing the Toolbar, TabBar, ViewPager, etc in Native Android)
Even with this addition, a developer will still need to host the fragment in an activity, but now that activity can contain native logic like a Drawer, Tabs, ViewPager, etc.
Test plan (required)
We have been using this class at Hudl for over a couple of months and have found it valuable.
If the community agrees on the addition, I can add documentation to the Android sections to include notes about the potential of this Fragment.
If the community agrees on the addition, I can update one or more of the examples in the /Examples folder and make use of the Fragment, or even create a new example that uses a native layout manager like Drawer, Tabs, Viewpager, etc)
Make sure tests pass on both Travis and Circle CI.
_To Note:_
There is also talk of using React Native inside Android Fragment's without any legit documentation, this could help remedy some of that with more documentation included in this PR https://facebook.github.io/react-native/releases/0.26/docs/embedded-app-android.html#sharing-a-reactinstance-across-multiple-activities-fragments-in-your-app
Others have also requested something similar and have a half-baked solution as well http://stackoverflow.com/questions/35221447/react-native-inside-a-fragment
Release Notes
[ANDROID][FEATURE][ReactAndroid/src/main/java/com/facebook/react/ReactFragment.java] - Adds support for Android's Fragment system. This allows for a more hybrid application.

Reviewed By: cpojer

Differential Revision: D15731340

fbshipit-source-id: 74b7aaedcfd6ad6e074ff911cd7f18a5111caf5c

ReactAndroid/src/main/java/com/facebook/react/ReactActivityDelegate.java

@@ -15,8 +15,7 @@ import android.view.KeyEvent;
 
 import com.facebook.infer.annotation.Assertions;
 import com.facebook.react.bridge.Callback;
-import com.facebook.react.devsupport.DoubleTapReloadRecognizer;
-import com.facebook.react.modules.core.DefaultHardwareBackBtnHandler;
+import com.facebook.react.uimanager.RootView;
 import com.facebook.react.modules.core.PermissionListener;
 
 import javax.annotation.Nullable;
@@ -31,10 +30,9 @@ public class ReactActivityDelegate {
   private final @Nullable Activity mActivity;
   private final @Nullable String mMainComponentName;
 
-  private @Nullable ReactRootView mReactRootView;
-  private @Nullable DoubleTapReloadRecognizer mDoubleTapReloadRecognizer;
   private @Nullable PermissionListener mPermissionListener;
   private @Nullable Callback mPermissionsCallback;
+  private ReactDelegate mReactDelegate;
 
   @Deprecated
   public ReactActivityDelegate(Activity activity, @Nullable String mainComponentName) {
@@ -52,7 +50,7 @@ public class ReactActivityDelegate {
   }
 
   protected ReactRootView createRootView() {
-    return new ReactRootView(getContext());
+    return mReactDelegate.createRootView();
   }
 
   /**
@@ -67,7 +65,7 @@ public class ReactActivityDelegate {
   }
 
   public ReactInstanceManager getReactInstanceManager() {
-    return getReactNativeHost().getReactInstanceManager();
+    return mReactDelegate.getReactInstanceManager();
   }
 
   public String getMainComponentName() {
@@ -76,36 +74,23 @@ public class ReactActivityDelegate {
 
   protected void onCreate(Bundle savedInstanceState) {
     String mainComponentName = getMainComponentName();
-    if (mainComponentName != null) {
+    mReactDelegate = new ReactDelegate(getPlainActivity(), getReactNativeHost(), mainComponentName, getLaunchOptions());
+    if (mMainComponentName != null) {
       loadApp(mainComponentName);
     }
-    mDoubleTapReloadRecognizer = new DoubleTapReloadRecognizer();
   }
 
   protected void loadApp(String appKey) {
-    if (mReactRootView != null) {
-      throw new IllegalStateException("Cannot loadApp while app is already running.");
-    }
-    mReactRootView = createRootView();
-    mReactRootView.startReactApplication(
-      getReactNativeHost().getReactInstanceManager(),
-      appKey,
-      getLaunchOptions());
-    getPlainActivity().setContentView(mReactRootView);
+    mReactDelegate.loadApp(appKey);
+    getPlainActivity().setContentView(mReactDelegate.getReactRootView());
   }
 
   protected void onPause() {
-    if (getReactNativeHost().hasInstance()) {
-      getReactNativeHost().getReactInstanceManager().onHostPause(getPlainActivity());
-    }
+    mReactDelegate.onHostPause();
   }
 
   protected void onResume() {
-    if (getReactNativeHost().hasInstance()) {
-      getReactNativeHost().getReactInstanceManager().onHostResume(
-        getPlainActivity(),
-        (DefaultHardwareBackBtnHandler) getPlainActivity());
-    }
+    mReactDelegate.onHostResume();
 
     if (mPermissionsCallback != null) {
       mPermissionsCallback.invoke();
@@ -114,20 +99,11 @@ public class ReactActivityDelegate {
   }
 
   protected void onDestroy() {
-    if (mReactRootView != null) {
-      mReactRootView.unmountReactApplication();
-      mReactRootView = null;
-    }
-    if (getReactNativeHost().hasInstance()) {
-      getReactNativeHost().getReactInstanceManager().onHostDestroy(getPlainActivity());
-    }
+    mReactDelegate.onHostDestroy();
   }
 
   public void onActivityResult(int requestCode, int resultCode, Intent data) {
-    if (getReactNativeHost().hasInstance()) {
-      getReactNativeHost().getReactInstanceManager()
-        .onActivityResult(getPlainActivity(), requestCode, resultCode, data);
-    }
+    mReactDelegate.onActivityResult(requestCode, resultCode, data, true);
   }
 
   public boolean onKeyDown(int keyCode, KeyEvent event) {
@@ -141,19 +117,7 @@ public class ReactActivityDelegate {
   }
 
   public boolean onKeyUp(int keyCode, KeyEvent event) {
-    if (getReactNativeHost().hasInstance() && getReactNativeHost().getUseDeveloperSupport()) {
-      if (keyCode == KeyEvent.KEYCODE_MENU) {
-        getReactNativeHost().getReactInstanceManager().showDevOptionsDialog();
-        return true;
-      }
-      boolean didDoubleTapR = Assertions.assertNotNull(mDoubleTapReloadRecognizer)
-        .didDoubleTapR(keyCode, getPlainActivity().getCurrentFocus());
-      if (didDoubleTapR) {
-        getReactNativeHost().getReactInstanceManager().getDevSupportManager().handleReloadJS();
-        return true;
-      }
-    }
-    return false;
+    return mReactDelegate.shouldShowDevMenuOrReload(keyCode, event);
   }
 
   public boolean onKeyLongPress(int keyCode, KeyEvent event) {
@@ -167,11 +131,7 @@ public class ReactActivityDelegate {
   }
 
   public boolean onBackPressed() {
-    if (getReactNativeHost().hasInstance()) {
-      getReactNativeHost().getReactInstanceManager().onBackPressed();
-      return true;
-    }
-    return false;
+    return mReactDelegate.onBackPressed();
   }
 
   public boolean onNewIntent(Intent intent) {