Implement onViewAppear by creating a new EventListener on ReactRootView listening for when it's attached to a RN Instance

Reviewed By: AaaChiuuu

Differential Revision: D4269272

fbshipit-source-id: 6dd54f8ab7befebd54e82ef355119ba302b9bfe5

Author: donyu

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

@@ -56,10 +56,21 @@
  */
 public class ReactRootView extends SizeMonitoringFrameLayout implements RootView {
 
+  /**
+   * Listener interface for react root view events
+   */
+  public interface ReactRootViewEventListener {
+    /**
+     * Called when the react context is attached to a ReactRootView.
+     */
+    void onAttachedToReactInstance(ReactRootView rootView);
+  }
+
   private @Nullable ReactInstanceManager mReactInstanceManager;
   private @Nullable String mJSModuleName;
   private @Nullable Bundle mLaunchOptions;
   private @Nullable CustomGlobalLayoutListener mCustomGlobalLayoutListener;
+  private @Nullable ReactRootViewEventListener mRootViewEventListener;
   private int mRootViewTag;
   private boolean mWasMeasured = false;
   private boolean mIsAttachedToInstance = false;
@@ -224,6 +235,16 @@ public void unmountReactApplication() {
     }
   }
 
+  public void onAttachedToReactInstance() {
+    if (mRootViewEventListener != null) {
+      mRootViewEventListener.onAttachedToReactInstance(this);
+    }
+  }
+
+  public void setEventListener(ReactRootViewEventListener eventListener) {
+    mRootViewEventListener = eventListener;
+  }
+
   /* package */ String getJSModuleName() {
     return Assertions.assertNotNull(mJSModuleName);
   }

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

@@ -810,6 +810,7 @@ private void attachMeasuredRootViewToInstance(
     appParams.putDouble("rootTag", rootTag);
     appParams.putMap("initialProps", initialProps);
     catalystInstance.getJSModule(AppRegistry.class).runApplication(jsAppModuleName, appParams);
+    rootView.onAttachedToReactInstance();
     Systrace.endSection(TRACE_TAG_REACT_JAVA_BRIDGE);
   }