Refactor Event to encourage and ease migration to `RCTModernEventEmitter` support

Summary:
The `Event` interface has been improved such that:

1. `getEventData` is now a default method on Event that returns null
2. `dispatch` has a default implementation that relies on getEventName() and getEventData()
3. `dispatchModern` can detect if surfaceId and event data are present; if not, it falls back to dispatch

This will dramatically ease future migrations: at some point in the (distant) future, we can simply delete RCTEventEmitter and
all use-cases will be supported by the current `Event` class without needing to introduce a 3rd transitional interface; and 99%
of all Event classes can be simplified, delet their `dispatch` implementation and need no further work.

At their core, all Events are simply: (1) a name, (2) data, (3) a target (surfaceId and tag). The interface now reflects that but still
allows for flexibility of the data and names being generated on-demand if necessary; but for the vast majority of Event classes, code
will be dramatically simplified.

I also migrate a single Event class, ContentSizeChangeEvent, to use this new method of dispatch.

Changelog: [Android][Changed] Added convenience methods to simplify native Event classes and ease migrations

Reviewed By: mdvacca

Differential Revision: D26043325

fbshipit-source-id: bc308105f7f6e654d45fd156dbf4a2bcbc45819c

ReactAndroid/src/main/java/com/facebook/react/uimanager/events/ContentSizeChangeEvent.java

@@ -19,8 +19,13 @@ public class ContentSizeChangeEvent extends Event<ContentSizeChangeEvent> {
   private final int mWidth;
   private final int mHeight;
 
+  @Deprecated
   public ContentSizeChangeEvent(int viewTag, int width, int height) {
-    super(viewTag);
+    this(-1, viewTag, width, height);
+  }
+
+  public ContentSizeChangeEvent(int surfaceId, int viewTag, int width, int height) {
+    super(surfaceId, viewTag);
     mWidth = width;
     mHeight = height;
   }
@@ -31,10 +36,10 @@ public class ContentSizeChangeEvent extends Event<ContentSizeChangeEvent> {
   }
 
   @Override
-  public void dispatch(RCTEventEmitter rctEventEmitter) {
+  protected WritableMap getEventData() {
     WritableMap data = Arguments.createMap();
     data.putDouble("width", PixelUtil.toDIPFromPixel(mWidth));
     data.putDouble("height", PixelUtil.toDIPFromPixel(mHeight));
-    rctEventEmitter.receiveEvent(getViewTag(), EVENT_NAME, data);
+    return data;
   }
 }

ReactAndroid/src/main/java/com/facebook/react/uimanager/events/Event.java

@@ -7,14 +7,26 @@
 
 package com.facebook.react.uimanager.events;
 
+import androidx.annotation.Nullable;
+import com.facebook.react.bridge.WritableMap;
 import com.facebook.react.common.SystemClock;
+import com.facebook.react.uimanager.IllegalViewOperationException;
 
 /**
  * A UI event that can be dispatched to JS.
  *
- * <p>For dispatching events {@link EventDispatcher#dispatchEvent} should be used. Once event object
- * is passed to the EventDispatched it should no longer be used as EventDispatcher may decide to
- * recycle that object (by calling {@link #dispose}).
+ * <p>For dispatching events {@code getEventData} should be used. Once event object is passed to the
+ * EventDispatched it should no longer be used as EventDispatcher may decide to recycle that object
+ * (by calling {@link #dispose}).
+ *
+ * <p>If you need advanced customizations and overriding only {@code getEventData} doesn't work for
+ * you, you must override both {@code dispatch} and {@code dispatchModern}. Both of these will be
+ * deleted in the distant future and it is highly recommended to use only {@code getEventData}.
+ *
+ * <p>Old, pre-Fabric Events only used viewTag as the identifier, but Fabric needs surfaceId as well
+ * as viewTag. You may use {@code UIManagerHelper.getSurfaceId} on a Fabric-managed View to get the
+ * surfaceId. Fabric will work without surfaceId - making {@code Event} backwards-compatible - but
+ * Events without SurfaceId are slightly slower to propagate.
  */
 public abstract class Event<T extends Event> {
 
@@ -119,15 +131,45 @@ public abstract class Event<T extends Event> {
 
   /**
    * Dispatch this event to JS using the given event emitter. Compatible with old and new renderer.
+   * Instead of using this or dispatchModern, it is recommended that you simply override
+   * `getEventData`. In the future
    */
   @Deprecated
-  public abstract void dispatch(RCTEventEmitter rctEventEmitter);
+  public void dispatch(RCTEventEmitter rctEventEmitter) {
+    WritableMap eventData = getEventData();
+    if (eventData == null) {
+      throw new IllegalViewOperationException(
+          "Event: you must return a valid, non-null value from `getEventData`, or override `dispatch` and `disatchModern`. Event: "
+              + getEventName());
+    }
+    rctEventEmitter.receiveEvent(getViewTag(), getEventName(), eventData);
+  }
 
   /**
-   * Dispatch this event to JS using a V2 EventEmitter. Events must explicitly override this, by
-   * default it uses the V1 dispatcher.
+   * Can be overridden by classes to make migrating to RCTModernEventEmitter support easier. If this
+   * class returns null, the RCTEventEmitter interface will be used instead of
+   * RCTModernEventEmitter. In the future, returning null here will be an error.
    */
+  @Nullable
+  protected WritableMap getEventData() {
+    return null;
+  }
+
+  /**
+   * Dispatch this event to JS using a V2 EventEmitter. If surfaceId is not -1 and `getEventData` is
+   * non-null, this will use the RCTModernEventEmitter API. Otherwise, it falls back to the
+   * old-style dispatch function. For Event classes that need to do something different, this method
+   * can always be overridden entirely, but it is not recommended.
+   */
+  @Deprecated
   public void dispatchModern(RCTModernEventEmitter rctEventEmitter) {
+    if (getSurfaceId() != -1) {
+      WritableMap eventData = getEventData();
+      if (eventData != null) {
+        rctEventEmitter.receiveEvent(getSurfaceId(), getViewTag(), getEventName(), getEventData());
+        return;
+      }
+    }
     dispatch(rctEventEmitter);
   }
 }

ReactAndroid/src/main/java/com/facebook/react/views/image/ImageLoadEvent.java

@@ -12,8 +12,6 @@ import androidx.annotation.Nullable;
 import com.facebook.react.bridge.Arguments;
 import com.facebook.react.bridge.WritableMap;
 import com.facebook.react.uimanager.events.Event;
-import com.facebook.react.uimanager.events.RCTEventEmitter;
-import com.facebook.react.uimanager.events.RCTModernEventEmitter;
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 
@@ -146,30 +144,18 @@ public class ImageLoadEvent extends Event<ImageLoadEvent> {
   }
 
   @Override
-  public void dispatch(RCTEventEmitter rctEventEmitter) {
-    rctEventEmitter.receiveEvent(getViewTag(), getEventName(), getEventData());
-  }
-
-  @Override
-  public void dispatchModern(RCTModernEventEmitter rctEventEmitter) {
-    rctEventEmitter.receiveEvent(getSurfaceId(), getViewTag(), getEventName(), getEventData());
-  }
-
-  private WritableMap getEventData() {
-    WritableMap eventData = null;
+  protected WritableMap getEventData() {
+    WritableMap eventData = Arguments.createMap();
 
     switch (mEventType) {
       case ON_PROGRESS:
-        eventData = Arguments.createMap();
         eventData.putInt("loaded", mLoaded);
         eventData.putInt("total", mTotal);
         break;
       case ON_LOAD:
-        eventData = Arguments.createMap();
         eventData.putMap("source", createEventDataSource());
         break;
       case ON_ERROR:
-        eventData = Arguments.createMap();
         eventData.putString("error", mErrorMessage);
         break;
     }