Add fabric support for maintainVisibleContentPosition on Android

ReactAndroid/src/main/java/com/facebook/react/animated/NativeAnimatedModule.java

@@ -298,6 +298,16 @@ public void didScheduleMountItems(UIManager uiManager) {
     mCurrentFrameNumber++;
   }
 
+  @Override
+  public void willMountItems(UIManager uiManager) {
+    // noop
+  }
+
+  @Override
+  public void didMountItems(UIManager uiManager) {
+    // noop
+  }
+
   // For FabricUIManager only
   @Override
   @UiThread

ReactAndroid/src/main/java/com/facebook/react/bridge/UIManagerListener.java

@@ -12,10 +12,33 @@ public interface UIManagerListener {
   /**
    * Called right before view updates are dispatched at the end of a batch. This is useful if a
    * module needs to add UIBlocks to the queue before it is flushed.
+   *
+   * This is called by Paper only.
    */
   void willDispatchViewUpdates(UIManager uiManager);
-  /* Called right after view updates are dispatched for a frame. */
+  /**
+   * Called on UIThread right before view updates are executed.
+   *
+   * This is called by Fabric only.
+   */
+  void willMountItems(UIManager uiManager);
+  /**
+   * Called on UIThread right after view updates are executed.
+   *
+   * This is called by Fabric only.
+   */
+  void didMountItems(UIManager uiManager);
+  /**
+   * Called on UIThread right after view updates are dispatched for a frame. Note that this will be called
+   * for every frame even if there are no updates.
+   *
+   * This is called by Fabric only.
+   */
   void didDispatchMountItems(UIManager uiManager);
-  /* Called right after scheduleMountItems is called in Fabric, after a new tree is committed. */
+  /**
+   * Called right after scheduleMountItems is called in Fabric, after a new tree is committed.
+   *
+   * This is called by Fabric only.
+   */
   void didScheduleMountItems(UIManager uiManager);
 }

ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java

@@ -1202,6 +1202,20 @@ public Map<String, Long> getPerformanceCounters() {
   }
 
   private class MountItemDispatchListener implements MountItemDispatcher.ItemDispatchListener {
+    @Override
+    public void willMountItems() {
+      for (UIManagerListener listener : mListeners) {
+        listener.willMountItems(FabricUIManager.this);
+      }
+    }
+
+    @Override
+    public void didMountItems() {
+      for (UIManagerListener listener : mListeners) {
+        listener.didMountItems(FabricUIManager.this);
+      }
+    }
+
     @Override
     public void didDispatchMountItems() {
       for (UIManagerListener listener : mListeners) {

ReactAndroid/src/main/java/com/facebook/react/fabric/mounting/MountItemDispatcher.java

@@ -193,6 +193,8 @@ private boolean dispatchMountItems() {
       return false;
     }
 
+    mItemDispatchListener.willMountItems();
+
     // As an optimization, execute all ViewCommands first
     // This should be:
     // 1) Performant: ViewCommands are often a replacement for SetNativeProps, which we've always
@@ -299,6 +301,9 @@ private boolean dispatchMountItems() {
       }
       mBatchedExecutionTime += SystemClock.uptimeMillis() - batchedExecutionStartTime;
     }
+
+    mItemDispatchListener.didMountItems();
+
     Systrace.endSection(Systrace.TRACE_TAG_REACT_JAVA_BRIDGE);
 
     return true;
@@ -415,6 +420,8 @@ private static void printMountItem(MountItem mountItem, String prefix) {
   }
 
   public interface ItemDispatchListener {
+    void willMountItems();
+    void didMountItems();
     void didDispatchMountItems();
   }
 }

ReactAndroid/src/main/java/com/facebook/react/views/scroll/MaintainVisibleScrollPositionHelper.java

@@ -18,6 +18,7 @@
 import com.facebook.react.bridge.UIManagerListener;
 import com.facebook.react.bridge.UiThreadUtil;
 import com.facebook.react.uimanager.UIManagerHelper;
+import com.facebook.react.uimanager.common.UIManagerType;
 import com.facebook.react.uimanager.common.ViewUtil;
 import com.facebook.react.views.scroll.ReactScrollViewHelper.HasSmoothScroll;
 import com.facebook.react.views.view.ReactViewGroup;
@@ -89,6 +90,14 @@ public void stop() {
    * been updated.
    */
   public void updateScrollPosition() {
+    // On Fabric this will be called internally in `didMountItems`.
+    if (ViewUtil.getUIManagerType(mScrollView.getId()) == UIManagerType.FABRIC) {
+      return;
+    }
+    updateScrollPositionInternal();
+  }
+
+  private void updateScrollPositionInternal() {
     if (mConfig == null || mFirstVisibleView == null || mPrevFirstVisibleFrame == null) {
       return;
     }
@@ -169,6 +178,16 @@ public void run() {
         });
   }
 
+  @Override
+  public void willMountItems(UIManager uiManager) {
+    computeTargetView();
+  }
+
+  @Override
+  public void didMountItems(UIManager uiManager) {
+    updateScrollPositionInternal();
+  }
+
   @Override
   public void didDispatchMountItems(UIManager uiManager) {
     // noop