Allow horizontal scroll view also retry side-effects when content vie…

…w is not mounted

Summary:
This diff fixed a NPE in horizontal scroll view when calling scrollToEnd API in side effect.

The problem here is we may trigger side-effects before the required view got mounted for performance reasons. We've been fixing this with retry logic on those side-effects and we've already done this in vertical scroll view. This is to fix on the horizontal scroll as well.

Changelog:
[Android][Fixed] - Fixed HorizontalScrollView API scrollToEnd causing NPE in side-effects.

Reviewed By: lunaleaps, JoshuaGross

Differential Revision: D37571847

fbshipit-source-id: 0a4dc38381008350fd09908aa3ebb64e3e65a424

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

@@ -8,10 +8,12 @@
 package com.facebook.react.views.scroll;
 
 import android.graphics.Color;
+import android.view.View;
 import androidx.annotation.Nullable;
 import androidx.core.view.ViewCompat;
 import com.facebook.react.bridge.ReadableArray;
 import com.facebook.react.bridge.ReadableMap;
+import com.facebook.react.bridge.RetryableMountingLayerException;
 import com.facebook.react.module.annotations.ReactModule;
 import com.facebook.react.uimanager.PixelUtil;
 import com.facebook.react.uimanager.PointerEvents;
@@ -208,8 +210,15 @@ public void scrollTo(
   public void scrollToEnd(
       ReactHorizontalScrollView scrollView,
       ReactScrollViewCommandHelper.ScrollToEndCommandData data) {
-    // ScrollView always has one child - the scrollable area
-    int right = scrollView.getChildAt(0).getWidth() + scrollView.getPaddingRight();
+    // ScrollView always has one child - the scrollable area. However, it's possible today that we
+    // execute this method as view command before the child view is mounted. Here we will retry the
+    // view commands as a workaround.
+    @Nullable View child = scrollView.getChildAt(0);
+    if (child == null) {
+      throw new RetryableMountingLayerException(
+          "scrollToEnd called on HorizontalScrollView without child");
+    }
+    int right = child.getWidth() + scrollView.getPaddingRight();
     if (data.mAnimated) {
       scrollView.reactSmoothScrollTo(right, scrollView.getScrollY());
     } else {

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

@@ -285,6 +285,9 @@ public void setOverflow(ReactScrollView view, @Nullable String overflow) {
   @Override
   public void scrollToEnd(
       ReactScrollView scrollView, ReactScrollViewCommandHelper.ScrollToEndCommandData data) {
+    // ScrollView always has one child - the scrollable area. However, it's possible today that we
+    // execute this method as view command before the child view is mounted. Here we will retry the
+    // view commands as a workaround.
     View child = scrollView.getChildAt(0);
     if (child == null) {
       throw new RetryableMountingLayerException("scrollToEnd called on ScrollView without child");