Fix fling and snap in recycler viewgroup where children views not fil…

…l up all the scrollable space

Summary:
This diff fixes an edge case where fling and snap failed to find the correct target position when children views not fill up all the scrollable space. In this case, the target position would be calculated as the end of the scrollable space, which case the snap logic to go to the end of the scrollable area, instead of stop at the expected snapping position.

Changelog:
[Android][Fixed] - Fix fling and snap with recycler viewgroup where fling to the end of scrollable distance when it goes over current rendered children views.

Reviewed By: mdvacca

Differential Revision: D32565459

fbshipit-source-id: 319ef6e2d4e1c4deb9e45ed02c1bff7d807575c3

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

@@ -968,6 +968,8 @@ private void flingAndSnap(int velocityX) {
                 maximumOffset);
       } else {
         ViewGroup contentView = (ViewGroup) getContentView();
+        int smallerChildOffset = largerOffset;
+        int largerChildOffset = smallerOffset;
         for (int i = 0; i < contentView.getChildCount(); i++) {
           View item = contentView.getChildAt(i);
           int itemStartOffset =
@@ -983,7 +985,16 @@ private void flingAndSnap(int velocityX) {
               largerOffset = itemStartOffset;
             }
           }
+
+          smallerChildOffset = Math.min(smallerChildOffset, itemStartOffset);
+          largerChildOffset = Math.max(largerChildOffset, itemStartOffset);
         }
+
+        // For Recycler ViewGroup, the maximumOffset can be much larger than the total heights of
+        // items in the layout. In this case snapping is not possible beyond the currently rendered
+        // children.
+        smallerOffset = Math.max(smallerOffset, smallerChildOffset);
+        largerOffset = Math.min(largerOffset, largerChildOffset);
       }
     } else {
       double interval = getSnapInterval();
@@ -994,7 +1005,9 @@ private void flingAndSnap(int velocityX) {
 
     // Calculate the nearest offset
     int nearestOffset =
-        targetOffset - smallerOffset < largerOffset - targetOffset ? smallerOffset : largerOffset;
+        Math.abs(targetOffset - smallerOffset) < Math.abs(largerOffset - targetOffset)
+            ? smallerOffset
+            : largerOffset;
 
     // if scrolling after the last snap offset and snapping to the
     // end of the list is disabled, then we allow free scrolling

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

@@ -747,6 +747,8 @@ private void flingAndSnap(int velocityY) {
                 maximumOffset);
       } else {
         ViewGroup contentView = (ViewGroup) getContentView();
+        int smallerChildOffset = largerOffset;
+        int largerChildOffset = smallerOffset;
         for (int i = 0; i < contentView.getChildCount(); i++) {
           View item = contentView.getChildAt(i);
           int itemStartOffset;
@@ -774,7 +776,16 @@ private void flingAndSnap(int velocityY) {
               largerOffset = itemStartOffset;
             }
           }
+
+          smallerChildOffset = Math.min(smallerChildOffset, itemStartOffset);
+          largerChildOffset = Math.max(largerChildOffset, itemStartOffset);
         }
+
+        // For Recycler ViewGroup, the maximumOffset can be much larger than the total heights of
+        // items in the layout. In this case snapping is not possible beyond the currently rendered
+        // children.
+        smallerOffset = Math.max(smallerOffset, smallerChildOffset);
+        largerOffset = Math.min(largerOffset, largerChildOffset);
       }
     } else {
       double interval = (double) getSnapInterval();
@@ -785,7 +796,9 @@ private void flingAndSnap(int velocityY) {
 
     // Calculate the nearest offset
     int nearestOffset =
-        targetOffset - smallerOffset < largerOffset - targetOffset ? smallerOffset : largerOffset;
+        Math.abs(targetOffset - smallerOffset) < Math.abs(largerOffset - targetOffset)
+            ? smallerOffset
+            : largerOffset;
 
     // if scrolling after the last snap offset and snapping to the
     // end of the list is disabled, then we allow free scrolling