Fix issue with onEndReached

Summary:
onEndReached can be triggered twice when more items are added to the end of the list. This change makes it so that a second call to onEndReached won't happen until the user scrolls down to the new end of the list.

Changelog:

[General] [Fixed] - Fix double call to onEndReached in VirtualizedList

Reviewed By: sahrens

Differential Revision: D20066740

fbshipit-source-id: 129d7ae6bfd241eeea18fe0bb12b82be67735874

Author: MartinSherburn

Libraries/Lists/VirtualizedList.js

@@ -1218,8 +1218,6 @@ class VirtualizedList extends React.PureComponent<Props, State> {
   componentDidUpdate(prevProps: Props) {
     const {data, extraData} = this.props;
     if (data !== prevProps.data || extraData !== prevProps.extraData) {
-      this._hasDataChangedSinceEndReached = true;
-
       // clear the viewableIndices cache to also trigger
       // the onViewableItemsChanged callback with the new data
       this._viewabilityTuples.forEach(tuple => {
@@ -1248,7 +1246,6 @@ class VirtualizedList extends React.PureComponent<Props, State> {
   _fillRateHelper: FillRateHelper;
   _frames = {};
   _footerLength = 0;
-  _hasDataChangedSinceEndReached = true;
   _hasDoneInitialScroll = false;
   _hasInteracted = false;
   _hasMore = false;
@@ -1546,20 +1543,22 @@ class VirtualizedList extends React.PureComponent<Props, State> {
     } = this.props;
     const {contentLength, visibleLength, offset} = this._scrollMetrics;
     const distanceFromEnd = contentLength - visibleLength - offset;
+    const threshold = onEndReachedThreshold
+      ? onEndReachedThreshold * visibleLength
+      : 0;
     if (
       onEndReached &&
       this.state.last === getItemCount(data) - 1 &&
-      /* $FlowFixMe(>=0.63.0 site=react_native_fb) This comment suppresses an
-       * error found when Flow v0.63 was deployed. To see the error delete this
-       * comment and run Flow. */
-      distanceFromEnd < onEndReachedThreshold * visibleLength &&
-      (this._hasDataChangedSinceEndReached ||
-        this._scrollMetrics.contentLength !== this._sentEndForContentLength)
+      distanceFromEnd < threshold &&
+      this._scrollMetrics.contentLength !== this._sentEndForContentLength
     ) {
-      // Only call onEndReached once for a given dataset + content length.
-      this._hasDataChangedSinceEndReached = false;
+      // Only call onEndReached once for a given content length
       this._sentEndForContentLength = this._scrollMetrics.contentLength;
       onEndReached({distanceFromEnd});
+    } else if (distanceFromEnd > threshold) {
+      // If the user scrolls away from the end and back again cause
+      // an onEndReached to be triggered again
+      this._sentEndForContentLength = 0;
     }
   }