Fixing onEndReachedThreshold === 0 not firing onEndReached function on Android.

Summary:
Changelog:
[Android][Fix] - When `onEndReachedThreshold` is set to 0 `onEndReached` function on `VirtualizedList` properly fires once the user scrolls to the bottom of the list.

Reviewed By: genkikondo

Differential Revision: D36488189

fbshipit-source-id: b95f3291f2957273280696d8840c1e000d669380

Author: Christian Ruink

Libraries/Lists/VirtualizedList.js

@@ -42,6 +42,8 @@ const FillRateHelper = require('./FillRateHelper');
 const ViewabilityHelper = require('./ViewabilityHelper');
 const invariant = require('invariant');
 
+const ON_END_REACHED_EPSILON = 0.001;
+
 type Item = any;
 
 export type Separators = {
@@ -218,7 +220,8 @@ type OptionalProps = {|
    * How far from the end (in units of visible length of the list) the bottom edge of the
    * list must be from the end of the content to trigger the `onEndReached` callback.
    * Thus a value of 0.5 will trigger `onEndReached` when the end of the content is
-   * within half the visible length of the list.
+   * within half the visible length of the list. A value of 0 will not trigger until scrolling
+   * to the very end of the list.
    */
   onEndReachedThreshold?: ?number,
   /**
@@ -1516,13 +1519,22 @@ class VirtualizedList extends React.PureComponent<Props, State> {
     const {data, getItemCount, onEndReached, onEndReachedThreshold} =
       this.props;
     const {contentLength, visibleLength, offset} = this._scrollMetrics;
-    const distanceFromEnd = contentLength - visibleLength - offset;
+    let distanceFromEnd = contentLength - visibleLength - offset;
+
+    // Especially when oERT is zero it's necessary to 'floor' very small distanceFromEnd values to be 0
+    // since debouncing causes us to not fire this event for every single "pixel" we scroll and can thus
+    // be at the "end" of the list with a distanceFromEnd approximating 0 but not quite there.
+    if (distanceFromEnd < ON_END_REACHED_EPSILON) {
+      distanceFromEnd = 0;
+    }
+
+    // TODO: T121172172 Look into why we're "defaulting" to a threshold of 2 when oERT is not present
     const threshold =
       onEndReachedThreshold != null ? onEndReachedThreshold * visibleLength : 2;
     if (
       onEndReached &&
       this.state.last === getItemCount(data) - 1 &&
-      distanceFromEnd < threshold &&
+      distanceFromEnd <= threshold &&
       this._scrollMetrics.contentLength !== this._sentEndForContentLength
     ) {
       // Only call onEndReached once for a given content length