Fix viewability for zero-sized lists

packages/virtualized-lists/Lists/VirtualizedList.js

@@ -1250,6 +1250,7 @@ class VirtualizedList extends StateSafePureComponent<
   _pendingViewabilityUpdate: boolean = false;
   _prevParentOffset: number = 0;
   _scrollMetrics: {
+    crossAxisLength: number,
     dOffset: number,
     dt: number,
     offset: number,
@@ -1258,6 +1259,7 @@ class VirtualizedList extends StateSafePureComponent<
     visibleLength: number,
     zoomScale: number,
   } = {
+    crossAxisLength: 0,
     dOffset: 0,
     dt: 10,
     offset: 0,
@@ -1381,6 +1383,7 @@ class VirtualizedList extends StateSafePureComponent<
         this.context.getOutermostParentListRef().getScrollRef(),
         (x, y, width, height) => {
           this._offsetFromParentVirtualizedList = this._selectOffset({x, y});
+          const crossAxisLength = this._selectCrossAxisLength({width, height});
           this._listMetrics.notifyListContentLayout({
             layout: {width, height},
             orientation: this._orientation(),
@@ -1390,10 +1393,12 @@ class VirtualizedList extends StateSafePureComponent<
           );
 
           const metricsChanged =
+            this._scrollMetrics.crossAxisLength !== crossAxisLength ||
             this._scrollMetrics.visibleLength !== scrollMetrics.visibleLength ||
             this._scrollMetrics.offset !== scrollMetrics.offset;
 
           if (metricsChanged) {
+            this._scrollMetrics.crossAxisLength = crossAxisLength;
             this._scrollMetrics.visibleLength = scrollMetrics.visibleLength;
             this._scrollMetrics.offset = scrollMetrics.offset;
 
@@ -1420,6 +1425,9 @@ class VirtualizedList extends StateSafePureComponent<
   }
 
   _onLayout = (e: LayoutChangeEvent) => {
+    this._scrollMetrics.crossAxisLength = this._selectCrossAxisLength(
+      e.nativeEvent.layout,
+    );
     if (this._isNestedWithSameOrientation()) {
       // Need to adjust our scroll metrics to be relative to our containing
       // VirtualizedList before we can make claims about list item viewability
@@ -1527,6 +1535,18 @@ class VirtualizedList extends StateSafePureComponent<
       : metrics.width;
   }
 
+  _selectCrossAxisLength(
+    metrics: Readonly<{
+      height: number,
+      width: number,
+      ...
+    }>,
+  ): number {
+    return !horizontalOrDefault(this.props.horizontal)
+      ? metrics.width
+      : metrics.height;
+  }
+
   _selectOffset({x, y}: Readonly<{x: number, y: number, ...}>): number {
     return this._orientation().horizontal ? x : y;
   }
@@ -1710,6 +1730,9 @@ class VirtualizedList extends StateSafePureComponent<
       this.props.onScroll(e);
     }
     const timestamp = e.timeStamp;
+    let crossAxisLength = this._selectCrossAxisLength(
+      e.nativeEvent.layoutMeasurement,
+    );
     let visibleLength = this._selectLength(e.nativeEvent.layoutMeasurement);
     let contentLength = this._selectLength(e.nativeEvent.contentSize);
     let offset = this._offsetFromScrollEvent(e);
@@ -1726,6 +1749,7 @@ class VirtualizedList extends StateSafePureComponent<
           visibleLength,
           offset,
         }));
+      crossAxisLength = this._scrollMetrics.crossAxisLength;
     }
 
     const dt = this._scrollMetrics.timestamp
@@ -1751,6 +1775,7 @@ class VirtualizedList extends StateSafePureComponent<
     // For invalid negative values (w/ RTL), set this to 1.
     const zoomScale = e.nativeEvent.zoomScale < 0 ? 1 : e.nativeEvent.zoomScale;
     this._scrollMetrics = {
+      crossAxisLength,
       dt,
       dOffset,
       offset,
@@ -2032,11 +2057,15 @@ class VirtualizedList extends StateSafePureComponent<
     if (this.state.pendingScrollUpdateCount > 0) {
       return;
     }
+    const visibleLength =
+      this._scrollMetrics.crossAxisLength > 0
+        ? this._scrollMetrics.visibleLength
+        : 0;
     this._viewabilityTuples.forEach(tuple => {
       tuple.viewabilityHelper.onUpdate(
         props,
         this._scrollMetrics.offset,
-        this._scrollMetrics.visibleLength,
+        visibleLength,
         this._listMetrics,
         this._createViewToken,
         tuple.onViewableItemsChanged,

packages/virtualized-lists/Lists/__tests__/VirtualizedList-test.js

@@ -468,6 +468,7 @@ describe('VirtualizedList', () => {
 
     const instance = component.getInstance();
 
+    simulateViewportLayout(component, {width: 300, height: 600});
     instance._onScrollBeginDrag({nativeEvent});
     instance._onScroll({
       timeStamp: 1000,
@@ -504,6 +505,56 @@ describe('VirtualizedList', () => {
     );
   });
 
+  it('does not report viewable items when scroll metrics have an empty cross-axis viewport', async () => {
+    const data = [{key: 'i1'}, {key: 'i2'}, {key: 'i3'}];
+    const onViewableItemsChanged = jest.fn();
+    let component;
+
+    await act(() => {
+      component = create(
+        <VirtualizedList
+          data={data}
+          getItem={(items, index) => items[index]}
+          getItemCount={items => items.length}
+          getItemLayout={(items, index) => ({
+            index,
+            length: 100,
+            offset: index * 100,
+          })}
+          horizontal={true}
+          onViewableItemsChanged={onViewableItemsChanged}
+          renderItem={({item}) => <item value={item.key} />}
+        />,
+      );
+    });
+
+    component.getInstance()._onScroll({
+      timeStamp: 1000,
+      nativeEvent: {
+        contentInset: {bottom: 0, left: 0, right: 0, top: 0},
+        contentOffset: {x: 0, y: 0},
+        contentSize: {width: 300, height: 0},
+        layoutMeasurement: {width: 300, height: 0},
+        zoomScale: 1,
+      },
+    });
+
+    expect(onViewableItemsChanged).not.toHaveBeenCalled();
+
+    component.getInstance()._onScroll({
+      timeStamp: 2000,
+      nativeEvent: {
+        contentInset: {bottom: 0, left: 0, right: 0, top: 0},
+        contentOffset: {x: 0, y: 0},
+        contentSize: {width: 300, height: 100},
+        layoutMeasurement: {width: 300, height: 100},
+        zoomScale: 1,
+      },
+    });
+
+    expect(onViewableItemsChanged).toHaveBeenCalledTimes(1);
+  });
+
   it('getScrollRef for case where it returns a ScrollView', async () => {
     const listRef = createRef(null);