VirtualizedList: fix bug where onViewableItemsChanged wouldn't trigger

Summary:
In the current implementation of the `VirtualizedList` the `onViewableItemsChanged` callback wouldn't trigger if the underlying list data changes. (see example snack https://snack.expo.io/Hk5703eBb)

I added a method in the `ViewabilityHelper` to invalidate the cached viewableIndices, which gets triggered when the list-data changes.
Closes facebook#14922

Differential Revision: D5864537

Pulled By: sahrens

fbshipit-source-id: 37f617763596244208548817d5b138dadc12c75d

Author: formatlos

Libraries/Lists/ViewabilityHelper.js

@@ -223,6 +223,13 @@ class ViewabilityHelper {
     }
   }
 
+  /**
+   * clean-up cached _viewableIndices to evaluate changed items on next update
+   */
+  resetViewableIndices() {
+    this._viewableIndices = [];
+  }
+
   /**
    * Records that an interaction has happened even if there has been no scroll.
    */

Libraries/Lists/VirtualizedList.js

@@ -516,6 +516,12 @@ class VirtualizedList extends React.PureComponent<Props, State> {
     });
     if (data !== this.props.data || extraData !== this.props.extraData) {
       this._hasDataChangedSinceEndReached = true;
+
+      // clear the viewableIndices cache to also trigger
+      // the onViewableItemsChanged callback with the new data
+      this._viewabilityTuples.forEach(tuple => {
+        tuple.viewabilityHelper.resetViewableIndices();
+      });
     }
   }
 

Libraries/Lists/__tests__/ViewabilityHelper-test.js

@@ -385,4 +385,54 @@ describe('onUpdate', function() {
       viewableItems: [{isViewable: true, key: 'a'}],
     });
   });
+
+  it('returns the right visible row after the underlying data changed', function() {
+    const helper = new ViewabilityHelper();
+    rowFrames = {
+      a: {y: 0, height: 200},
+      b: {y: 200, height: 200},
+    };
+    data = [{key: 'a'}, {key: 'b'}];
+    const onViewableItemsChanged = jest.fn();
+    helper.onUpdate(
+      data.length,
+      0,
+      200,
+      getFrameMetrics,
+      createViewToken,
+      onViewableItemsChanged,
+    );
+    expect(onViewableItemsChanged.mock.calls.length).toBe(1);
+    expect(onViewableItemsChanged.mock.calls[0][0]).toEqual({
+      changed: [{isViewable: true, key: 'a'}],
+      viewabilityConfig: {viewAreaCoveragePercentThreshold: 0},
+      viewableItems: [{isViewable: true, key: 'a'}],
+    });
+
+    // update data
+    rowFrames = {
+      c: {y: 0, height: 200},
+      a: {y: 200, height: 200},
+      b: {y: 400, height: 200},
+    };
+    data = [{key: 'c'}, {key: 'a'}, {key: 'b'}];
+
+    helper.resetViewableIndices();
+
+    helper.onUpdate(
+      data.length,
+      0,
+      200,
+      getFrameMetrics,
+      createViewToken,
+      onViewableItemsChanged,
+    );
+
+    expect(onViewableItemsChanged.mock.calls.length).toBe(2);
+    expect(onViewableItemsChanged.mock.calls[1][0]).toEqual({
+      changed: [{isViewable: true, key: 'c'}, {isViewable: false, key: 'a'}],
+      viewabilityConfig: {viewAreaCoveragePercentThreshold: 0},
+      viewableItems: [{isViewable: true, key: 'c'}],
+    });
+  });
 });

Libraries/Lists/__tests__/VirtualizedList-test.js

@@ -160,4 +160,63 @@ describe('VirtualizedList', () => {
     );
     expect(component).toMatchSnapshot();
   });
+
+  it('returns the viewableItems correctly in the onViewableItemsChanged callback after changing the data', () => {
+    const ITEM_HEIGHT = 800;
+    let data = [{key: 'i1'}, {key: 'i2'}, {key: 'i3'}];
+    const nativeEvent = {
+      contentOffset: {y: 0, x: 0},
+      layoutMeasurement: {width: 300, height: 600},
+      contentSize: {width: 300, height: data.length * ITEM_HEIGHT},
+      zoomScale: 1,
+      contentInset: {right: 0, top: 0, left: 0, bottom: 0},
+    };
+    const onViewableItemsChanged = jest.fn();
+    const props = {
+      data,
+      renderItem: ({item}) => <item value={item.key} />,
+      getItem: (data, index) => data[index],
+      getItemCount: data => data.length,
+      getItemLayout: (data, index) => ({
+        length: ITEM_HEIGHT,
+        offset: ITEM_HEIGHT * index,
+        index,
+      }),
+      onViewableItemsChanged,
+    };
+
+    const component = ReactTestRenderer.create(<VirtualizedList {...props} />);
+
+    const instance = component.getInstance();
+
+    instance._onScrollBeginDrag({nativeEvent});
+    instance._onScroll({
+      timeStamp: 1000,
+      nativeEvent,
+    });
+
+    expect(onViewableItemsChanged).toHaveBeenCalledTimes(1);
+    expect(onViewableItemsChanged).toHaveBeenLastCalledWith(
+      expect.objectContaining({
+        viewableItems: [expect.objectContaining({isViewable: true, key: 'i1'})],
+      }),
+    );
+    data = [{key: 'i4'}, ...data];
+    component.update(<VirtualizedList {...props} data={data} />);
+
+    instance._onScroll({
+      timeStamp: 2000,
+      nativeEvent: {
+        ...nativeEvent,
+        contentOffset: {y: 100, x: 0},
+      },
+    });
+
+    expect(onViewableItemsChanged).toHaveBeenCalledTimes(2);
+    expect(onViewableItemsChanged).toHaveBeenLastCalledWith(
+      expect.objectContaining({
+        viewableItems: [expect.objectContaining({isViewable: true, key: 'i4'})],
+      }),
+    );
+  });
 });