Add getNativeScrollRef to FlatList

Summary:
Add a method to get the underlying host component of `FlatList`. Fix flow types in `FlatList` and `VirtualizedList`. Add test cases to test the behavior of the new function in all cases.

Changelog: [General] [Added] - Add getNativeScrollRef method to FlatList component

Reviewed By: TheSavior

Differential Revision: D18302202

fbshipit-source-id: 7005a2bc1dab207434be3f1f4d8fde0b11b3bb4d

Libraries/Lists/FlatList.js

@@ -19,7 +19,10 @@ const StyleSheet = require('../StyleSheet/StyleSheet');
 
 const invariant = require('invariant');
 
-import type {ScrollResponderType} from '../Components/ScrollView/ScrollView';
+import ScrollView, {
+  type ScrollResponderType,
+} from '../Components/ScrollView/ScrollView';
+import type {ScrollViewNativeComponentType} from '../Components/ScrollView/ScrollViewNativeComponentType.js';
 import type {ViewStyleProp} from '../StyleSheet/StyleSheet';
 import type {
   ViewToken,
@@ -367,6 +370,24 @@ class FlatList<ItemT> extends React.PureComponent<Props<ItemT>, void> {
     }
   }
 
+  /**
+   * Provides a reference to the underlying host component
+   */
+  getNativeScrollRef():
+    | ?React.ElementRef<typeof View>
+    | ?React.ElementRef<ScrollViewNativeComponentType> {
+    if (this._listRef) {
+      const scrollRef = this._listRef.getScrollRef();
+      if (scrollRef != null) {
+        if (scrollRef instanceof ScrollView) {
+          return scrollRef.getNativeScrollRef();
+        } else {
+          return scrollRef;
+        }
+      }
+    }
+  }
+
   getScrollableNode(): any {
     if (this._listRef) {
       return this._listRef.getScrollableNode();

Libraries/Lists/VirtualizedList.js

@@ -520,7 +520,9 @@ class VirtualizedList extends React.PureComponent<Props, State> {
     }
   }
 
-  getScrollRef(): ?React.ElementRef<typeof ScrollView> {
+  getScrollRef():
+    | ?React.ElementRef<typeof ScrollView>
+    | ?React.ElementRef<typeof View> {
     if (this._scrollRef && this._scrollRef.getScrollRef) {
       return this._scrollRef.getScrollRef();
     } else {

Libraries/Lists/__tests__/FlatList-test.js

@@ -93,4 +93,63 @@ describe('FlatList', () => {
     );
     expect(component).toMatchSnapshot();
   });
+  it('getNativeScrollRef for case where it returns a native view', () => {
+    jest.resetModules();
+    jest.unmock('../../Components/ScrollView/ScrollView');
+
+    const listRef = React.createRef(null);
+
+    ReactTestRenderer.create(
+      <FlatList
+        data={[{key: 'outer0'}, {key: 'outer1'}]}
+        renderItem={outerInfo => (
+          <FlatList
+            data={[
+              {key: outerInfo.item.key + ':inner0'},
+              {key: outerInfo.item.key + ':inner1'},
+            ]}
+            renderItem={innerInfo => {
+              return <item title={innerInfo.item.key} />;
+            }}
+            ref={listRef}
+          />
+        )}
+      />,
+    );
+
+    const scrollRef = listRef.current.getNativeScrollRef();
+
+    // This is checking if the ref acts like a host component. If we had an
+    // `isHostComponent(ref)` method, that would be preferred.
+    expect(scrollRef.measure).toBeInstanceOf(jest.fn().constructor);
+    expect(scrollRef.measureLayout).toBeInstanceOf(jest.fn().constructor);
+    expect(scrollRef.measureInWindow).toBeInstanceOf(jest.fn().constructor);
+  });
+
+  it('getNativeScrollRef for case where it returns a native scroll view', () => {
+    jest.resetModules();
+    jest.unmock('../../Components/ScrollView/ScrollView');
+
+    function ListItemComponent({item}) {
+      return <item value={item.key} />;
+    }
+    const listRef = React.createRef(null);
+
+    ReactTestRenderer.create(
+      <FlatList
+        data={[{key: 'i4'}, {key: 'i2'}, {key: 'i3'}]}
+        ListItemComponent={ListItemComponent}
+        numColumns={2}
+        ref={listRef}
+      />,
+    );
+
+    const scrollRef = listRef.current.getNativeScrollRef();
+
+    // This is checking if the ref acts like a host component. If we had an
+    // `isHostComponent(ref)` method, that would be preferred.
+    expect(scrollRef.measure).toBeInstanceOf(jest.fn().constructor);
+    expect(scrollRef.measureLayout).toBeInstanceOf(jest.fn().constructor);
+    expect(scrollRef.measureInWindow).toBeInstanceOf(jest.fn().constructor);
+  });
 });

Libraries/Lists/__tests__/VirtualizedList-test.js

@@ -281,4 +281,57 @@ describe('VirtualizedList', () => {
       }),
     );
   });
+
+  it('getScrollRef for case where it returns a ScrollView', () => {
+    const listRef = React.createRef(null);
+
+    ReactTestRenderer.create(
+      <VirtualizedList
+        data={[{key: 'i1'}, {key: 'i2'}, {key: 'i3'}]}
+        renderItem={({item}) => <item value={item.key} />}
+        getItem={(data, index) => data[index]}
+        getItemCount={data => data.length}
+        ref={listRef}
+      />,
+    );
+
+    const scrollRef = listRef.current.getScrollRef();
+
+    // This is checking if the ref acts like a ScrollView. If we had an
+    // `isScrollView(ref)` method, that would be preferred.
+    expect(scrollRef.scrollTo).toBeInstanceOf(Function);
+  });
+
+  it('getScrollRef for case where it returns a View', () => {
+    const listRef = React.createRef(null);
+
+    ReactTestRenderer.create(
+      <VirtualizedList
+        data={[{key: 'outer0'}, {key: 'outer1'}]}
+        renderItem={outerInfo => (
+          <VirtualizedList
+            data={[
+              {key: outerInfo.item.key + ':inner0'},
+              {key: outerInfo.item.key + ':inner1'},
+            ]}
+            renderItem={innerInfo => {
+              return <item title={innerInfo.item.key} />;
+            }}
+            getItem={(data, index) => data[index]}
+            getItemCount={data => data.length}
+            ref={listRef}
+          />
+        )}
+        getItem={(data, index) => data[index]}
+        getItemCount={data => data.length}
+      />,
+    );
+    const scrollRef = listRef.current.getScrollRef();
+
+    // This is checking if the ref acts like a host component. If we had an
+    // `isHostComponent(ref)` method, that would be preferred.
+    expect(scrollRef.measure).toBeInstanceOf(jest.fn().constructor);
+    expect(scrollRef.measureLayout).toBeInstanceOf(jest.fn().constructor);
+    expect(scrollRef.measureInWindow).toBeInstanceOf(jest.fn().constructor);
+  });
 });