Implement fading edges for Scrollview and it's dependent FlatList

This should add props for enabling horizontal and vertical fading
edges for Scrollview and FlatList.
These fading edges are used to communicate to the user that there is more content to see.

Libraries/Components/ScrollView/ScrollView.js

@@ -335,6 +335,18 @@ type AndroidProps = $ReadOnly<{|
    * @platform android
    */
   persistentScrollbar?: ?boolean,
+  /**
+   * Fades out the edges of the the scroll content.
+   *
+   * If the value is greater than 0, the fading edges will be set accordingly
+   * to the current scroll direction and position,
+   * indicating if there is more content to show.
+   *
+   * The default value is 0.
+   *
+   * @platform android
+   */
+  fadingEdgeLength?: ?number,
 |}>;
 
 type VRProps = $ReadOnly<{|

Libraries/Lists/FlatList.js

@@ -233,6 +233,10 @@ type OptionalProps<ItemT> = {
    * will be called when its corresponding ViewabilityConfig's conditions are met.
    */
   viewabilityConfigCallbackPairs?: Array<ViewabilityConfigCallbackPair>,
+  /**
+   * See `ScrollView` for flow type and further documentation.
+   */
+  fadingEdgeLength?: ?number,
 };
 export type Props<ItemT> = RequiredProps<ItemT> &
   OptionalProps<ItemT> &

RNTester/js/examples/FlatList/FlatListExample.js

@@ -29,7 +29,14 @@ const {
   pressItem,
   renderSmallSwitchOption,
 } = require('../../components/ListExampleShared');
-const {Alert, Animated, StyleSheet, View} = require('react-native');
+const {
+  Alert,
+  Animated,
+  Platform,
+  StyleSheet,
+  TextInput,
+  View,
+} = require('react-native');
 
 import type {Item} from '../../components/ListExampleShared';
 
@@ -51,6 +58,7 @@ type State = {|
   virtualized: boolean,
   empty: boolean,
   useFlatListItemComponent: boolean,
+  fadingEdgeLength: number,
 |};
 
 class FlatListExample extends React.PureComponent<Props, State> {
@@ -65,6 +73,7 @@ class FlatListExample extends React.PureComponent<Props, State> {
     virtualized: true,
     empty: false,
     useFlatListItemComponent: false,
+    fadingEdgeLength: 0,
   };
 
   _onChangeFilterText = filterText => {
@@ -124,11 +133,26 @@ class FlatListExample extends React.PureComponent<Props, State> {
               {renderSmallSwitchOption(this, 'empty')}
               {renderSmallSwitchOption(this, 'debug')}
               {renderSmallSwitchOption(this, 'useFlatListItemComponent')}
+              {Platform.OS === 'android' && (
+                <View>
+                  <TextInput
+                    placeholder="Fading edge length"
+                    underlineColorAndroid="black"
+                    keyboardType={'numeric'}
+                    onChange={event =>
+                      this.setState({
+                        fadingEdgeLength: Number(event.nativeEvent.text),
+                      })
+                    }
+                  />
+                </View>
+              )}
               <Spindicator value={this._scrollPos} />
             </View>
           </View>
           <SeparatorComponent />
           <Animated.FlatList
+            fadingEdgeLength={this.state.fadingEdgeLength}
             ItemSeparatorComponent={ItemSeparatorComponent}
             ListHeaderComponent={<HeaderComponent />}
             ListFooterComponent={FooterComponent}

ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactHorizontalScrollViewManager.java

@@ -276,4 +276,15 @@ public void setOverflow(ReactHorizontalScrollView view, @Nullable String overflo
   public void setPersistentScrollbar(ReactHorizontalScrollView view, boolean value) {
     view.setScrollbarFadingEnabled(!value);
   }
+
+  @ReactProp(name = "fadingEdgeLength")
+  public void setFadingEdgeLength(ReactHorizontalScrollView view, int value) {
+    if (value > 0) {
+      view.setHorizontalFadingEdgeEnabled(true);
+      view.setFadingEdgeLength(value);
+    } else {
+      view.setHorizontalFadingEdgeEnabled(false);
+      view.setFadingEdgeLength(0);
+    }
+  }
 }

ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactScrollViewManager.java

@@ -278,6 +278,17 @@ public void setPersistentScrollbar(ReactScrollView view, boolean value) {
     view.setScrollbarFadingEnabled(!value);
   }
 
+  @ReactProp(name = "fadingEdgeLength")
+  public void setFadingEdgeLength(ReactScrollView view, int value) {
+    if (value > 0) {
+      view.setVerticalFadingEdgeEnabled(true);
+      view.setFadingEdgeLength(value);
+    } else {
+      view.setVerticalFadingEdgeEnabled(false);
+      view.setFadingEdgeLength(0);
+    }
+  }
+
   @Override
   public @Nullable Map<String, Object> getExportedCustomDirectEventTypeConstants() {
     return createExportedCustomDirectEventTypeConstants();