Implement fading edges for ScrollView and it's dependent FlatList (#26163)

Summary:
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.

## Changelog

[Android] [Added] - fading edges props to the FlatList and ScrollView components
Pull Request resolved: #26163

Test Plan:
Open the React Native test app and navigate to the FlatList section.
Enable the `useFadingEdges` switch and insert a number into `Fading edge length`.

![device-2019-08-23-123745](https://user-images.githubusercontent.com/222393/63587150-7385cb00-c5a3-11e9-98dc-bffe8276d30c.png)
![device-2019-08-23-123844](https://user-images.githubusercontent.com/222393/63587156-75e82500-c5a3-11e9-9e9f-66876ac8f506.png)

Differential Revision: D17080676

Pulled By: TheSavior

fbshipit-source-id: 91df629c17052d43c99145672e9084e1379a4113

Author: André Krüger

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();