Add onPageScrollStateChanged for ViewPagerAndroid

Summary:
I have an issue when combining `PullToRefreshViewAndroid` and `ViewPagerAndroid`.
`ViewPagerAndroid` will not able to scroll that gesture handler is being taken by `PullToRefreshViewAndroid`
One solution is to disable `PullToRefreshViewAndroid` if `ViewPagerAndroid` is scrolling (i.e. not idle).
[Reference solution here](http://stackoverflow.com/a/29946734/2590265)

So here need to expose the `onPageScrollStateChanged` event.
Some code referenced from  DrawerLayoutAndroid, especially the `VIEWPAGER_PAGE_SCROLL_STATES` array.
Please feel free give me comments.
Thanks.
Closes facebook#5026

Reviewed By: svcscm

Differential Revision: D2830623

Pulled By: andreicoman11

fb-gh-sync-id: c2a6920c6f4c7daab0115f13864db83b93b31abf

Examples/UIExplorer/ViewPagerAndroidExample.android.js

@@ -25,6 +25,8 @@ var {
   ViewPagerAndroid,
 } = React;
 
+import type { ViewPagerScrollState } from 'ViewPagerAndroid';
+
 var PAGES = 5;
 var BGCOLOR = ['#fdc08e', '#fff6b9', '#99d1b7', '#dde5fe', '#f79273'];
 var IMAGE_URIS = [
@@ -114,6 +116,10 @@ var ViewPagerAndroidExample = React.createClass({
     this.setState({progress: e.nativeEvent});
   },
 
+  onPageScrollStateChanged: function(state : ViewPagerScrollState) {
+    this.setState({scrollState: state});
+  },
+
   move: function(delta) {
     var page = this.state.page + delta;
     this.go(page);
@@ -155,6 +161,7 @@ var ViewPagerAndroidExample = React.createClass({
           initialPage={0}
           onPageScroll={this.onPageScroll}
           onPageSelected={this.onPageSelected}
+          onPageScrollStateChanged={this.onPageScrollStateChanged}
           ref={viewPager => { this.viewPager = viewPager; }}>
           {pages}
         </ViewPagerAndroid>
@@ -170,6 +177,7 @@ var ViewPagerAndroidExample = React.createClass({
               enabled={true}
               onPress={() => this.setState({animationsAreEnabled: true})}
             /> }
+          <Text style={styles.scrollStateText}>ScrollState[ {this.state.scrollState} ]</Text>
         </View>
         <View style={styles.buttons}>
           <Button text="Start" enabled={page > 0} onPress={() => this.go(0)}/>
@@ -207,6 +215,9 @@ var styles = StyleSheet.create({
   buttonText: {
     color: 'white',
   },
+  scrollStateText: {
+    color: '#99d1b7',
+  },
   container: {
     flex: 1,
     backgroundColor: 'white',

Libraries/Components/ViewPager/ViewPagerAndroid.android.js

@@ -19,6 +19,14 @@ var requireNativeComponent = require('requireNativeComponent');
 
 var VIEWPAGER_REF = 'viewPager';
 
+type Event = Object;
+
+export type ViewPagerScrollState = $Enum<{
+  idle: string;
+  dragging: string;
+  settling: string;
+}>;
+
 /**
  * Container that allows to flip left and right between child views. Each
  * child view of the `ViewPagerAndroid` will be treated as a separate page
@@ -78,6 +86,16 @@ var ViewPagerAndroid = React.createClass({
      */
     onPageScroll: ReactPropTypes.func,
 
+    /**
+     * Function called when the page scrolling state has changed.
+     * The page scrolling state can be in 3 states:
+     * - idle, meaning there is no interaction with the page scroller happening at the time
+     * - dragging, meaning there is currently an interaction with the page scroller
+     * - settling, meaning that there was an interaction with the page scroller, and the
+     *   page scroller is now finishing it's closing or opening animation
+     */
+    onPageScrollStateChanged: ReactPropTypes.func,
+
     /**
      * This callback will be called once ViewPager finish navigating to selected page
      * (when user swipes between pages). The `event.nativeEvent` object passed to this
@@ -147,6 +165,12 @@ var ViewPagerAndroid = React.createClass({
     }
   },
 
+  _onPageScrollStateChanged: function(e: Event) {
+    if (this.props.onPageScrollStateChanged) {
+      this.props.onPageScrollStateChanged(e.nativeEvent.pageScrollState);
+    }
+  },
+
   _onPageSelected: function(e: Event) {
     if (this.props.onPageSelected) {
       this.props.onPageSelected(e);
@@ -183,6 +207,7 @@ var ViewPagerAndroid = React.createClass({
         ref={VIEWPAGER_REF}
         style={this.props.style}
         onPageScroll={this._onPageScroll}
+        onPageScrollStateChanged={this._onPageScrollStateChanged}
         onPageSelected={this._onPageSelected}
         children={this._childrenWithOverridenStyle()}
       />

ReactAndroid/src/main/java/com/facebook/react/views/viewpager/PageScrollStateChangedEvent.java

@@ -0,0 +1,49 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+package com.facebook.react.views.viewpager;
+
+import com.facebook.react.bridge.Arguments;
+import com.facebook.react.bridge.WritableMap;
+import com.facebook.react.uimanager.events.Event;
+import com.facebook.react.uimanager.events.RCTEventEmitter;
+
+/**
+ * Event emitted by {@link ReactViewPager} when user scrolling state changed.
+ *
+ * Additional data provided by this event:
+ *  - pageScrollState - {Idle,Dragging,Settling}
+ */
+class PageScrollStateChangedEvent extends Event<PageScrollStateChangedEvent> {
+
+  public static final String EVENT_NAME = "topPageScrollStateChanged";
+
+  private final String mPageScrollState;
+
+  protected PageScrollStateChangedEvent(int viewTag, long timestampMs, String pageScrollState) {
+    super(viewTag, timestampMs);
+    mPageScrollState = pageScrollState;
+  }
+
+  @Override
+  public String getEventName() {
+    return EVENT_NAME;
+  }
+
+  @Override
+  public void dispatch(RCTEventEmitter rctEventEmitter) {
+    rctEventEmitter.receiveEvent(getViewTag(), getEventName(), serializeEventData());
+  }
+
+  private WritableMap serializeEventData() {
+    WritableMap eventData = Arguments.createMap();
+    eventData.putString("pageScrollState", mPageScrollState);
+    return eventData;
+  }
+}

ReactAndroid/src/main/java/com/facebook/react/views/viewpager/ReactViewPager.java

@@ -104,7 +104,22 @@ public void onPageSelected(int position) {
 
     @Override
     public void onPageScrollStateChanged(int state) {
-      // don't send events
+      String pageScrollState;
+      switch (state) {
+        case SCROLL_STATE_IDLE:
+          pageScrollState = "idle";
+          break;
+        case SCROLL_STATE_DRAGGING:
+          pageScrollState = "dragging";
+          break;
+        case SCROLL_STATE_SETTLING:
+          pageScrollState = "settling";
+          break;
+        default:
+          throw new IllegalStateException("Unsupported pageScrollState");
+      }
+      mEventDispatcher.dispatchEvent(
+        new PageScrollStateChangedEvent(getId(), SystemClock.uptimeMillis(), pageScrollState));
     }
   }
 

ReactAndroid/src/main/java/com/facebook/react/views/viewpager/ReactViewPagerManager.java

@@ -50,6 +50,7 @@ public boolean needsCustomLayoutForChildren() {
   public Map getExportedCustomDirectEventTypeConstants() {
     return MapBuilder.of(
         PageScrollEvent.EVENT_NAME, MapBuilder.of("registrationName", "onPageScroll"),
+        PageScrollStateChangedEvent.EVENT_NAME, MapBuilder.of("registrationName", "onPageScrollStateChanged"),
         PageSelectedEvent.EVENT_NAME, MapBuilder.of("registrationName", "onPageSelected")
     );
   }