message: Handled back to page 0 of ViewPager
diff: import React from 'react';
  -import {StyleSheet, Text, TouchableHighlight, ToolbarAndroid, View, ScrollView} from 'react-native';
  +import {StyleSheet, BackHandler, Text, TouchableHighlight, ToolbarAndroid, View, ScrollView} from 'react-native';
  import ViewPager from '@react-native-community/viewpager';
  import Tweets from './Tweets';
  import Notifications from './Notifications';
  -export default ({tweets, follows}) => (
  +export default class Home extends React.Component {
  + constructor(props, context) {
  + super(props, context);
  + this.state = {page: 0};
  + this.handleBack = this.handleBack.bind(this);
  + }
  + componentDidMount() {
  + BackHandler.addEventListener('hardwareBackPress', this.handleBack);
  + }
  + componentWillUnmount() {
  + BackHandler.removeEventListener('hardwareBackPress', this.handleBack);
  + }
  + handleBack() {
  + if (this.state.page === 1) {
  + this.setPage(0);
  + return true;
  + }
  + return false;
  + }
  + setPage(page) {
  + this.setState({page});
  + this.viewPager.setPage(page);
  + }
  + render() {
  + var {tweets, follows} = this.props;
  + return (
  <View style={{flex: 1}}>
  <ToolbarAndroid title="Home" style={styles.toolbar} />
  <View style={styles.tabs}>
  <TouchableHighlight
  underlayColor="white"
  style={styles.tab}
  - onPress={() => this.viewPager.setPage(0)}>
  + onPress={() => this.setPage(0)}>
  <Text style={styles.text}>Home</Text>
  </TouchableHighlight>
  <TouchableHighlight
  underlayColor="white"
  style={styles.tab}
  - onPress={() => this.viewPager.setPage(1)}>
  + onPress={() => this.setPage(1)}>
  <Text style={styles.text}>Notifications</Text>
  </TouchableHighlight>
  </View>
  - <ViewPager style={{flex: 1}} ref={el => this.viewPager = el}>
  + <ViewPager
  + ref={el => this.viewPager = el}
  + onPageSelected={({nativeEvent}) => {
  + this.setState({page: nativeEvent.position})
  + }}
  + style={{flex: 1}}>
  <View key={1}>
  <ScrollView style={styles.view}>
  <Tweets tweets={tweets} />
  @@ -35,6 +65,8 @@ export default ({tweets, follows}) => (
  </ViewPager>
  </View>
  );
  + }
  +};
  const styles = StyleSheet.create({
  toolbar: {
  