componentWillMount() {

   BackHandler.addEventListener('hardwareBackPress', this.backPressed);

}

​

componentWillUnmount() {

   BackHandler.removeEventListener('hardwareBackPress', this.backPressed);

}

​

backPressed = () => {

  Alert.alert(

    'Exit App',

    'Do you want to exit?',

    [

      {text: 'No', onPress: () => console.log('Cancel Pressed'), style: 'cancel'},

      {text: 'Yes', onPress: () => BackHandler.exitApp()},

    ],

    { cancelable: false });

    return true;

}