React Native - Device back button handling

问题:

I want to check if there are more than one screens are on stack when device back button is hit. If yes, I want to show previous screen and if no, I want to exit app.

I have checked number of examples but those use BackAndroid and Navigator. But both of them are deprecated. BackHandler is replacement for BackAndroid. And I can show previous screen by using props.navigation.goBack(null).

But I am unable to find code for finding screen count in stack. I don't want to use deprecated Navigator!

回答1:

This example will show you back navigation which is expected generally in most of the flows. You will have to add following code to every screen depending on expected behavior. There are 2 cases: 1. If there are more than 1 screen on stack, device back button will show previous screen. 2. If there is only 1 screen on stack, device back button will exit app.

Case 1: Show previous screen

import { BackHandler } from 'react-native';

constructor(props) {
    super(props)
    this.handleBackButtonClick = this.handleBackButtonClick.bind(this);
}

componentWillMount() {
    BackHandler.addEventListener('hardwareBackPress', this.handleBackButtonClick);
}

componentWillUnmount() {
    BackHandler.removeEventListener('hardwareBackPress', this.handleBackButtonClick);
}

handleBackButtonClick() {
    this.props.navigation.goBack(null);
    return true;
}

Important: Don't forget to bind method in constructor and to remove listener in componentWillUnmount.

Case 2: Exit App

In this case, no need to handle anything on that screen where you want to exit app.

Important: This should be only screen on stack.

回答2:

In a case where there are more than one screens stacked in the stack, the default back button behavior in react-native is to navigate back to the previous screen in the stack. Handling the device back button press when having only one screen to exit the app requires a custom setting. Yet this can be achieved without having to add back handling code to each and every screen by modifying the getStateForAction method of the particular StackNavigator's router.

Suppose you have the following StackNavigator used in the application

const ScreenStack = StackNavigator(
  {
    'Screen1': {
      screen: Screen1
    },
    'Screen2': {
      screen: Screen2
    },
  },
  {
    initialRouteName: 'Screen1'
  }
);

The getStateForAction method of the stack navigator's router can be modified as follows to achieve the expected back behavior.

const defaultStackGetStateForAction =
  ScreenStack.router.getStateForAction;

ScreenStack.router.getStateForAction = (action, state) => {
  if(state.index === 0 && action.type === NavigationActions.BACK){
    BackHandler.exitApp();
    return null;
  }

  return defaultStackGetStateForAction(action, state);
};

the state.index becomes 0 only when there is one screen in the stack.

回答3:

Guyz please do understand it might not only be the problem with react native. Be careful while integrating it with firebase. The recent firebase version has the problem of integrating back button in react-native apps!! Please downgrade the firebase version to firebase-version @5.0.3 and then recheck whether it works or not! I had the same issue and was worried for days. I finally downgraded to @5.0.3 version and now the back button works perfectly fine! You may downgrade to lower versions if still facing the problem.

回答4:

I am on v0.46.0 of react-native and had the same issue. I tracked the issue down to this file in the react-native code base

https://github.com/facebook/react-native/blob/master/Libraries/Utilities/BackHandler.android.js#L25

When running with the chrome debugger turned off the line

var subscriptions = Array.from(_backPressSubscriptions.values()).reverse()

always returns an empty array for subscriptions which in turn causes the invokeDefault variable to stay true and the .exitApp() function to be called.

After more investigation, I think the issue was discovered and discussed in the following PR #15182.

Even after copy/pasting the PR change in an older version of RN it did not work most likely caused by the issue described in the PR.

After some very slight modifications I got it working by changing to

RCTDeviceEventEmitter.addListener(DEVICE_BACK_EVENT, function() {
  var invokeDefault = true;
  var subscriptions = []
  _backPressSubscriptions.forEach(sub => subscriptions.push(sub))

  for (var i = 0; i < subscriptions.reverse().length; ++i) {
    if (subscriptions[i]()) {
      invokeDefault = false;
      break;
    }
  }

  if (invokeDefault) {
    BackHandler.exitApp();
  }
});

Simply using a .forEach which was the original implementation on the PR before the amended Array.from syntax works throughout.

So you could fork react-native and use a modified version, submit a PR though I imagine that will take a little while to be approved and merged upstream, or you can do something similar to what I did which was to override the RCTDeviceEventEmitter.addListener(...) for the hardwareBackPress event.

// other imports
import { BackHandler, DeviceEventEmitter } from 'react-native'

class MyApp extends Component {
  constructor(props) {
    super(props)
    this.backPressSubscriptions = new Set()
  }

  componentDidMount = () => {
    DeviceEventEmitter.removeAllListeners('hardwareBackPress')
    DeviceEventEmitter.addListener('hardwareBackPress', () => {
      let invokeDefault = true
      const subscriptions = []

      this.backPressSubscriptions.forEach(sub => subscriptions.push(sub))

      for (let i = 0; i < subscriptions.reverse().length; i += 1) {
        if (subscriptions[i]()) {
          invokeDefault = false
          break
        }
      }

      if (invokeDefault) {
        BackHandler.exitApp()
      }
    })

    this.backPressSubscriptions.add(this.handleHardwareBack)
  }

  componentWillUnmount = () => {
    DeviceEventEmitter.removeAllListeners('hardwareBackPress')
    this.backPressSubscriptions.clear()
  }

  handleHardwareBack = () => { /* do your thing */ }

  render() { return <YourApp /> }
}

回答5:

constructor(props){
    super(props)
    this.onBackPress = this.onBackPress.bind(this);
}

componentWillMount() {
        BackHandler.addEventListener('hardwareBackPress', this.onBackPress);

}

componentWillUnmount(){
    BackHandler.removeEventListener('hardwareBackPress', this.onBackPress);
}

onBackPress(){
    const {dispatch, nav} = this.props;
    if (nav.index < 0) {
        return false;
    }
    dispatch(NavigationActions.back());
    return true;
}

render(){
    const {dispatch, nav} = this.props;
    return(
        <DrawerRouter
            navigation= {
                addNavigationHelpers({
                    dispatch,
                    state: nav,
                    addListener,
                })
            }
        />
    );
}

回答6:

I used flux for navigation.

    const RouterComp = () => {

    let backLoginScene=false;

    return (

        <Router
        backAndroidHandler={() => {
            const back_button_prohibited = ['login','userInfo','dashboard'];
            if (back_button_prohibited.includes(Actions.currentScene) ) {
                if (backLoginScene == false) {
                    ToastAndroid.show("Click back again to exit.", ToastAndroid.SHORT);
                    backLoginScene = !backLoginScene;
                    setTimeout(() => {
                        backLoginScene = false;
                    }, 2000);
                    return true;
                } else {
                    backLoginScene = false;
                    BackHandler.exitApp();
                }
                return false;
            }}}>
            <Scene key='root' hideNavBar>
                <Scene key='guest' hideNavBar >
                    <Scene key='login' component={Login} ></Scene>
                    <Scene key='userInfo' component={UserInfo}></Scene>
                </Scene>

                <Scene key='user' hideNavBar>
                    <Scene key='dashboard' component={Dashboard} title='Dashboard' initial />
                    <Scene key='newAd' component={NewAd} title='New Ad' />

                </Scene>



            </Scene>
        </Router>
    )
}

export default RouterComp;