React Native - Device back button handling

Question

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.

Answer 1

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  }
}