[Android] [BackAndroid] New listeners registered while back event is handled also receive the current event

[fabianeichinger]

I just faced for me unexpected behavior when (indirectly) registering a listener to BackAndroid while an event is currently handled.

In my project I use a Root component type, which calls a function provided to it via the onBackPress prop. It looks something like this:

export default class Root extends React.Component {

  _handleBackButton = () => {
    if (this.props.onBackPress) {
      this.props.onBackPress();
      return true;
    }
    return false;
  };

  componentDidMount() {
    BackAndroid.addEventListener('hardwareBackPress', this._handleBackButton);
  }

  componentWillUnmount() {
    BackAndroid.removeEventListener('hardwareBackPress', this._handleBackButton);
  }

  render() {
    return (
      …
  }
}

Now if the back button is pressed and, as part of the onBackPress function, the current Root element is replaced by a new one, the new Root element's componentDidMount is executed before onBackPress returns, adding the new event listener to BackAndroid.
Problem is: BackAndroid will add the new listener to the Set of listeners it loops over and eventually call the newly added listener for the same event it was created by.

I think the better behavior would be not to modify the Set of listeners while looping over them to dispatch an event. In case the Set needs to be modified create a copy, apply the modification there and continue to use the original Set to complete handling of the current event.

[facebook-github-bot]

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[mkonicek]

Hi there! This issue is being closed because it has been inactive for a while.

But don't worry, it will live on with ProductPains! Check out its new home: https://productpains.com/post/react-native/android-backandroid-new-listeners-registered-while-back-event-is-handled-also-receive-the-current-event

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