By default, when user presses the Android hardware back button, react-navigation will pop a screen or exit the app if there are no screens to pop. This is a sensible default behavior, but there are situations when you might want to implement custom handling.
As an example, consider a screen where user is selecting items in a list, and a "selection mode" is active. On a back button press, you would first want the "selection mode" to be deactivated, and the screen should be popped only on the second back button press. The following code snippet demonstrates the situation. We make use of
BackHandler which comes with react-native, along with the
useFocusEffect hook to add our custom
onBackPress denotes that we have handled the event, and react-navigation's listener will not get called, thus not popping the screen. Returning
false will cause the event to bubble up and react-navigation's listener will pop the screen.
The presented approach will work well for screens that are shown in a
StackNavigator. Custom back button handling in other situations may not be supported at the moment (eg. A known case when this does not work is when you want to handle back button press in an open drawer. PRs for such use cases are welcome!).
If instead of overriding system back button, you'd like to prevent going back from the screen, see docs for preventing going back.
Why not use component lifecycle methods
At first, you may be inclined to use
componentDidMount to subscribe for the back press event and
componentWillUnmount to unsubscribe, or use
useEffect to add the listener. This approach will not work - learn more about this in navigation lifecycle.