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Version: 7.x

Navigation lifecycle

In a previous section, we worked with a stack navigator that has two screens (Home and Profile) and learned how to use navigation.navigate('RouteName') to navigate between the screens.

An important question in this context is: what happens with Home when we navigate away from it, or when we come back to it? How does a screen find out that a user is leaving it or coming back to it?

If you are coming to react-navigation from a web background, you may assume that when the user navigates from route A to route B, A will unmount (its componentWillUnmount is called) and A will mount again when the user comes back to it. While these React lifecycle methods are still valid and are used in React Navigation, their usage differs from the web. This is driven by the more complex needs of mobile navigation.

Example scenario​

Consider a stack navigator with 2 screens: Home and Profile. When we first render the navigator, the Home screen is mounted, i.e. its useEffect or componentDidMount is called. When we navigate to Profile, now Profile is mounted and its useEffect or componentDidMount is called. But nothing happens to Home - it remains mounted in the stack. The cleanup function returned by useEffect or componentWillUnmount is not called.

When we go back from Profile to Home, Profile is unmounted and its useEffect cleanup or componentWillUnmount is called. But Home is not mounted again - it remained mounted the whole time - and its useEffect or componentDidMount is not called.

Similar results can be observed (in combination) with other navigators as well. Consider a tab navigator with two tabs, where each tab is a stack navigator:

const SettingsStack = createNativeStackNavigator({
  screens: {
    Settings: SettingsScreen,
    Profile: ProfileScreen,
  },
});

const HomeStack = createNativeStackNavigator({
  screens: {
    Home: HomeScreen,
    Details: DetailsScreen,
  },
});

const MyTabs = createBottomTabNavigator({
  screenOptions: {
    headerShown: false,
  },
  screens: {
    First: SettingsStack,
    Second: HomeStack,
  },
});
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We start on the HomeScreen and navigate to DetailsScreen. Then we use the tab bar to switch to the SettingsScreen and navigate to ProfileScreen. After this sequence of operations is done, all 4 of the screens are mounted! If you use the tab bar to switch back to the HomeStack, you'll notice you'll be presented with the DetailsScreen - the navigation state of the HomeStack has been preserved!

React Navigation lifecycle events​

Now that we understand how React lifecycle methods work in React Navigation, let's answer the question we asked at the beginning: "How do we find out that a user is leaving (blur) it or coming back to it (focus)?"

React Navigation emits events to screen components that subscribe to them. We can listen to focus and blur events to know when a screen comes into focus or goes out of focus respectively.

Example:

function ProfileScreen() {
  const navigation = useNavigation();

  React.useEffect(() => {
    const unsubscribe = navigation.addListener('focus', () => {
      console.log('ProfileScreen focused');
    });

    return unsubscribe;
  }, [navigation]);

  React.useEffect(() => {
    const unsubscribe = navigation.addListener('blur', () => {
      console.log('ProfileScreen blurred');
    });

    return unsubscribe;
  }, [navigation]);

  return (
    <View style={{ flex: 1, alignItems: 'center', justifyContent: 'center' }}>
      <Text>Profile Screen</Text>
    </View>
  );
}
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See Navigation events for more details on the available events and the API usage.

For performing side effects, we can use the useFocusEffect hook instead of subscribing to events. It's like React's useEffect hook, but it ties into the navigation lifecycle.

Example:

import { useFocusEffect } from '@react-navigation/native';

function ProfileScreen() {
  useFocusEffect(
    React.useCallback(() => {
      // Do something when the screen is focused
      console.log('ProfileScreen focus effect');

      return () => {
        // Do something when the screen is unfocused
        // Useful for cleanup functions
        console.log('ProfileScreen focus effect cleanup');
      };
    }, [])
  );

  return (
    <View style={{ flex: 1, alignItems: 'center', justifyContent: 'center' }}>
      <Text>Profile Screen</Text>
    </View>
  );
}
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If you want to render different things based on if the screen is focused or not, you can use the useIsFocused hook which returns a boolean indicating whether the screen is focused.

If you want to know if the screen is focused or not inside of an event listener, you can use the navigation.isFocused() method. Note that using this method doesn't trigger a re-render like the useIsFocused hook does, so it is not suitable for rendering different things based on focus state.

Summary​

  • React Navigation does not unmount screens when navigating away from them
  • The useFocusEffect hook is analogous to React's useEffect but is tied to the navigation lifecycle instead of the component lifecycle.
  • The useIsFocused hook and navigation.isFocused() method can be used to determine if a screen is currently focused.
  • React Navigation emits focus and blur events that can be listened to when a screen comes into focus or goes out of focus.