Navigation lifecycle

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

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 route find out that a user is leaving it or coming back to it?

Coming to react-navigation from the web, you may assume that when user navigates from route A to route B, A will unmount (its componentWillUnmount is called) and A will mount again when 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 more complex needs of mobile navigation.

Example scenario

Consider a stack navigator with screens A and B. After navigating to A, its componentDidMount is called. When pushing B, its componentDidMount is also called, but A remains mounted on the stack and its componentWillUnmount is therefore not called.

When going back from B to A, componentWillUnmount of B is called, but componentDidMount of A is not because A remained mounted the whole time.

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 HomeStack = createStackNavigator({
  Home: HomeScreen,
  Details: DetailsScreen,
});

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

const TabNavigator = createBottomTabNavigator(
  {
    Home: HomeStack,
    Settings: SettingsStack,
  }
);

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 it or coming back to it?"

React Navigation emits events to screen components that subscribe to them. There are four different events that you can subscribe to: willFocus, willBlur, didFocus and didBlur. Read more about them in the API reference.

Many of your use cases may be covered with the withNavigationFocus HOC or the <NavigationEvents /> component which are a little more straightforward to use.

Summary

• while React's lifecycle methods are still valid, React Navigation adds more lifecycle events that you can subscribe to through the navigation prop.
• you may also use the withNavigationFocus HOC or <NavigationEvents /> component to react to lifecycle changes