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Learn about the mechanism that allows your library to hook into Android Activity and Application functions using Expo modules API.
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To respond to certain Android system events relevant to an app, such as inbound links and configuration changes, it is necessary to override the corresponding lifecycle callbacks in MainActivity.java and/or MainApplication.java.
The React Native module API does not provide any mechanism to hook into these, and so setup instructions for React Native libraries often include steps to copy code into these files. To simplify and automate setup and maintenance, the Expo Modules API provides a mechanism that allows your library to hook into Activity
or Application
functions.
First, you need to have created an Expo module or integrated the Expo modules API in the library using the React Native module API. Learn more.
Inside your module, create a concrete class that implements the Package
interface. For most cases, you only need to implement the createReactActivityLifecycleListeners
or createApplicationLifecycleListeners
methods.
Activity
lifecycle listenersYou can hook into the Activity
lifecycle using ReactActivityLifecycleListener
. ReactActivityLifecycleListener
hooks into React Native's ReactActivity
lifecycle using its ReactActivityDelegate
and provides a similar experience to the Android Activity
lifecycle.
The following Activity
lifecycle callbacks are currently supported:
onCreate
onResume
onPause
onDestroy
onNewIntent
onBackPressed
To create a ReactActivityLifecycleListener
, you should implement createReactActivityLifecycleListeners
in your derived Package
class. For example, MyLibPackage
.
// android/src/main/java/expo/modules/mylib/MyLibPackage.kt
package expo.modules.mylib
import android.content.Context
import expo.modules.core.interfaces.Package
import expo.modules.core.interfaces.ReactActivityLifecycleListener
class MyLibPackage : Package {
override fun createReactActivityLifecycleListeners(activityContext: Context): List<ReactActivityLifecycleListener> {
return listOf(MyLibReactActivityLifecycleListener())
}
}
// android/src/main/java/expo/modules/mylib/MyLibPackage.java
package expo.modules.mylib;
import android.content.Context;
import expo.modules.core.interfaces.Package;
import expo.modules.core.interfaces.ReactActivityLifecycleListener;
import java.util.Collections;
import java.util.List;
public class MyLibPackage implements Package {
@Override
public List<? extends ReactActivityLifecycleListener> createReactActivityLifecycleListeners(Context activityContext) {
return Collections.singletonList(new MyLibReactActivityLifecycleListener());
}
}
MyLibReactActivityLifecycleListener
is a ReactActivityLifecycleListener
derived class that you can hook into the lifecycles. You can only override the methods you need.
// android/src/main/java/expo/modules/mylib/MyLibReactActivityLifecycleListener.kt
package expo.modules.mylib
import android.app.Activity
import android.os.Bundle
import expo.modules.core.interfaces.ReactActivityLifecycleListener
class MyLibReactActivityLifecycleListener : ReactActivityLifecycleListener {
override fun onCreate(activity: Activity, savedInstanceState: Bundle?) {
// Your setup code in `Activity.onCreate`.
doSomeSetupInActivityOnCreate(activity)
}
}
// android/src/main/java/expo/modules/mylib/MyLibReactActivityLifecycleListener.java
package expo.modules.mylib;
import android.app.Activity;
import android.os.Bundle;
import expo.modules.core.interfaces.ReactActivityLifecycleListener;
public class MyLibReactActivityLifecycleListener implements ReactActivityLifecycleListener {
@Override
public void onCreate(Activity activity, Bundle savedInstanceState) {
// Your setup code in `Activity.onCreate`.
doSomeSetupInActivityOnCreate(activity);
}
}
Application
lifecycle listenersYou can hook into the Application
lifecycle using ApplicationLifecycleListener
.
The following Application
lifecycle callbacks are currently supported:
onCreate
onConfigurationChanged
To create an ApplicationLifecycleListener
, you should implement createApplicationLifecycleListeners
in your derived Package
class. For example, MyLibPackage
.
// android/src/main/java/expo/modules/mylib/MyLibPackage.kt
package expo.modules.mylib
import android.content.Context
import expo.modules.core.interfaces.ApplicationLifecycleListener
import expo.modules.core.interfaces.Package
class MyLibPackage : Package {
override fun createApplicationLifecycleListeners(context: Context): List<ApplicationLifecycleListener> {
return listOf(MyLibApplicationLifecycleListener())
}
}
// android/src/main/java/expo/modules/mylib/MyLibPackage.java
import android.content.Context;
import java.util.Collections;
import java.util.List;
import expo.modules.core.interfaces.ApplicationLifecycleListener;
import expo.modules.core.interfaces.Package;
public class MyLibPackage implements Package {
@Override
public List<? extends ApplicationLifecycleListener> createApplicationLifecycleListeners(Context context) {
return Collections.singletonList(new MyLibApplicationLifecycleListener());
}
}
MyLibApplicationLifecycleListener
is an ApplicationLifecycleListener
derived class that can hook into the Application
lifecycle callbacks. You should only override the methods you need (due to possible maintenance costs).
// android/src/main/java/expo/modules/mylib/MyLibApplicationLifecycleListener.kt
package expo.modules.mylib
import android.app.Application
import expo.modules.core.interfaces.ApplicationLifecycleListener
class MyLibApplicationLifecycleListener : ApplicationLifecycleListener {
override fun onCreate(application: Application) {
// Your setup code in `Application.onCreate`.
doSomeSetupInApplicationOnCreate(application)
}
}
// android/src/main/java/expo/modules/mylib/MyLibApplicationLifecycleListener.java
package expo.modules.mylib;
import android.app.Application;
import expo.modules.core.interfaces.ApplicationLifecycleListener;
public class MyLibApplicationLifecycleListener implements ApplicationLifecycleListener {
@Override
public void onCreate(Application application) {
// Your setup code in `Application.onCreate`.
doSomeSetupInApplicationOnCreate(application);
}
}
onStart
and onStop
Activity listenersIn the current implementation, we do not set up the hooks from MainActivity
but from ReactActivityDelegate
. There are some slight differences between MainActivity
and ReactActivityDelegate
. Since ReactActivityDelegate
does not have onStart
and onStop
, we don't yet support them here.
The listener interfaces may change from time to time between Expo SDK releases. Our strategy for backward compatibility is always to add new interfaces and add @Deprecated
annotation for interfaces we plan to remove. Our interfaces are all based on Java 8 interface default method; you don't have to and should not implement all methods. Doing this will benefit your module's maintenance cost between Expo SDKs.