Expo Modules API for adding native modules
The Expo Modules API is used to add and use native modules in your app.
Expo & React Native · all subjects
modules & native code
12 notes, read out of this brain and free to use. Each one was extracted from a source and is re-checked against its exam.
Package interface for lifecycle listeners
Create a concrete class implementing the Package interface to hook into Android lifecycle events. Implement either createReactActivityLifecycleListeners or createApplicationLifecycleListeners methods depending on whether you need Activity or Application lifecycle hooks.
Expo Autolinking discovers Package classes automatically
Expo Autolinking scans each linked module's Android sources for classes in files ending in Package.java or Package.kt that implement the Package interface. During Android build, these classes are added to the generated ExpoModulesPackageList. You do not need to register the Package class in MainApplication.java, MainApplication.kt, or expo-module.config.json if the module already supports Android.
ReactActivityLifecycleListener supported callbacks
ReactActivityLifecycleListener hooks into React Native's ReactActivity lifecycle and supports the following Activity lifecycle callbacks: onCreate, onResume, onPause, onDestroy, onNewIntent, and onBackPressed.
Override only needed Activity lifecycle methods
When implementing ReactActivityLifecycleListener, you can override only the lifecycle methods you need. The interface uses Java 8 default methods, so you don't have to implement all methods. This reduces maintenance costs between Expo SDK versions.
Communicating from Activity lifecycle listener to JavaScript
Lifecycle listeners are singleton classes that exist independently of module instances. To communicate between a lifecycle listener and your module or JavaScript code, use the observer pattern: lifecycle listeners capture system events and notify module observers, which then send structured events to JavaScript via sendEvent. Use weak references to prevent memory leaks.
ApplicationLifecycleListener supported callbacks
ApplicationLifecycleListener supports the following Application lifecycle callbacks: onCreate and onConfigurationChanged.
Create ApplicationLifecycleListener in Package class
Implement createApplicationLifecycleListeners in your Package class, returning a list of ApplicationLifecycleListener instances. The method receives a context parameter of type Context.
Activity lifecycle listener example with deep links
Example of DeepLinkHandlerActivityLifecycleListener implementing onCreate and onNewIntent to capture deep links: onCreate calls handleIntent with activity.getIntent(), onNewIntent calls handleIntent with the received intent. handleIntent extracts the URI from intent.getData() and notifies observers.
Module with event sending and observer management
A module can maintain a static set of observer callbacks (e.g., urlReceivedObservers) that lifecycle listeners notify. The module should implement OnStartObserving and OnStopObserving to add and remove observers. Use WeakReference to store module references in observers to prevent memory leaks. sendEvent is called with event name and Bundle containing event data.
Why onStart and onStop Activity listeners are not supported
Lifecycle hooks are set up from ReactActivityDelegate rather than MainActivity. ReactActivityDelegate does not have onStart and onStop callbacks, so these are not yet supported in the Expo Modules API.
Interface stability and backward compatibility strategy
Listener interfaces may change between Expo SDK releases. The strategy for backward compatibility is to add new interfaces and use @Deprecated annotation for interfaces planned for removal. All interfaces are based on Java 8 interface default methods, so you don't have to implement all methods.