react-native/docs/HeadlessJSAndroid.md

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Headless JS is a way to run tasks in JavaScript while your app is in the background. It can be used, for example, to sync fresh data, handle push notifications, or play music.

The JS API

A task is a simple async function that you register on AppRegistry, similar to registering React applications:

AppRegistry.registerHeadlessTask('SomeTaskName', () => require('SomeTaskName'));

Then, in SomeTaskName.js:

module.exports = async (taskData) => {
  // do stuff
}

You can do anything in your task as long as it doesn't touch UI: network requests, timers and so on. Once your task completes (i.e. the promise is resolved), React Native will go into "paused" mode (unless there are other tasks running, or there is a foreground app).

The Java API

Yes, this does still require some native code, but it's pretty thin. You need to extend HeadlessJsTaskService and override getTaskConfig, e.g.:

public class MyTaskService extends HeadlessJsTaskService {

  @Override
  protected @Nullable HeadlessJsTaskConfig getTaskConfig(Intent intent) {
    Bundle extras = intent.getExtras();
    if (extras != null) {
      return new HeadlessJsTaskConfig(
          "SomeTaskName",
          Arguments.fromBundle(extras),
          5000);
    }
    return null;
  }
}

Then add the service to your AndroidManifest.xml file:

<service android:name="com.example.MyTaskService" />

Now, whenever you start your service, e.g. as a periodic task or in response to some system event / broadcast, JS will spin up, run your task, then spin down.

Example:

Intent service = new Intent(getApplicationContext(), MyTaskService.class);
Bundle bundle = new Bundle();

bundle.putString("foo", "bar");
service.putExtras(bundle);

getApplicationContext().startService(service);

Caveats

• By default, your app will crash if you try to run a task while the app is in the foreground. This is to prevent developers from shooting themselves in the foot by doing a lot of work in a task and slowing the UI. You can pass a fourth boolean argument to control this behaviour.
• If you start your service from a BroadcastReceiver, make sure to call HeadlessJsTaskService.acquireWakelockNow() before returning from onReceive().