---
id: headless-js-android
title: Headless JS
layout: docs
category: Guides (Android)
permalink: docs/headless-js-android.html
banner: ejected
next: signed-apk-android
previous: custom-webview-android
---

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:

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

Then, in `SomeTaskName.js`:

```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.:

```java
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, // timeout for the task
          false // optional: defines whether or not  the task is allowed in foreground. Default is false
        );
    }
    return null;
  }
}
```

Then add the service to your `AndroidManifest.xml` file:

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

Now, whenever you [start your service][0], 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:

```java
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()`.

## Example Usage

Service can be started from Java API. First you need to decide when the service should be started and implement your solution accordingly. Here is a simple example that reacts to network connection change.

Following lines shows part of Android manifest file for registering broadcast receiver.   
```xml
<receiver android:name=".NetworkChangeReceiver" >
  <intent-filter>
    <action android:name="android.net.conn.CONNECTIVITY_CHANGE" />
  </intent-filter>
</receiver>
```

Broadcast receiver then handles intent that was broadcasted in onReceive function. This is a great place to check whether your app is on foreground or not. If app is not on foreground we can prepare our intent to be started, with no information or additional information bundled using putExta (keep in mind bundle can handle only parcelable values). In the end service is started and wakelock is acquired.

```java
public class NetworkChangeReceiver extends BroadcastReceiver {

    @Override
    public void onReceive(final Context context, final Intent intent) {
        /**
          This part will be called everytime network connection is changed
          e.g. Connected -> Not Connected
        **/
        if (!isAppOnForeground((context))) {
            /**
              We will start our service and send extra info about 
              network connections
            **/
            boolean hasInternet = isNetworkAvailable(context);
            Intent serviceIntent = new Intent(context, MyTaskService.class);
            serviceIntent.putExtra("hasInternet", hasInternet);
            context.startService(serviceIntent);
            HeadlessJsTaskService.acquireWakeLockNow(context);
        }
    }

    private boolean isAppOnForeground(Context context) {
        /**
          We need to check if app is in foreground otherwise the app will crash.
         http://stackoverflow.com/questions/8489993/check-android-application-is-in-foreground-or-not
        **/
        ActivityManager activityManager = (ActivityManager) context.getSystemService(Context.ACTIVITY_SERVICE);
        List<ActivityManager.RunningAppProcessInfo> appProcesses = 
        activityManager.getRunningAppProcesses();
        if (appProcesses == null) {
            return false;
        }
        final String packageName = context.getPackageName();
        for (ActivityManager.RunningAppProcessInfo appProcess : appProcesses) {
            if (appProcess.importance == 
            ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND &&
             appProcess.processName.equals(packageName)) {
                return true;
            }
        }
        return false;
    }

    public static boolean isNetworkAvailable(Context context) {
        ConnectivityManager cm = (ConnectivityManager) 
        context.getSystemService(Context.CONNECTIVITY_SERVICE);
        NetworkInfo netInfo = cm.getActiveNetworkInfo();
        return (netInfo != null && netInfo.isConnected());
    }


}
```

[0]: https://developer.android.com/reference/android/content/Context.html#startService(android.content.Intent)