@capacitor/background-runner

Background Runner provides an event-based standalone JavaScript environment for executing your Javascript code outside of the web view.

Install

npm install @capacitor/background-runner
npx cap sync

Background Runner has support for various device APIs that require permission from the user prior to use.

iOS

On iOS you must enable the Background Modes capability.

Once added, you must enable the Background fetch and Background processing modes at a minimum to enable the ability to register and schedule your background tasks.

If you will be making use of Geolocation or Push Notifications, enable Location updates or Remote notifications respectively.

After enabling the Background Modes capability, add the following to your app's AppDelegate.swift:

At the top of the file, under import Capacitor add:

import CapacitorBackgroundRunner

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

    // ....
    BackgroundRunnerPlugin.registerBackgroundTask()
    BackgroundRunnerPlugin.handleApplicationDidFinishLaunching(launchOptions: launchOptions)
    // ....

    return true
}

To allow the Background Runner to handle remote notifications, add the following:

func application(_ application: UIApplication, didReceiveRemoteNotification userInfo: [AnyHashable : Any], fetchCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {
        // ....
        BackgroundRunnerPlugin.dispatchEvent(event: "remoteNotification", eventArgs: userInfo) { result in
            switch result {
            case .success:
                completionHandler(.newData)
            case .failure:
                completionHandler(.failed)
            }
        }
    }

Geolocation

Apple requires privacy descriptions to be specified in Info.plist for location information:

• NSLocationAlwaysUsageDescription (Privacy - Location Always Usage Description)
• NSLocationWhenInUseUsageDescription (Privacy - Location When In Use Usage Description)

Read about Configuring Info.plist in the iOS Guide for more information on setting iOS permissions in Xcode

Android

Insert the following line to android/app/build.gradle:

...

repositories {
    flatDir{
        dirs '../capacitor-cordova-android-plugins/src/main/libs', 'libs'
+       dirs '../../node_modules/@capacitor/background-runner/android/src/main/libs', 'libs'
    }
}
...

If you are upgrading from 1.0.5 with an existing Android project, be sure to delete the android-js-engine-release.aar from android/src/main/libs.

Geolocation

This API requires the following permissions be added to your AndroidManifest.xml:

<!-- Geolocation API -->
<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
<uses-feature android:name="android.hardware.location.gps" />

The first two permissions ask for location data, both fine and coarse, and the last line is optional but necessary if your app requires GPS to function. You may leave it out, though keep in mind that this may mean your app is installed on devices lacking GPS hardware.

Local Notifications

Android 13 requires a permission check in order to send notifications. You are required to call checkPermissions() and requestPermissions() accordingly.

On Android 12 and older it won't show a prompt and will just return as granted.

Starting on Android 12, scheduled notifications won't be exact unless this permission is added to your AndroidManifest.xml:

<uses-permission android:name="android.permission.SCHEDULE_EXACT_ALARM" />

Note that even if the permission is present, users can still disable exact notifications from the app settings.

Read about Setting Permissions in the Android Guide for more information on setting Android permissions.

About Background Runner

During the course of building complex applications, its sometimes necessary to perform work while the application is not in the foreground. The challenge with standard Capacitor applications is that the webview is not available when these background events occur, requiring you to write native code to handle these events. This is where the Background Runner plugin comes in.

Background Runner makes it easy to write JavaScript code to handle native background events. All you need to do is create your runner JavaScript file and define your configuration, then the Background Runner plugin will automatically configure and schedule a native background task that will be executed according to your config and the rules of the platform. No modification to your UI code is necessary.

Using Background Runner

Background Runner contains a headless JavaScript environment that calls event handlers in javascript file that you designate in your capacitor.config.ts file. If the runner finds a event handler corresponding to incoming event in your runner file, it will execute the event handler, then shutdown once resolve() or reject() are called (or if the OS force kills your process).

Example Runner JS File

addEventListener('myCustomEvent', (resolve, reject, args) => {
  console.log('do something to update the system here');
  resolve();
});

addEventListener('myCustomEventWithReturnData', (resolve, reject, args) => {
  try {
    console.log('accepted this data: ' + JSON.stringify(args.user));

    const updatedUser = args.user;
    updatedUser.firstName = updatedUser.firstName + ' HELLO';
    updatedUser.lastName = updatedUser.lastName + ' WORLD';

    resolve(updatedUser);
  } catch (err) {
    reject(err);
  }
});

addEventListener('remoteNotification', (resolve, reject, args) => {
  try {
    console.log('received silent push notification');

    CapacitorNotifications.schedule([
      {
        id: 100,
        title: 'Enterprise Background Runner',
        body: 'Received silent push notification',
      },
    ]);

    resolve();
  } catch (err) {
    reject();
  }
});

Calling resolve() \ reject() is required within every event handler called by the runner. Failure to do this could result in your runner being killed by the OS if your event is called while the app is in the background. If the app is in the foreground, async calls to dispatchEvent may not resolve.

For more real world examples of using Background Runner, check out the Background Runner Test App.

Configuring Background Runner

JavaScript API

Background Runner does not execute your Javascript code in a browser or web view, therefore the typical Web APIs you may be used to may not be available. This includes DOM APIs nor ability to interact with your application's DOM.

Below is a list of the available Web APIs provided in Background Runner:

• console
  • only info, log, warn, error , and debug are available
• TextDecoder
  • only decode is available
• TextEncoder
  • only encode is available
• addEventListener
  • Event Listener options and useCapture not supported
• setTimeout
• setInterval
• clearTimeout
• clearInterval
• crypto
• fetch
  • Request object not yet supported
  • Only method, headers and body supported in options object

In addition to the standard Web APIs, Background Runner also supports a number of custom Capacitor APIs custom APIs that expose relevant mobile device functionality

Runner Lifetimes

Currently, the runners are designed for performing periodic bursts of work while your app is in the background, or for executing asynchronous work in a thread separate from your UI while your app is in the foreground. As a result, runners are not long lived. State is not maintained between calls to events in the runner. Each call to dispatchEvent() creates a new context in which your runner code is loaded and executed, and once resolve() or reject() is called, the context is destroyed.

Android Battery Optimizations

Some Android vendors offer built-in battery optimization settings that go beyond what stock Android provides. Some of these optimizations must be disabled by your end users in order for your background tasks to work properly.

Visit Don't kill my app! for more information on the affected manufacturers and steps required by your users to adjust the settings.

Limitations of Background Tasks

It’s not possible to run persistent, always running background services on mobile operating systems. Due to the limitations imposed by iOS and Android designed to reduce battery and data consumption, background tasks are constrained with various limitations that you must keep in mind while designing and implementing your background task.

iOS

• Each invocation of your task has approximately up to 30 seconds of runtime before you must call completed() or your task is killed.
• While you can set an interval to define when your task runs after the app is backgrounded, or how often it should run, this is not guaranteed. iOS will determine when and how often you task will ultimately run, determined in part by how often you app is used.
• Background tasks are not executed in the simulator.

Android

• Your task has a maximum of 10 minutes to perform work, but to keep your task cross platform compatible, you should limit your work to 30 seconds at most.
• Repeating background tasks have a minimal interval of at least 15 minutes. Similar to iOS, any interval you request may not be hit exactly - actual execution time is subject to OS battery optimizations and other heuristics.

API

Capacitor API