Quick Start Guide to using BackgroundTaskScheduler

Background

In Android M+ it is encouraged to use JobScheduler for all background jobs, instead of using things like IntentService or polling using alarms. Using the system API is beneficial as it has a full view of what goes on in the system and can schedule jobs accordingly.

However, that leaves an API gap for Android L and below. Prior to Android L, the JobScheduler API was not available at all. It was introduced in Android L; but is not recommended on that platform, because it limits task execution time to 1 minute. This is not really practically usable. For example, merely setting up a network connection will often burn through much of that budget. Android M+ extends this execution time limit to 10 minutes.

For these older platforms, we can leverage the GcmNetworkManager API provided by Google Play services to implement a suitable replacement for the JobScheduler API. The background_task_scheduler component provides a new framework for use within Chromium to schedule and execute background jobs using the frameworks available on a given version of Android. The public API of the framework is similar to that of the Android JobScheduler, but it is backed by either the system JobScheduler API or by GcmNetworkManager. What service is used to back the framework remains a black box to callers of the API.

In practice, we prefer to use system APIs, since they do not require including external libraries, which would bloat the APK size of Chrome and add unnecessary complexity. Thus, the GcmNetworkManager is only used when the system API is not available (or available but not considered stable enough). That is, the JobScheduler API is used on Android M+; and the GcmNetworkManager is used otherwise.

NOTE: Some of the pre-M devices do not include Google Play services and therefore remain unsupported by background_task_scheduler. Ultimately, this component hopes to provide a full compatibility layer on top of JobScheduler. However, until that is implemented, please be thoughtful about whether this component provides the coverage that your background task needs.

What is a task

A task is defined as a class that implements the BackgroundTask interface, which looks like this:

interface BackgroundTask {
  interface TaskFinishedCallback {
    void taskFinished(boolean needsReschedule);
  }

  boolean onStartTask(Context context,
                      TaskParameters taskParameters,
                      TaskFinishedCallback callback);
  boolean onStopTask(Context context,
                     TaskParameters taskParameters);
}

Any class implementing this interface must have a public constructor which takes no arguments.

A task must also have a unique ID, and it must be listed in TaskIds to ensure there is no overlap between different tasks.

How to schedule a task

A task is scheduled by creating an object containing information about the task, such as when to run it, whether it requires battery, and other similar constraints. This object is called TaskInfo and has a builder you can use to set all the relevant fields.

There are two main types of tasks; one-off tasks and periodic tasks. One-off tasks are only executed once, whereas periodic tasks are executed once per a defined interval.

As an example for how to create a one-off task that executes in 200 minutes, you can do the following:

TaskInfo.createOneOffTask(TaskIds.YOUR_FEATURE,
                            MyBackgroundTask.class,
                            TimeUnit.MINUTES.toMillis(200)).build();

For a periodic task that executes every 200 minutes, you can call:

TaskInfo.createPeriodicTask(TaskIds.YOUR_FEATURE,
                              MyBackgroundTask.class,
                              TimeUnit.MINUTES.toMillis(200)).build();

Typically you will also set other required parameters such as what type of network conditions are necessary and whether the task requires the device to be charging. They can be set on the builder like this:

TaskInfo.createOneOffTask(TaskIds.YOUR_FEATURE,
                            MyBackgroundTask.class,
                            TimeUnit.MINUTES.toMillis(100)
                            TimeUnit.MINUTES.toMillis(200))
                          .setRequiresCharging(true)
                          .setRequiredNetworkType(TaskInfo.NETWORK_TYPE_UNMETERED)
                          .build();

When the task is ready for scheduling, you use the BackgroundTaskSchedulerFactory to get the current instance of the BackgroundTaskScheduler and use it to schedule the job.

BackgroundTaskSchedulerFactory.getScheduler().schedule(myTaskInfo);

If you ever need to cancel a task, you can do that by calling cancel, and passing in the task ID:

BackgroundTaskSchedulerFactory.getScheduler().cancel(TaskIds.YOUR_FEATURE);

Passing task arguments

A TaskInfo supports passing in arguments through a Bundle, but only values that can be part of an Android BaseBundle are allowed. You can pass them in using the TaskInfo.Builder:

Bundle myBundle = new Bundle();
myBundle.putString("foo", "bar");
myBundle.putLong("number", 1337L);

TaskInfo.createOneOffTask(TaskIds.YOUR_FEATURE,
                            MyBackgroundTask.class,
                            TimeUnit.MINUTES.toMillis(100)
                            TimeUnit.MINUTES.toMillis(200))
                          .setExtras(myBundle)
                          .build();

These arguments will be readable for the task through the TaskParameters object that is passed to both onStartTask(...) and onStopTask(...), by doing the following:

boolean onStartTask(Context context,
                    TaskParameters taskParameters,
                    TaskFinishedCallback callback) {
  Bundle myExtras = taskParameters.getExtras();
  // Use |myExtras|.
  ...
}

Background processing

Even though the BackgroundTaskScheduler provides functionality for invoking code while the application is in the background, the BackgroundTask instance is still invoked on the application main thread.

This means that unless the operation is extremely quick, processing must happen asynchronously, and the call to onStartJob(...) must return before the task has finished processing. In that case, onStartJob(...) must return true, and instead invoke the TaskFinishedCallback when the processing is finished, which typically happens on a different Thread, Handler or using an AsyncTask.

If the task finishes while still being on the main thread, onStartJob(...) should return false, indicating that no further processsing is required.

If at any time the constraints given through the TaskInfo object does not hold anymore, or if the system deems it necessary, onStopTask(...) will be invoked, requiring all activity to cease immediately. The task can return true if the task needs to be rescheduled since it was canceled, or false otherwise.

The system will hold a wakelock from the time onStartTask(...) is invoked until either the task itself invokes the TaskFinishedCallback, or onStopTask(...) is invoked.

Loading Native parts

Some of the tasks running in the background require native parts of the browser to be initialized. In order to simplify implementation of such tasks, we provide an base NativeBackgroundTask implementation in the browser layer. It requires extending classes to implement 4 methods:

• onStartTaskBeforeNativeLoaded(...) where the background task can decide whether conditions are correct to proceed with native initialization;
• onStartTaskWithNative(...) where the background task can be sure that native initialization was completed, therefore it can depend on that part of the browser;
• onStopTaskBeforeNativeLoaded(...) which is delivered to the background task just like onStopTask(...) and the native parts of the browser are not loaded;
• onStopTaskWithNative(...) which is delivered to the background task just like onStopTask(...) and the native parts of the browser are loaded.

While in a normal execution, both onStart... methods are called, only one of the stopping methods will be triggered, depending on whether the native parts of the browser are loaded at the time the underlying scheduler decides to stop the task.