CWAC Wakeful: Staying Awake At Work

The recommended pattern for Android's equivalent to cron jobs and Windows scheduled tasks is to use AlarmManager. This works well when coupled with an IntentService, as the service will do its work on a background thread and shut down when there is no more work to do.

There's one small problem: IntentService does nothing to keep the device awake. If the alarm was a WAKEUP variant, the phone will only stay awake on its own while the BroadcastReceiver handling the alarm is in its onReceive() method. Otherwise, the phone may fall back asleep.

WakefulIntentService attempts to combat this by combining the ease of IntentService with a partial WakeLock.

This is available as a JAR file from the downloads area of this GitHub repo. The project itself is set up as an Android library project, in case you wish to use the source code in that fashion.

NOTE: WakefulIntentService v0.4.0 and newer requires Android 2.0+, so it can take advantage of onStartCommand() for better handling of crashed services. Use earlier versions of WakefulIntentService if you wish to try to use it on older versions of Android, though this is not supported.

Basic Usage

Any component that wants to send work to a WakefulIntentService subclass needs to call either:

WakefulIntentService.sendWakefulWork(context, MyService.class);

(where MyService.class is the WakefulIntentService subclass)

or:

WakefulIntentService.sendWakefulWork(context, intentOfWork);

(where intentOfWork is an Intent that will be used to call startService() on your WakefulIntentService subclass)

Implementations of WakefulIntentService must override doWakefulWork() instead of onHandleIntent(). doWakefulWork() will be processed within the bounds of a WakeLock. Otherwise, the semantics of doWakefulWork() are identical to onHandleIntent(). doWakefulWork() will be passed the Intent supplied to sendWakefulWork() (or an Intent created by the sendWakefulWork() method, depending on which flavor of that method you use).

And that's it. WakefulIntentService handles the rest.

NOTE: this only works with local services. You have no means of accessing the static WakeLock of a remote service.

NOTE #2: Your application must hold the WAKE_LOCK permission.

NOTE #3: If you get an "WakeLock under-locked" exception, make sure that you are not starting your service by some means other than sendWakefulWork().

Alarm Usage

If you want to slightly simplify your use of WakefulIntentService in conjunction with AlarmManager, you can do the following:

First, implement your WakefulIntentService and doWakefulWork() as described above.

Next, create a class implementing the WakefulIntentService.AlarmListener interface. This class needs to have a no-argument public constructor in addition to the interface method implementations. One method is scheduleAlarms(), where you are passed in an AlarmManager, a PendingIntent, and a Context, and your mission is to schedule your alarms using the supplied PendingIntent. You also implement sendWakefulWork(), which is passed a Context, and is where you call sendWakefulWork() upon your WakefulIntentService implementation. And, you need to implement getMaxAge(), which should return the time in milliseconds after which, if we have not seen an alarm go off, we should assume that the alarms were canceled (e.g., application was force-stopped by the user), and should reschedule them.

Then, create an XML metadata file where you identify the class that implements WakefulIntentService.AlarmListener from the previous step, akin to:

<WakefulIntentService
  listener="com.commonsware.cwac.wakeful.demo.AppListener"
/>

Next, register com.commonsware.cwac.wakeful.AlarmReceiver as a <receiver> in your manifest, set to respond to ACTION_BOOT_COMPLETED broadcasts, and with a com.commonsware.cwac.wakeful <meta-data> element pointing to the XML resource from the previous step, akin to:

<receiver android:name="com.commonsware.cwac.wakeful.AlarmReceiver">
  <intent-filter>
    <action android:name="android.intent.action.BOOT_COMPLETED"/>
  </intent-filter>

  <meta-data
    android:name="com.commonsware.cwac.wakeful"
    android:resource="@xml/wakeful"/>
</receiver>

Also, add the RECEIVE_BOOT_COMPLETED permission to your manifest.

Finally, when you wish to manually set up the alarms (e.g., on first run of your app), create an instance of your AlarmListener and call scheduleAlarms() on the WakefulIntentService class, passing in the AlarmListener and a Context (e.g., the activity that is trying to set up the alarms). If you are only scheduling alarms using the single provided PendingIntent, you can also call cancelAlarms() on the WakefulIntentService class to cancel any outstanding alarms.

For production use, ProGuard may rename your AlarmListener class, which will foul up access to your metadata. To stop this from happening, you will need to add a -keep line to your ProGuard configuration file (e.g., proguard.cfg) to stop ProGuard from renaming it.

Over time, this portion of the framework will be expanded further to help consolidate a good usage pattern for managing alarms.

Additional documentation can be found in the "AlarmManager: Making the Services Run On Time" section of this free excerpt from The Busy Coder's Guide to Advanced Android Development.

Dependencies

None.

Version

This is version v0.6.0 of this module, meaning it is proving to be surprisingly popular.

Demo

In the demo/ project directory and com.commonsware.cwac.wakeful.demo package you will find an AppListener, which is an implementation of AlarmListener, and AppService, which extends WakefulIntentService. AppService pretends to do some work in a background thread. All of this is set up via a DemoActivity (required to move the application out of the "stopped" state on Android 3.1+), and if needed on a reboot.

Note that when you build the JAR via ant jar, the sample activity is not included, nor any resources -- only the compiled classes for the actual library are put into the JAR.

License

The code in this project is licensed under the Apache Software License 2.0, per the terms of the included LICENSE file.

Questions

If you have questions regarding the use of this code, please post a question on StackOverflow tagged with commonsware and android. Be sure to indicate what CWAC module you are having issues with, and be sure to include source code and stack traces if you are encountering crashes.

If you have encountered what is clearly a bug, please post an issue. Be certain to include complete steps for reproducing the issue.

Do not ask for help via Twitter. Release Notes

• v0.6.1: replaced AlarmListener Log lines with RuntimeExceptions
• v0.6.0: added cancelAlarms() to WakefulIntentService
• v0.5.1: semi-automatically handle canceled alarms (e.g., app force-stopped)
• v0.5.0: added the AlarmListener portion of the framework
• v0.4.5: completed switch to Application as the Context for the WakeLock
• v0.4.4: switched to Application as the Context for the WakeLock
• v0.4.3: added better recovery from an Intent redelivery condition
• v0.4.2: added volatile keyword to static WakeLock for better double-checked locking implementation
• v0.4.1: added setIntentRedelivery() call, nuked extraneous permissions check
• v0.4.0: switched to onStartCommand(), requiring Android 2.0+ (API level 5 or higher)
• v0.3.0: converted to Android library project, added test for WAKE_LOCK permission