Friday, December 4, 2009

Creating a boot-loading, always-on service

Let's say you've got an app that you'd like to run in the background. You'd like it to get started as soon as possible, and stay on whenever the device is powered on. It's an achievable goal, with one caveat. After installing the app from the Android Market or wherever, the user will need to run it at least once to give you a chance to execute some code that puts your foot in the door. Of course, once your app starts up, you're free to set up the system to make sure that your code is always running. There are really two parts to the issue: running the service, and turning the service on at startup. For today, let's talk about startup. If you poke around in the Android developer docs enough, you'll probably encounter the BroadcastReceiver class. The OS supports the idea of broadcast events, which can be caught by subclasses of BroadcastReceiver. Setting up that subclass is fairly simple - just extend BroadcastReceiver and override onReceive(), like this:
public class ServiceLauncher extends BroadcastReceiver {

public void onReceive(Context context, Intent intent) {

        // Start the service here

It's possible for a BroadcastReceiver to pull extra duty by listening for several types of broadcasts. If that's what you want to do, open up the Intent passed to onReceive() and check for the proper Action. Now, that's only half of the story. At this point, we have a class ready and able to listen, but the OS doesn't know that. To make the final connection, we need to add this snippet inside the application tag of AndroidManifest.xml:
<receiver android:enabled="true" android:exported="true" android:name="your.package.ServiceLauncher"> <intent-filter> <action android:name="android.intent.action.BOOT_COMPLETED"/> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter> </receiver>
There seems to be a lot going on here, but when you really look at it, it's just an intent and an action, which you've most likely seen before, wrapped in this new receiver tag. All this is really saying is "We've got a class called ServiceLauncher, and it's going to get fired when the system boots." To sum up, there are two small parts, and that's all we need:
  • Create a subclass BroadcastReceiver and override onReceive().
  • Add a receiver tag for your subclass.
I hope this is helpful, and I'd be happy to take requests or questions about particular topics.

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