CWAC LocationPoller: Asking "Where Am I?" Over and Over Again

Some applications wish to know where the device is at on a periodic basis, whether the device is awake or asleep. This is a bit challenging, since you have to keep the device awake (via a WakeLock) long enough to get the location fix, but not forever in case the location simply is unavailable (e.g., user is in a place where GPS signals cannot be received).

LocationPoller wraps up some code to implement this in a reusable package. You simply set up an AlarmManager alarm to contact LocationPoller on whatever frequency you wish, and it will handle all of the location work from there, sending you the results via a broadcast Intent. Your BroadcastReceiver can then use the location data as needed.

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.

Alex Birkett is maintaining a more advanced version of the LocationPoller concept in his fork.

Usage

First, you need to add the JAR or Android library project to your project.

Second, you need to add the following to your manifest:

		<receiver android:name="com.commonsware.cwac.locpoll.LocationPoller" />
		<service android:name="com.commonsware.cwac.locpoll.LocationPollerService" />

Your manifest will also need the WAKE_LOCK permission along with whatever permission is required for the location provider you wish to use (e.g., ACCESS_FINE_LOCATION).

Next, you need to create an alarm via AlarmManager, so you can control how frequently the location is retrieved and whether it should wake up the device if it is asleep. Please do not request location updates frequently, as this will drain the user's battery, particularly if you are using GPS.

The PendingIntent you use for the alarm should be a getBroadcast() PendingIntent, wrapping an Intent destined for com.commonsware.cwac.locpoll.LocationPoller. That Intent should have one extra, keyed by LocationPoller.EXTRA_INTENT, that represents an Intent to be "broadcast" when a location is found. It should have another extra, keyed by LocationPoller.EXTRA_PROVIDER, with the name of the location provider you wish to use.

For example, this sets up such an alarm:

	mgr=(AlarmManager)getSystemService(ALARM_SERVICE);
	
	Intent i=new Intent(this, LocationPoller.class);
	
	i.putExtra(LocationPoller.EXTRA_INTENT,
						 new Intent(this, LocationReceiver.class));
	i.putExtra(LocationPoller.EXTRA_PROVIDER,
						 LocationManager.NETWORK_PROVIDER);
	
	pi=PendingIntent.getBroadcast(this, 0, i, 0);
	mgr.setRepeating(AlarmManager.ELAPSED_REALTIME_WAKEUP,
										SystemClock.elapsedRealtime(),
										PERIOD,
										pi);

Finally, you need a BroadcastReceiver set up to respond to the Intent you supplied via the LocationPoller.EXTRA_INTENT extra. The Intent received by the receiver in onReceive() will have either a LocationPoller.EXTRA_LOCATION extra (containing a Location) or a LocationPoller.EXTRA_ERROR extra (containing a String with an error message). For example:

		Location loc=(Location)intent.getExtras().get(LocationPoller.EXTRA_LOCATION);
		String msg;
		
		if (loc==null) {
			msg=intent.getStringExtra(LocationPoller.EXTRA_ERROR);
		}
		else {
			msg=loc.toString();
		}
		
		if (msg==null) {
			msg="Invalid broadcast received!";
		}

In the case where you get an error message via EXTRA_ERROR, there will also be an extra named EXTRA_LASTKNOWN, containing the results of getLastKnownLocation() for your selected provider. This may be null — if not, it will be a Location object. There may also be EXTRA_ERROR_PROVIDER_DISABLED, with a boolean indicating if the provider in question was disabled (thereby preventing getting any readings).

Dependencies

This project has no dependencies.

Version

This is version 0.2.3 of this module, meaning it is even less new than before.

Demo

In the demo/ sub-project you will find a sample activity that demonstrates the use of LocationPoller.

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

• v0.2.3: merged pull request that added EXTRA_ERROR_PROVIDER_DISABLED
• v0.2.2: added defense against emulator issue
• v0.2.1: added more protection against under-locked WakeLocks
• v0.2.0: added EXTRA_LASTKNOWN support
• v0.1.0: initial release

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