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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.

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. The Intent should have a Bundle managed by a com.commonsware.cwac.locpoll.LocationPollerParameter attached to it that contains:

  • One or more location providers that will be attempted to be used in order of which they are defined
  • An Intent to be "broadcast" when a location is found or a all the location providers have timed out
  • A timeout how long to wait for the provider to respond (optional - defaults to 2 minutes)

For example, this sets up such an alarm:

	mgr=(AlarmManager)getSystemService(ALARM_SERVICE);
	
	Intent i=new Intent(this, LocationPoller.class);
	
	Bundle bundle = new Bundle();
	LocationPollerParameter parameter = new LocationPollerParameter(bundle);
	parameter.setIntentToBroadcastOnCompletion(new Intent(this, LocationReceiver.class));
	// try GPS and fall back to NETWORK_PROVIDER
	parameter.setProviders(new String[] {LocationManager.GPS_PROVIDER, LocationManager.NETWORK_PROVIDER});
	parameter.setTimeout(60000);
	i.putExtras(bundle);
	
	
	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 com.commonsware.cwac.locpoll.LocationPollerParameter extra. The Intent received by the receiver in onReceive() contains an extra that should be decoded using a com.commonsware.cwac.locpoll.LocationPollerResult. For example:

  Bundle b=intent.getExtras();
  
  LocationPollerResult locationResult = new LocationPollerResult(b);
  
  Location loc=locationResult.getLocation();
  String msg;

  if (loc==null) {
    loc=locationResult.getLastKnownLocation();

    if (loc==null) {
      msg=locationResult.getError();
    }
    else {
      msg="TIMEOUT, lastKnown="+loc.toString();
    }
  }
  else {
    msg=loc.toString();
  }

  if (msg==null) {
    msg="Invalid broadcast received!";
  }

In the case where you get an error message via getError(), getLastKnownLocation() will contain the results of getLastKnownLocation() for your selected provider. This may be null — if not, it will be a Location object.

Dependencies

This project has no dependencies.

Version

This is version 0.3 of this module.

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

v0.3.0: added configurable timeout and location provider fallback mechanism

v0.2.0: added EXTRA_LASTKNOWN support

v0.1.0: initial release