android | Al's Blog

I recently encountered the need to have an AsyncTask in an Android app that gets the current GPS position, then fetches some data from the network upon getting the position. This isn’t very straightforward to do, since the GPS stuff uses a callback driven model. I did a quick search and came up with a lot of Stack Overflow solutions to the problem that were really bad. Most of them suggested launching a GPS request, then having some sort of loop that spins until the location comes in. At least one of them didn’t even have any sort of delay in the loop, making it burn up your phone’s battery while it waited.

It turns out to be pretty simple to solve this problem, using the Looper class, which is a bit of a confusing thing. The looper will establish a loop which waits for events/messages within the AsyncTask thread. Once the position comes in, you have the code exit the loop, and then continue on with your network request. Here’s the basic idea:

	public class AsyncTaskGps extends AsyncTask implements LocationListener {
	private Location location;

	@Override
	protected Object doInBackground(Object… arg0) {
	LocationManager lm = (LocationManager) arg0[0];
	Looper.prepare();
	// Request GPS updates. The third param is the looper to use, which defaults the the one for
	// the current thread.
	lm.requestSingleUpdate(LocationManager.GPS_PROVIDER, this, null);
	Looper.loop(); // start waiting…when this is done, we'll have the location in this.location
	try {
	// now go use the location to load some data
	URL url = new URL("…?latitude="+location.getLatitude()+"&longitude="+location.getLongitude());
	URLConnection con = url.openConnection();
	InputStream is = con.getInputStream();
	byte[] data = new byte[1024];
	ByteArrayOutputStream baos = new ByteArrayOutputStream();
	int len = –1;
	do {
	len = is.read(data);
	if (len > 0) {
	baos.write(data, 0, len);
	}
	} while (len > –1);

	// parse data and do stuff…

	} catch (IOException e) {
	Log.e("LoadScheduleTask", "Error", e);
	} catch (JSONException e) {
	Log.e("LoadScheduleTask", "Error", e);
	}

	return null;
	}

	@Override
	protected void onPostExecute(Object result) {
	// notify someone we are done…
	}

	@Override
	public void onLocationChanged(Location location) {
	// Store the location, then get the current thread's looper and tell it to
	// quit looping so it can continue on doing work with the new location.
	this.location = location;
	Looper.myLooper().quit();
	}

	}

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AsyncTaskGps.java

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If you are in the process of developing an android app that uses SQLite, you probably have found yourself in the situation where you wanted to restructure the database. Instead of bumping the version on the helper class and putting in upgrade logic, it is best to just start over with a fresh database when you haven’t yet released your new version of the app. Here’s how you zap your database in the emulator’s file system.

First, find out the serial number of the emulator as it is running:

$ adb devices
List of devices attached
emulator-5554    device

Now, connect a shell and delete the file from the filesystem:

$ adb emulator-5554 shell
# rm /data/data/<yourpackage>/databases/<databaseName>
# exit

After that, your database should be gone and will be recreated next time you run the helper to get a connection.

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Category Archives: android

GPS in an AsyncTask in Android

Deleting SQLite database in android