How the Android Gps Tracker Client Works

April 23rd, 2014 | Nick Fox

I finally had the time to rewrite the android client and to get the background service working as I’ve wanted to for a long time. Gps Tracker is now based on work I had done previously on my other website, mycelltracker.com. The big difference between the old mycelltracker app and the new Gps Tracker is the introduction of Google Play location services.

With that I want to go into an in-depth discussion of how Gps Tracker now works on Android devices. We’ll start by looking at the structure of the application beginning with the five classes used in the app. I’ll first give a brief description of each. Here are the class files on github if you want to follow along:

GpsTracker android client class files on github

GpsTrackerActivity – This is the one and only screen of the application. It displays two text fields. One for the user name and one for the upload website. There are a set of radio buttons that allow the user to set the interval (from one minute, 5, 15, 30, 60). And there is a start tracking button.

Android Gps Tracker

GpsTrackerAlarmReceiver – This class has an onReceive method that is called periodically from a repeating AlarmManager. When the method is called, the background service is started.

GpsTrackerBootReceiver – This class also has an onReceive method that is called but this one is called when the phone is rebooted. When the method is called, it creates a new AlarmManager. The AlarmManager starts the background service periodically using the receiver class above.

LocationService – This background service is called periodically from the AlarmManager above. When it starts, it uses google play location services to get the location of the device and send it to the update website using an open source asynchronous http client library. The service then terminates itself.

LoopjHttpClient – This class wraps a third party library that does asynchronous http calls.

The first thing that happens is that the main activity starts up and in onCreate, a sharedPreference file is opened and the currentlyTracking variable is set:

This is an important variable and will be used throughout the application. When onResume is called, two other methods are called. The first is displayUserSettings which just restores the UI to its previous state and the second is setTrackingButtonState which displays the correct button state (either tracking or not tracking). When a user taps on the start tracking button, the currentlyTracking variable is set to true and the method startAlarmManager is called:

In this method, a repeating AlarmManager is created and the interval is set to the variable intervalInMinutes. intervalInMinutes is chosen by the user using the radio buttons on the screen. The interval is set to 1 minute, 5, 15, 30 or 60. The AlarmManager class is an interface to the phone’s system alarm services and allows us to run code at a specific time even if the GpsTracker app is not running. In this method, we create a pending intent which allows AlarmManager to run our code using our application’s permissions. So what happens here is that every one minute, for instance, the alarmManager will execute the onReceive method of the GpsTrackerAlarmReceiver class.

Let’s take a look at the GpsTrackerAlarmReceiver class. First of all you’ll see that it extends the WakefulBroadcastReceiver class. This is very important. This is a helper class that creates a partial wake lock. Creating this wake lock insures that the cpu doesn’t go back to sleep while the background service is running. You need to set this permission in the AndroidManifest file:

As you can see the GpsTrackerAlarmReceiver class is very simple, with only one method.

When the alarmManager calls the onReceive method it starts the LocationService background service. If it’s already started, like if you have the interval set to one minute, then it only calls the onStartCommand method in the LocationService class. As long as the phone is running, the alarmManager will continue to start this service over and over again, get the phone’s location and then send the location to your update website.

What happens if somebody turns the phone off and then back on again, will we still get location updates? Well, glad you asked and the answer is yes, so let me show you how it’s done. There is another class called GpsTrackerBootReceiver which extends BroadcastReceiver. This time we will not use the wakeful receiver because all we are doing is creating a repeating alarmManager. This is a very fast operation and since we are not starting a service as in the previous class, we do not need to create an additional wake lock.

Note also that we are checking the variable currentlyTracking:

if we are currently tracking (as set by the user in the gpsTrackerActivity screen when they tap on the start tracking button), then start the alarmManager, otherwise cancel it.

This broadcast receiver’s onResume method is called every time the phone is rebooted. We’ll need to look at the AndroidManifest.xml file for a moment to see how that happens.

Here you can see we have registered a receiver with the name of our class, .GpsTrackerBootReceiver (don’t forget the period before the class name), and this receiver has a filter that says only listen to BOOT_COMPLETED events. So every time the phone is rebooted, call my onReceive method.

While we are looking in the AndroidManifest, note that GpsTrackerAlarmReceiver is also registered and the LocationService as well:

This brings us to the next class in the application, the LocationService. This class extends Service and runs on the main thread of the application. It runs in the background and will continue running even if the application shuts down. Since it runs on the main thread, any long running operations should be on their own threads or else the user could experience having the app’s UI freeze up (something you want to avoid…). This service does two things:

a) gets the phone’s location using Google play location services
b) sends the location data to the upload website

The first operation uses the requestLocationUpdates method of the LocationClient class to start getting location data from the phone. When the phone has a location that meets the criteria that we specify then it returns that location back to a callback method. That method’s name is onLocationChanged. This all happens asynchronously so we never have to worry about freezing up the UI with this operation.

The second operation uses this cool open source library:

Android Asynchronous Http Client

written by James Smith. As you can see from the name of the library, this allows us to make http calls asynchronously so once again, we don’t have to worry about tying up the UI. Also, this library handles being called from a background service. One less thing we have to worry about.

So when the service starts, it calls onCreate (like an activity) and then calls onStartCommand. If the service is already running and something else tries to start the service, a new service is not created, what happens is onStartCommand is called again. This is where I call the startTracking method from:

I first check the variable currentlyProcessingLocation. If the service has already been started (which might happen when we have an interval of one minute) then we don’t want to call startTracking twice. Sometimes it can take more than a minute to get that first satellite fix if we are using gps. So, the moment we start tracking, we set that variable to true and that solves the problem. START_STICKY means that if the service is somehow killed, then the system will restart the service and the service can continue doing what it’s supposed to do.

The next thing we do is create our locationClient. Once the client is connected we can then create a LocationRequest object in the onConnected callback method. Let’s take a look at the properties we are setting on the locationRequest object:

We set our interval to 1 second and our priority to PRIORITY_HIGH_ACCURACY. This means get the highest accuracy possible as quickly as possible. So the phone will be trying to get it’s location from the google play’s fused location provider. Fused means that the location can be coming from either gps, wifi, cell towers or other means. In the new google play location services, you no longer worry about which provider to use, you simply tell it what accuracy you want and it will figure it out on it’s own. I tell you this is a welcome relief from how things have been in the past. Stackoverflow has countless questions about which provider to use (gps vs. wifi vs. cell tower etc…) and it has always been a serious point of confusion for lots of developers, so good riddance.

You’ll notice that I’m using requestLocationUpdates instead of something like getLastLocation. I wanted the ability to filter my location by accuracy. That’s why I have the if block here in the onLocationChanged method:

to only accept a location that has an accuracy of 100 meters or better. This can be adjusted to fit your own needs. onLocationChanged is going to be called about once a second (since we’re using requestLocationUpdates), so once we have a location that meets our criteria, let’s send that location to the update website and shut down the service.

In the sendLocationDataToWebsite method, we create a RequestParams object, which is part of the loopj async http library and set all of our parameters for our POST http operation. We do our async post and on either success or failure, we shut down the service with this line:

One thing you’ll notice within the sendLocationDataToWebsite method is a little if/else block that checks if firstTimeGettingPosition is true. If it is true, then we need to save our latitude and longitude to shared preferences only. If it’s not true, then we need to create a new location object with our previously saved latitude and longitude. Once we have this, we can use the distanceTo method of the Location class to figure out how far we have moved since the last location update. This allows us to calculate our total distance traveled.

When the alarmManager reaches its interval again, the background service is recreated and the whole process starts again. This process works well on larger intervals such as five minutes, on shorter intervals like one minute, there is a chance that it could take google play location services a longer time to get a location fix that meets our criteria. What I’m getting at is that for shorter intervals, it might be less of an energy drain to keep the background service running continuously and keep google play location services running continuously. It may be less of a battery drain. The two scenarios need to be tested.

As the application stands now, I think it’s a good starting point for any further work. It works reliably and periodically in the background and restarts when the phone reboots. This would be a good starter app for other apps. If you want to see this app in action, you can download it from google play:

https://play.google.com/store/apps/details?id=com.websmithing.gpstracker

and then start tracking. Then go to the test webpage and search for your username in the routes dropdown box below the map. It should be near the top of the list:

https://www.websmithing.com/gpstracker/displaymap.php

The full source code can be found in my repo on github in the phoneClients android directory.