Restful API service

If your service is going to be part of you application then you are making it way more complex than it needs to be. Since you have a simple use case of getting some data from a RESTful Web Service, you should look into ResultReceiver and IntentService.

This Service + ResultReceiver pattern works by starting or binding to the service with startService() when you want to do some action. You can specify the operation to perform and pass in your ResultReceiver (the activity) through the extras in the Intent.

In the service you implement onHandleIntent to do the operation that is specified in the Intent. When the operation is completed you use the passed in ResultReceiver to send a message back to the Activity at which point onReceiveResult will be called.

So for example, you want to pull some data from your Web Service.

You create the intent and call startService. The operation in the service starts and it sends the activity a message saying it started The activity processes the message and shows a progress. The service finishes the operation and sends some data back to your activity. Your activity processes the data and puts in in a list view The service sends you a message saying that it is done, and it kills itself. The activity gets the finish message and hides the progress dialog.

I know you mentioned you didn't want a code base but the open source Google I/O 2010 app uses a service in this way I am describing.

Updated to add sample code:

The activity.

public class HomeActivity extends Activity implements MyResultReceiver.Receiver {

    public MyResultReceiver mReceiver;

    public void onCreate(Bundle savedInstanceState) {
        mReceiver = new MyResultReceiver(new Handler());
        mReceiver.setReceiver(this);
        ...
        final Intent intent = new Intent(Intent.ACTION_SYNC, null, this, QueryService.class);
        intent.putExtra("receiver", mReceiver);
        intent.putExtra("command", "query");
        startService(intent);
    }

    public void onPause() {
        mReceiver.setReceiver(null); // clear receiver so no leaks.
    }

    public void onReceiveResult(int resultCode, Bundle resultData) {
        switch (resultCode) {
        case RUNNING:
            //show progress
            break;
        case FINISHED:
            List results = resultData.getParcelableList("results");
            // do something interesting
            // hide progress
            break;
        case ERROR:
            // handle the error;
            break;
    }
}

The Service:

public class QueryService extends IntentService {
    protected void onHandleIntent(Intent intent) {
        final ResultReceiver receiver = intent.getParcelableExtra("receiver");
        String command = intent.getStringExtra("command");
        Bundle b = new Bundle();
        if(command.equals("query") {
            receiver.send(STATUS_RUNNING, Bundle.EMPTY);
            try {
                // get some data or something           
                b.putParcelableArrayList("results", results);
                receiver.send(STATUS_FINISHED, b)
            } catch(Exception e) {
                b.putString(Intent.EXTRA_TEXT, e.toString());
                receiver.send(STATUS_ERROR, b);
            }    
        }
    }
}

ResultReceiver extension - edited about to implement MyResultReceiver.Receiver

public class MyResultReceiver implements ResultReceiver {
    private Receiver mReceiver;

    public MyResultReceiver(Handler handler) {
        super(handler);
    }

    public void setReceiver(Receiver receiver) {
        mReceiver = receiver;
    }

    public interface Receiver {
        public void onReceiveResult(int resultCode, Bundle resultData);
    }

    @Override
    protected void onReceiveResult(int resultCode, Bundle resultData) {
        if (mReceiver != null) {
            mReceiver.onReceiveResult(resultCode, resultData);
        }
    }
}

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Developing Android REST client applications has been an awesome resource for me. The speaker does not show any code, he just goes over design considerations and techniques in putting together a rock solid Rest Api in android. If your a podcast kinda person or not, I'd recommend giving this one at least one listen but, personally I've listened to it like 4 or five times thus far and I'm probably going to listen to it again.

Developing Android REST client applications
Author: Virgil Dobjanschi
Description:

This session will present architectural considerations for developing RESTful applications on the Android platform. It focuses on design patterns, platform integration and performance issues specific to the Android platform.

And there are so many considerations I really hadn't made in the first version of my api that I've had to refactor

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Also when I hit the post(Config.getURL("login"), values) the app seems to pause for a while (seems weird - thought the idea behind a service was that it runs on a different thread!)

No you have to create a thread yourself, a Local service runs in the UI thread by default.

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I know @Martyn does not want full code, but I think this annotation its good for this question:

10 Open Source Android Apps which every Android developer must look into

Foursquared for Android is open-source, and have an interesting code pattern interacting with the foursquare REST API.

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I would highly recommend the REST client Retrofit.

I have found this well written blog post extremely helpful, it also contains simple example code. The author uses Retrofit to make the network calls and Otto to implement a data bus pattern:

http://www.mdswanson.com/blog/2014/04/07/durable-android-rest-clients.html

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Just wanted to point you all in the direction of an standalone class I rolled that incorporates all of the functionality.

http://github.com/StlTenny/RestService

It executes the request as non-blocking, and returns the results in an easy to implement handler. Even comes with an example implementation.

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Lets say I want to start the service on an event - onItemClicked() of a button. The Receiver mechanism would not work in that case because :-
a) I passed the Receiver to the service (as in Intent extra) from onItemClicked()
b) Activity moves to the background. In onPause() I set the receiver reference within the ResultReceiver to null to avoid leaking the Activity.
c) Activity gets destroyed.
d) Activity gets created again. However at this point the Service will not be able to make a callback to the Activity as that receiver reference is lost.
The mechanism of a limited broadcast or a PendingIntent seems to be more usefull in such scenarios- refer to Notify activity from service

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Note that the solution from Robby Pond is somehow lacking: in this way you only allow todo one api call at a time since the IntentService only handles one intent at a time. Often you want to perform parallel api calls. If you want todo this you have to extend Service instead of IntentService and create your own thread.

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Also when I hit the post(Config.getURL("login"), values) the app seems to pause for a while (seems weird - thought the idea behind a service was that it runs on a different thread!)

In this case its better to use asynctask, which runs on a different thread and return result back to the ui thread on completion.

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Robby provides a great answer, though I can see you still looking for more information. I implemented REST api calls the easy BUT wrong way. It wasn't until watching this Google I/O video that I understood where I went wrong. It's not as simple as putting together an AsyncTask with a HttpUrlConnection get/put call.

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There is another approach here which basically helps you to forget about the whole management of the requests. It is based on an async queue method and a callable/callback based response. The main advantage is that by using this method you'll be able to make the whole process (request, get and parse response, sabe to db) completely transparent for you. Once you get the response code the work is already done. After that you just need to make a call to your db and you are done. It helps as well with the problematic of what happens when your activity is not active. What will happen here is that you'll have all your data saved in your local database but the response won't be processed by your activity, that's the ideal way.

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