Working with Google Docs and OAuth on Android (part two)

In part one I have described the process of obtaining an OAuth access token for Google Apps. Part two will focus on the implementation of that process, and then - in part three - I will show you how to use the OAuth credentials to retrieve data from Google Docs. If you're not bothered reading and just want the sample code click here. I am skipping a lot of things below, so you will want to download that sample code anyway.

Let's start with creating a simple layout containing two buttons and a text view. Call the buttons B1 and B2, and the text view T1. I will leave this part as an exercise.


Next create a new class called OAuthStageOne, extending AsyncTask. This class will provide the functionality required for authenticating the user and retrieving the initial authorization.

public class OAuthStageOne extends AsyncTask<Void, Void, Void> { [..] }

This class will require access to an OAuth provider and consumer, and it also needs to know the context it is running in. Let's gather those details in the constructor:

public OAuthStageOne(Context context, CommonsHttpOAuthProvider provider, CommonsHttpOAuthConsumer consumer) { [..] }

Note that we are using the CommonsHttpOAuth provider and consumer classes, rather than the DefaultOAuth classes. The later ones have issues when working with Google Apps.

Next, we need to override the doInBackground() method of AsyncTask and put our stage one code there. All this code does is create a token request URL and then start the default web browser to open the Google Apps authentication/authorization site. This could just as well be done from within the main controller activity, but using AsyncTask will safeguard the application from being force closed due to a slow network connection etc.

The token request URL is obtained using the retrieveRequestToken() method of the OAuth provider. We pass the consumer and the callback URL as parameters:

final String url = provider.retrieveRequestToken(consumer, C.OAuth.CALLBACK_URL);

Now start the browser using this URL.

Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse(url));

intent.setFlags(
Intent.FLAG_ACTIVITY_SINGLE_TOP | 
Intent.FLAG_ACTIVITY_NO_HISTORY | 
Intent.FLAG_FROM_BACKGROUND);

context.startActivity(intent);

That's stage one done. The browser will open with the standard Google login box and once the user has logged-in they will be offered to either grant or deny our application access to their account. Once the user grants access the browser will call our callback URL. What happens now is the OAuthMain activity will be called with a new intent, hence the onNewIntent() method will fire. In that method we check if the intent uri scheme is the one our callback is registered with, and if yes, we launch stage two.

@Override
public void onNewIntent(Intent intent) {
 super.onNewIntent(intent);
 final Uri uri = intent.getData();
 if (uri != null && uri.getScheme().equals(C.OAuth.CALLBACK_SCHEME)) {
  new OAuthStageTwo(this, provider, consumer).execute(uri);
  finish(); 
 }
}

Notice that we again pass the provider and consumer. You may be wondering where these came from, since there's no initialization code in that method. If you have a look at the AndroidManifext.xml file in the sample project you will notice the OAuthMain activity marked as "singleTask". This way we prevent multiple copies of the activity being started (which would happen with the callback!), and all previously initialized class variables are available as a result.

Next create a new class called OAuthStageTwo, again extending AsyncTask. This class will provide the functionality required for retrieving the long-term access token and storing it in application preferences. The process is very simple - retrieve the OAuth verifier string from callback URI, and pass it along with the consumer instance to the retrieveAccessToken() method of the provider. Then store the retrieved token and token secret in application preferences. This will fire the onSharedPreferencesChanged() listener in the controller activity and finish the process.

[..]
provider.retrieveAccessToken(consumer, oauth_verifier);

final Editor edit = prefs.edit();
edit.putString(OAuth.OAUTH_TOKEN, consumer.getToken());
edit.putString(OAuth.OAUTH_TOKEN_SECRET, consumer.getTokenSecret());
edit.commit();
[..]

And that's about it for OAuth. In part three I will explain how to use the access token to retrieve data from Google Docs. Alternatively you can download the entire sample project from Google.