Simon's Wonder World

Once you figured out how OAuth works, you might ask yourself how this principle works from within a native Android app. It’s actually easier then expected at first. There exist some libraries that can make our life somehow easier, but I decided not to use any of them and to do everything from scratch.

This is the OAuth workflow for Foursquare:

• A call to the Foursquare server is made with the client id and a redirect URL (has to be in a browser)
• The user accepts the access request of the application
• Foursquare calls the redirect URL with special code as parameter
• The Foursquare server has to be called again with the client id, client secret, the code and the redirect URL (not in the browser anymore)
• Foursquare returns now the access token which has to be saved by the app
• Further calls to the Foursquare API can be made with the token

The only problem in this workflow is how we can make the first call in the browser and then return to our app.

OK, we’re going to register out app first as an OAuth Consumer at Foursquare. Define the Callback URL as “simontest-android-app://”. You can do so here.

Now, we create a new Android project and give it the INTERNET permission.

Then open the Manifest file and add the following filter to the <application> tag:

<application android:icon="@drawable/icon" android:label="@string/app_name">
	<activity android:name=".LoginTest" android:label="@string/app_name">
		<intent-filter>
			<action android:name="android.intent.action.MAIN" />
			<category android:name="android.intent.category.LAUNCHER" />
		</intent-filter>

		<!-- Add this filter -->
		<intent-filter>
 			<action android:name="android.intent.action.VIEW" />
 			<category android:name="android.intent.category.DEFAULT" />
 			<category android:name="android.intent.category.BROWSABLE" />
 			<data android:scheme="simontest-android-app" />
 		</intent-filter>

 	</activity>
</application>

This maps the schema “simontest-android-app” to this activity.

Now it’s time to start the coding. Add a button or something to your UI and add a click listener to it, which calls this method:

private void loginToForsquare() {
	// Call the webbrowser with the Foursquare OAuth login URL
	Intent intent = new Intent(Intent.ACTION_VIEW);
	intent.setData(
	      Uri.parse(
	    		  "https://foursquare.com/oauth2/authenticate" +
	    		  "?client_id=" +CLIENT_ID +
	    		  "&response_type=code" +
	    		  "&redirect_uri=simontest-android-app://"));
	startActivity(intent);
}

This will open the browser where the user can accept or deny our app to access his Foursquare data. After that the browser will automatically be redirected to the Callback URL “simontest-android-app://” which we have previously mapped to our activity. The activity will be called and we can check for the expected parameter in the onResume() method:

public void onResume() {
	super.onResume();

    // extract the OAUTH access token if it exists
    Uri uri = this.getIntent().getData();
    if(uri != null) {
    	String code = null;

    	// Check if ther is a parameter called "code"
    	if((code = uri.getQueryParameter("code")) != null){

			try {
				// Call Foursquare again to get the access token
				JSONObject tokenJson = executeHttpGet(
						"https://foursquare.com/oauth2/access_token" +
						"?client_id=" + CLIENT_ID +
						"&client_secret=" + CLIENT_SECRET +
						"&grant_type=authorization_code" +
						"&redirect_uri=simontest-android-app://" +
						"&code=" + code);

				String token = tokenJson.getString("access_token");

				// TODO save token

				// Get userdata of myself
				JSONObject userJson = executeHttpGet(
						"https://api.foursquare.com/v2/" +
						"users/self" +
						"?oauth_token=" +token);

				// Get return code
				int returnCode = Integer.parseInt(userJson.getJSONObject("meta").getString("code"));

				// 200 = OK
				if(returnCode == 200){
					// output data
					Log.i("LoginTest", userJson.getJSONObject("response").getJSONObject("user").toString());
				}else{
					Toast.makeText(this, "Wrong return code: " +code, Toast.LENGTH_SHORT).show();
				}
			} catch (Exception exp) {
				Log.e("LoginTest", "Login to Foursquare failed", exp);
			}
    	}else if((code = uri.getQueryParameter("error")) != null){
    		Toast.makeText(this, code, Toast.LENGTH_SHORT).show();
    	}else{
    		Toast.makeText(this, "Unknown login error", Toast.LENGTH_SHORT).show();
    	}
    }
}

// Calls a URI and returns the answer as a JSON object
private JSONObject executeHttpGet(String uri) throws Exception{
	HttpGet req = new HttpGet(uri);

	HttpClient client = new DefaultHttpClient();
	HttpResponse resLogin = client.execute(req);
	BufferedReader r = new BufferedReader(
			new InputStreamReader(resLogin.getEntity()
					.getContent()));
	StringBuilder sb = new StringBuilder();
	String s = null;
	while ((s = r.readLine()) != null) {
		sb.append(s);
	}

	return new JSONObject(sb.toString());
}

And that is already everything. You just might wanna save the token after you’ve got it so you don’t have to login to Foursquare the next time the user starts your app.

Happy Coding!