Implementing GCM Client

A Google Cloud Messaging (GCM) client is a GCM-enabled app that runs on an Android device. To write your client code, we recommend that you use the GCM APIs. The client helper library that was offered in previous versions of GCM still works, but it has been superseded by the more efficient GCM APIs.

A full GCM implementation requires both a client implementation and a server implementation. For more information about implementing the server side, see Implementing GCM Server.

The following sections walk you through the steps involved in writing a GCM client-side application. Your client app can be arbitrarily complex, but at bare minimum, a GCM client app must include code to register (and thereby get a registration ID), and a broadcast receiver to receive messages sent by GCM.

Step 1: Set Up Google Play Services

To write your client application, use the GCM APIs. To use this API, you must set up your project to use the Google Play services SDK, as described in Setup Google Play Services SDK.

Caution: When you add the Play Services library to your project, be sure to add it with resources, as described in Setup Google Play Services SDK. The key point is that you must reference the library—simply adding a .jar file to your Eclipse project will not work. You must follow the directions for referencing a library, or your app won't be able to access the library's resources, and it won't run properly. If you're using Android Studio, this is the string to add to the dependency section of your application's build.gradle file:

dependencies {
  compile "com.google.android.gms:play-services:3.1.+"
}

Step 2: Edit Your Application's Manifest

Add the following to your application's manifest:

  • The com.google.android.c2dm.permission.RECEIVE permission so the Android application can register and receive messages.
  • The android.permission.INTERNET permission so the Android application can send the registration ID to the 3rd party server.
  • The android.permission.GET_ACCOUNTS permission as GCM requires a Google account (necessary only if if the device is running a version lower than Android 4.0.4)
  • The android.permission.WAKE_LOCK permission so the application can keep the processor from sleeping when a message is received. Optional—use only if the app wants to keep the device from sleeping.
  • An applicationPackage + ".permission.C2D_MESSAGE" permission to prevent other Android applications from registering and receiving the Android application's messages. The permission name must exactly match this pattern—otherwise the Android application will not receive the messages.
  • A receiver for com.google.android.c2dm.intent.RECEIVE, with the category set as applicationPackage. The receiver should require the com.google.android.c2dm.SEND permission, so that only the GCM Framework can send a message to it. If your app uses an IntentService (not required, but a common pattern), this receiver should be an instance of WakefulBroadcastReceiver. A WakefulBroadcastReceiver takes care of creating and managing a partial wake lock for your app.
  • A Service (typically an IntentService) to which the WakefulBroadcastReceiver passes off the work of handling the GCM message, while ensuring that the device does not go back to sleep in the process. Including an IntentService is optional—you could choose to process your messages in a regular BroadcastReceiver instead, but realistically, most apps will use a IntentService.
  • If the GCM feature is critical to the Android application's function, be sure to set android:minSdkVersion="8" or higher in the manifest. This ensures that the Android application cannot be installed in an environment in which it could not run properly.

Here are excerpts from a sample manifest that supports GCM:

<manifest package="com.example.gcm" ...>

    <uses-sdk android:minSdkVersion="8" android:targetSdkVersion="17"/>
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.GET_ACCOUNTS" />
    <uses-permission android:name="android.permission.WAKE_LOCK" />
    <uses-permission android:name="com.google.android.c2dm.permission.RECEIVE" />

    <permission android:name="com.example.gcm.permission.C2D_MESSAGE"
        android:protectionLevel="signature" />
    <uses-permission android:name="com.example.gcm.permission.C2D_MESSAGE" />

    <application ...>
        <receiver
            android:name=".GcmBroadcastReceiver"
            android:permission="com.google.android.c2dm.permission.SEND" >
            <intent-filter>
                <action android:name="com.google.android.c2dm.intent.RECEIVE" />
                <category android:name="com.example.gcm" />
            </intent-filter>
        </receiver>
        <service android:name=".GcmIntentService" />
    </application>

</manifest>

Step 3: Write Your Application

Finally, write your application. This section features a sample client application that illustrates how to use the GoogleCloudMessaging API. The sample consists of a main activity (DemoActivity), a WakefulBroadcastReceiver (GcmBroadcastReceiver), and an IntentService (GcmIntentService). You can find the complete source code for this sample at the open source site.

Note the following:

  • Among other things, the sample illustrates registration and upstream (device-to-cloud) messaging. Upstream messaging only applies to apps that are running against a CCS (XMPP) server; HTTP-based servers don't support upstream messaging.
  • The GoogleCloudMessaging registration APIs replace the old registration process, which was based on the now-obsolete client helper library. While the old registration process still works, we encourage you to use the newer GoogleCloudMessaging registration APIs, regardless of your underlying server.

Check for Google Play Services APK

As described in Setup Google Play Services SDK, apps that rely on the Play Services SDK should always check the device for a compatible Google Play services APK before accessing Google Play services features. In the sample app this check is done in two places: in the main activity's onCreate() method, and in its onResume() method. The check in onCreate() ensures that the app can't be used without a successful check. The check in onResume() ensures that if the user returns to the running app through some other means, such as through the back button, the check is still performed. If the device doesn't have a compatible Google Play services APK, your app can call GooglePlayServicesUtil.getErrorDialog() to allow users to download the APK from the Google Play Store or enable it in the device's system settings. For example:

private final static int PLAY_SERVICES_RESOLUTION_REQUEST = 9000;
...
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    setContentView(R.layout.main);
    mDisplay = (TextView) findViewById(R.id.display);

    context = getApplicationContext();

    // Check device for Play Services APK.
    if (checkPlayServices()) {
        // If this check succeeds, proceed with normal processing.
        // Otherwise, prompt user to get valid Play Services APK.
        ...
    }
}

// You need to do the Play Services APK check here too.
@Override
protected void onResume() {
    super.onResume();
    checkPlayServices();
}

/**
 * Check the device to make sure it has the Google Play Services APK. If
 * it doesn't, display a dialog that allows users to download the APK from
 * the Google Play Store or enable it in the device's system settings.
 */
private boolean checkPlayServices() {
    int resultCode = GooglePlayServicesUtil.isGooglePlayServicesAvailable(this);
    if (resultCode != ConnectionResult.SUCCESS) {
        if (GooglePlayServicesUtil.isUserRecoverableError(resultCode)) {
            GooglePlayServicesUtil.getErrorDialog(resultCode, this,
                    PLAY_SERVICES_RESOLUTION_REQUEST).show();
        } else {
            Log.i(TAG, "This device is not supported.");
            finish();
        }
        return false;
    }
    return true;
}

Register for GCM

An Android application needs to register with GCM servers before it can receive messages. When an app registers, it receives a registration ID, which it can then store for future use (note that registration IDs must be kept secret). In the following snippet the onCreate() method in the sample app's main activity checks to see if the app is already registered with GCM and with the server:

/**
 * Main UI for the demo app.
 */
public class DemoActivity extends Activity {

    public static final String EXTRA_MESSAGE = "message";
    public static final String PROPERTY_REG_ID = "registration_id";
    private static final String PROPERTY_APP_VERSION = "appVersion";
    private final static int PLAY_SERVICES_RESOLUTION_REQUEST = 9000;

    /**
     * Substitute you own sender ID here. This is the project number you got
     * from the API Console, as described in "Getting Started."
     */
    String SENDER_ID = "Your-Sender-ID";

    /**
     * Tag used on log messages.
     */
    static final String TAG = "GCMDemo";

    TextView mDisplay;
    GoogleCloudMessaging gcm;
    AtomicInteger msgId = new AtomicInteger();
    SharedPreferences prefs;
    Context context;

    String regid;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.main);
        mDisplay = (TextView) findViewById(R.id.display);

        context = getApplicationContext();

        // Check device for Play Services APK. If check succeeds, proceed with
        //  GCM registration.
        if (checkPlayServices()) {
            gcm = GoogleCloudMessaging.getInstance(this);
            regid = getRegistrationId(context);

            if (regid.isEmpty()) {
                registerInBackground();
            }
        } else {
            Log.i(TAG, "No valid Google Play Services APK found.");
        }
    }
...
}

The app calls getRegistrationId() to see whether there is an existing registration ID stored in shared preferences:

/**
 * Gets the current registration ID for application on GCM service.
 * <p>
 * If result is empty, the app needs to register.
 *
 * @return registration ID, or empty string if there is no existing
 *         registration ID.
 */
private String getRegistrationId(Context context) {
    final SharedPreferences prefs = getGCMPreferences(context);
    String registrationId = prefs.getString(PROPERTY_REG_ID, "");
    if (registrationId.isEmpty()) {
        Log.i(TAG, "Registration not found.");
        return "";
    }
    // Check if app was updated; if so, it must clear the registration ID
    // since the existing regID is not guaranteed to work with the new
    // app version.
    int registeredVersion = prefs.getInt(PROPERTY_APP_VERSION, Integer.MIN_VALUE);
    int currentVersion = getAppVersion(context);
    if (registeredVersion != currentVersion) {
        Log.i(TAG, "App version changed.");
        return "";
    }
    return registrationId;
}
...
/**
 * @return Application's {@code SharedPreferences}.
 */
private SharedPreferences getGCMPreferences(Context context) {
    // This sample app persists the registration ID in shared preferences, but
    // how you store the regID in your app is up to you.
    return getSharedPreferences(DemoActivity.class.getSimpleName(),
            Context.MODE_PRIVATE);
}

If the registration ID doesn't exist or the app was updated, getRegistrationId() returns an empty string to indicate that the app needs to get a new regID. getRegistrationId() calls the following method to check the app version:

/**
 * @return Application's version code from the {@code PackageManager}.
 */
private static int getAppVersion(Context context) {
    try {
        PackageInfo packageInfo = context.getPackageManager()
                .getPackageInfo(context.getPackageName(), 0);
        return packageInfo.versionCode;
    } catch (NameNotFoundException e) {
        // should never happen
        throw new RuntimeException("Could not get package name: " + e);
    }
}

If there isn't a valid existing registration ID, DemoActivity calls the following registerInBackground() method to register. Note that because the GCM methods register() and unregister() are blocking, this has to take place on a background thread. This sample uses AsyncTask to accomplish this:

/**
 * Registers the application with GCM servers asynchronously.
 * <p>
 * Stores the registration ID and app versionCode in the application's
 * shared preferences.
 */
private void registerInBackground() {
    new AsyncTask() {
        @Override
        protected String doInBackground(Void... params) {
            String msg = "";
            try {
                if (gcm == null) {
                    gcm = GoogleCloudMessaging.getInstance(context);
                }
                regid = gcm.register(SENDER_ID);
                msg = "Device registered, registration ID=" + regid;

                // You should send the registration ID to your server over HTTP,
                // so it can use GCM/HTTP or CCS to send messages to your app.
                // The request to your server should be authenticated if your app
                // is using accounts.
                sendRegistrationIdToBackend();

                // For this demo: we don't need to send it because the device
                // will send upstream messages to a server that echo back the
                // message using the 'from' address in the message.

                // Persist the regID - no need to register again.
                storeRegistrationId(context, regid);
            } catch (IOException ex) {
                msg = "Error :" + ex.getMessage();
                // If there is an error, don't just keep trying to register.
                // Require the user to click a button again, or perform
                // exponential back-off.
            }
            return msg;
        }

        @Override
        protected void onPostExecute(String msg) {
            mDisplay.append(msg + "\n");
        }
    }.execute(null, null, null);
    ...
}

Once you've received your registration ID, send it to your server:

/**
 * Sends the registration ID to your server over HTTP, so it can use GCM/HTTP
 * or CCS to send messages to your app. Not needed for this demo since the
 * device sends upstream messages to a server that echoes back the message
 * using the 'from' address in the message.
 */
private void sendRegistrationIdToBackend() {
    // Your implementation here.
}

After registering, the app calls storeRegistrationId() to store the registration ID in shared preferences for future use. This is just one way of persisting a regID. You might choose to use a different approach in your app:

/**
 * Stores the registration ID and app versionCode in the application's
 * {@code SharedPreferences}.
 *
 * @param context application's context.
 * @param regId registration ID
 */
private void storeRegistrationId(Context context, String regId) {
    final SharedPreferences prefs = getGCMPreferences(context);
    int appVersion = getAppVersion(context);
    Log.i(TAG, "Saving regId on app version " + appVersion);
    SharedPreferences.Editor editor = prefs.edit();
    editor.putString(PROPERTY_REG_ID, regId);
    editor.putInt(PROPERTY_APP_VERSION, appVersion);
    editor.commit();
}

Handle registration errors

As stated above, an Android app must register with GCM servers and get a registration ID (regID) before it can receive messages. A given regID is not guaranteed to last indefinitely, so the first thing your app should always do is check to make sure it has a valid regID (as shown in the code snippets above).

In addition to confirming that it has a valid regID, your app should be prepared to handle the registration error TOO_MANY_REGISTRATIONS. This error indicates that the device has too many apps registered with GCM. The error only occurs in cases where there are extreme numbers of apps, so it should not affect the average user. The remedy is to prompt the user to delete some of the other GCM-enabled apps from the device to make room for the new one.

Send a message

When the user clicks the app's Send button, the app sends an upstream message using the GoogleCloudMessaging API. In order to receive the upstream message, your server should be connected to CCS. You can use one of the demo servers in Implementing an XMPP-based App Server to run the sample and connect to CCS.

public void onClick(final View view) {
    if (view == findViewById(R.id.send)) {
        new AsyncTask() {
            @Override
            protected String doInBackground(Void... params) {
                String msg = "";
                try {
                    Bundle data = new Bundle();
                        data.putString("my_message", "Hello World");
                        data.putString("my_action",
                                "com.google.android.gcm.demo.app.ECHO_NOW");
                        String id = Integer.toString(msgId.incrementAndGet());
                        gcm.send(SENDER_ID + "@gcm.googleapis.com", id, data);
                        msg = "Sent message";
                } catch (IOException ex) {
                    msg = "Error :" + ex.getMessage();
                }
                return msg;
            }

            @Override
            protected void onPostExecute(String msg) {
                mDisplay.append(msg + "\n");
            }
        }.execute(null, null, null);
    } else if (view == findViewById(R.id.clear)) {
        mDisplay.setText("");
    }
}

Receive a message

As described above in Step 2, the app includes a WakefulBroadcastReceiver for the com.google.android.c2dm.intent.RECEIVE intent. A broadcast receiver is the mechanism GCM uses to deliver messages. When onClick() calls gcm.send(), it triggers the broadcast receiver's onReceive() method, which has the responsibility of making sure that the GCM message gets handled.

A WakefulBroadcastReceiver is a special type of broadcast receiver that takes care of creating and managing a partial wake lock for your app. It passes off the work of processing the GCM message to a Service (typically an IntentService), while ensuring that the device does not go back to sleep in the transition. If you don't hold a wake lock while transitioning the work to a service, you are effectively allowing the device to go back to sleep before the work completes. The net result is that the app might not finish processing the GCM message until some arbitrary point in the future, which is not what you want.

Note: Using WakefulBroadcastReceiver is not a requirement. If you have a relatively simple app that doesn't require a service, you can intercept the GCM message in a regular BroadcastReceiver and do your processing there. Once you get the intent that GCM passes into your broadcast receiver's onReceive() method, what you do with it is up to you.

This snippet starts GcmIntentService with the method startWakefulService(). This method is comparable to startService(), except that the WakefulBroadcastReceiver is holding a wake lock when the service starts. The intent that is passed with startWakefulService() holds an extra identifying the wake lock:

public class GcmBroadcastReceiver extends WakefulBroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
        // Explicitly specify that GcmIntentService will handle the intent.
        ComponentName comp = new ComponentName(context.getPackageName(),
                GcmIntentService.class.getName());
        // Start the service, keeping the device awake while it is launching.
        startWakefulService(context, (intent.setComponent(comp)));
        setResultCode(Activity.RESULT_OK);
    }
}

The intent service shown below does the actual work of handling the GCM message. When the service is finished, it calls GcmBroadcastReceiver.completeWakefulIntent() to release the wake lock. The completeWakefulIntent() method has as its parameter the same intent that was passed in from the WakefulBroadcastReceiver.

This snippet processes the GCM message based on message type, and posts the result in a notification. But what you do with GCM messages in your app is up to you—the possibilities are endless. For example, the message might be a ping, telling the app to sync to a server to retrieve new content, or it might be a chat message that you display in the UI.

public class GcmIntentService extends IntentService {
    public static final int NOTIFICATION_ID = 1;
    private NotificationManager mNotificationManager;
    NotificationCompat.Builder builder;

    public GcmIntentService() {
        super("GcmIntentService");
    }

    @Override
    protected void onHandleIntent(Intent intent) {
        Bundle extras = intent.getExtras();
        GoogleCloudMessaging gcm = GoogleCloudMessaging.getInstance(this);
        // The getMessageType() intent parameter must be the intent you received
        // in your BroadcastReceiver.
        String messageType = gcm.getMessageType(intent);

        if (!extras.isEmpty()) {  // has effect of unparcelling Bundle
            /*
             * Filter messages based on message type. Since it is likely that GCM
             * will be extended in the future with new message types, just ignore
             * any message types you're not interested in, or that you don't
             * recognize.
             */
            if (GoogleCloudMessaging.
                    MESSAGE_TYPE_SEND_ERROR.equals(messageType)) {
                sendNotification("Send error: " + extras.toString());
            } else if (GoogleCloudMessaging.
                    MESSAGE_TYPE_DELETED.equals(messageType)) {
                sendNotification("Deleted messages on server: " +
                        extras.toString());
            // If it's a regular GCM message, do some work.
            } else if (GoogleCloudMessaging.
                    MESSAGE_TYPE_MESSAGE.equals(messageType)) {
                // This loop represents the service doing some work.
                for (int i=0; i<5; i++) {
                    Log.i(TAG, "Working... " + (i+1)
                            + "/5 @ " + SystemClock.elapsedRealtime());
                    try {
                        Thread.sleep(5000);
                    } catch (InterruptedException e) {
                    }
                }
                Log.i(TAG, "Completed work @ " + SystemClock.elapsedRealtime());
                // Post notification of received message.
                sendNotification("Received: " + extras.toString());
                Log.i(TAG, "Received: " + extras.toString());
            }
        }
        // Release the wake lock provided by the WakefulBroadcastReceiver.
        GcmBroadcastReceiver.completeWakefulIntent(intent);
    }

    // Put the message into a notification and post it.
    // This is just one simple example of what you might choose to do with
    // a GCM message.
    private void sendNotification(String msg) {
        mNotificationManager = (NotificationManager)
                this.getSystemService(Context.NOTIFICATION_SERVICE);

        PendingIntent contentIntent = PendingIntent.getActivity(this, 0,
                new Intent(this, DemoActivity.class), 0);

        NotificationCompat.Builder mBuilder =
                new NotificationCompat.Builder(this)
        .setSmallIcon(R.drawable.ic_stat_gcm)
        .setContentTitle("GCM Notification")
        .setStyle(new NotificationCompat.BigTextStyle()
        .bigText(msg))
        .setContentText(msg);

        mBuilder.setContentIntent(contentIntent);
        mNotificationManager.notify(NOTIFICATION_ID, mBuilder.build());
    }
}

Running the Sample

To run the sample:

  1. Follow the instructions in Getting Started to get your sender ID and API key.
  2. Implement your client app, as described in this document. You can find the complete source code for the client app at the open source site.
  3. Run one of the demo servers (Java or Python) provided in Implementing an XMPP-based App Server. Whichever demo server you choose, don't forget to edit its code before running it to supply your sender ID and API key.

Viewing Statistics

To view statistics and any error messages for your GCM applications:

  1. Go to the Developer Console.
  2. Login with your developer account.

    You will see a page that has a list of all of your apps.

  3. Click on the "statistics" link next to the app for which you want to view GCM stats.

    Now you are on the statistics page.

  4. Go to the drop-down menu and select the GCM metric you want to view.

Note: Stats on the Google API Console are not enabled for GCM. You must use the Developer Console.