Using Google Cloud Messaging (GCM) on Android and iOS in Xamarin

Assuming you've already got a Xamarin Forms project set up and ready to go, we'll begin with installing the Xamarin GCM service into your app and then talk about adding the code to interface with it.

Installing GCM:

You'll want to begin by right clicking the Portable or Shared project within the solution and clicking 'Manage NuGet Packages'. Once open, you'll need to click 'Browse' at the top left and then search for 'Xamarin.GooglePlayServices.Gcm' and install the package. Note: Make sure you've got the correct package, the 'Author' label displays as 'Xamarin Inc'. You'll want to repeat this process for both the Android and iOS projects in the solution.

Before you link GCM to your app, you'll need to make Google aware of it by going to the Google Developer Console and creating your app, then adding GCM to it. You might find this page useful for helping you create it: http://dev.tapjoy.com/faq/how-to-find-sender-id-and-api-key-for-gcm/

Android:

Within your 'AndroidManifest.xml' file add the following permissions to enable your app to receive notifications:

<uses-permission android:name="com.google.android.c2dm.permission.RECEIVE" />
<uses-permission android:name="android.permission.WAKE_LOCK" />
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="{MYAPPNAMESPACE}.permission.C2D_MESSAGE" />

Make sure to add them above <application...>...</application>. And replace {MYAPPNAMESPACE} with the root namespace that your app runs in.

Then add the following within the application tags:

<receiver android:name="com.google.android.gms.gcm.GcmReceiver" android:exported="true" android:permission="com.google.android.c2dm.permission.SEND">
  <intent-filter>
    <action android:name="com.google.android.c2dm.intent.RECEIVE" />
    <action android:name="com.google.android.c2dm.intent.REGISTRATION" />
    <category android:name="bifa.droid" />
  </intent-filter>
</receiver>

This tells the Android phone to hook in a receiver to look for information sent by GCM, you'll want to create a file called GCMListenerService.cs and add it to it.

namespace {MYAPPNAMESPACE}
{
    [Service (Exported = false), IntentFilter (new [] { "com.google.android.c2dm.intent.RECEIVE" })]
    public class MyGcmListenerService : GcmListenerService
    {
        public override void OnMessageReceived (string from, Bundle data)
        {
            string id = data.GetString ("id");
            string title = data.GetString ("title");
            string message = data.GetString ("message");

            Log.Debug ("MyGcmListenerService", "From:    " + from);
            Log.Debug ("MyGcmListenerService", "Message: " + message);

            SendNotification (id, title, message);
        }

        void SendNotification (string id, string title, string message)
        {

                var intent = new Intent (this, typeof(MainActivity));
                intent.AddFlags (ActivityFlags.ClearTop);
                intent.PutExtra("id", int.Parse(id));
                var pendingIntent = PendingIntent.GetActivity (this, 0, intent, PendingIntentFlags.OneShot);

                var notificationBuilder = new Notification.Builder(this)
                    .SetSmallIcon (Resource.Drawable.icon)
                    .SetVibrate(new long[] { 0, 200 })
                    .SetContentTitle (title)
                    .SetContentText (message)
                    .SetAutoCancel (true)
                    .SetContentIntent (pendingIntent);

                var notificationManager = (NotificationManager)GetSystemService(Context.NotificationService);
                notificationManager.Notify (0, notificationBuilder.Build());
        }
    }
}

iOS: