Exemplo de BroadcastReceiver

MainActivity.cs

[Activity(Label = "Cronometro", MainLauncher = true)]
    public class MainActivity : Activity
    {
        Receiver1 receiver;
        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);

            // Set our view from the "main" layout resource
            SetContentView(Resource.Layout.Main);            

            receiver = new Receiver1();

            RegisterReceiver(receiver, new IntentFilter("com.xamarin.example.TEST"));
            Intent intent = new Intent("com.xamarin.example.TEST");
            intent.PutExtra("recado", "Funcionou comigo!");
            SendBroadcast(intent);

        }

        protected override void OnResume()
        {
            base.OnResume();
            RegisterReceiver(receiver, new IntentFilter("com.xamarin.example.TEST"));
            // Code omitted for clarity
        }
    }

Receiver1.cs

[BroadcastReceiver]
    [IntentFilter(new[] { "com.xamarin.example.TEST" })]

    public class Receiver1 : BroadcastReceiver
    {
        public override void OnReceive(Context context, Intent intent)
        {
            string recado = intent.GetStringExtra("recado");
            Toast.MakeText(context, "Received intent! RECADO: "+recado, ToastLength.Short).Show();
        }
    }