Xamarin Developer - Alert Center

AlertCenter displays notifications in a manner similar to native iOS banner notifications.

iOS Example

using Xamarin.Controls;
...

public override void ViewDidLoad ()
{
    base.ViewDidLoad ();

    AlertCenter.Default.PostMessage ("Knock knock!", "Who's there?");
    AlertCenter.Default.PostMessage ("Interrupting cow.", "Interrupting cow who?",
                                UIImage.FromFile ("cow.png"), delegate {
        Console.WriteLine ("Moo!");
    });
}

Android Example

Using AlertCenter on Android is similar to iOS, except that you need to initialize the AlertCenter with a Context. The permission SYSTEM_ALERT_WINDOW will also need to be declared in your AndroidManfiest.xml.

using Xamarin.Controls;
...

protected override void OnCreate (Bundle bundle)
{
    base.OnCreate (bundle);

    AlertCenter.Default.Init (Application);

    AlertCenter.Default.PostMessage ("Knock knock!", "Who's there?", Resource.Drawable.Icon);
    AlertCenter.Default.PostMessage ("Interrupting cow.", "Interrupting cow who?",
                                Resource.Drawable.Icon, () => {
        Console.WriteLine ("Moo!");
    });
}

Source: https://components.xamarin.com/gettingstarted/alert-center/true