MessagingCenter

public class SubscriberThing 
{
    public int IntProperty { get; set; }
}

var subscriber = new SubscriberThing();
MessagingCenter.Subscribe<MyPage, int> (subscriber, "IntPropertyMessage", (s, e) => {
    subscriber.IntProperty = e; 
});

//...later...

MessagingCenter.Send<MyPage, int>(this, "IntPropertyMessage", 2);
Assert.AreEqual(2, subscriber.IntProperty);