Subscribe to DisplayAlert in Xamarin.Forms - testing

Question

I'd like to get notified whenever DisplayAlert is called somewhere in my app. The Xamarin.Forms source code suggests to use the MessagingCenter, since it is using it to send a message within DisplayAlert():
MessagingCenter.Send(this, AlertSignalName, args);
But I haven't been able to receive anything, yet. This is one of the lines I tried so far:
MessagingCenter.Subscribe<Page>(this, Page.AlertSignalName, arg => {
Console.WriteLine("Message received: " + arg);
});
Is this the right direction? Or do you have an alternative solution? I'd even consider some hacky reflection-based approach, since I need it for testing purposes only.

Answer 1

This works for me:
MessagingCenter.Subscribe<Page, Xamarin.Forms.Internals.AlertArguments>(this, Page.AlertSignalName, (obj, obj1) => {
int aa = 0;
});
But it is raised when DisplayAlert is displayed. (I think it's correct...)
This is my page:
using System;
using Xamarin.Forms;
namespace Test
{
public class MyPage : ContentPage
{
public MyPage()
{
Button b = new Button {Text = "Press" };
b.Clicked += async (object sender, EventArgs e) => {
await DisplayAlert("Attention", "AAA", "Ok");
MessagingCenter.Send<MyPage>(this, "AAA");
};
Content = new StackLayout
{
Children = {
new Label { Text = "Hello ContentPage" },
b
}
};
MessagingCenter.Subscribe<MyPage>(this, "AAA", (obj) => {
int aa = 0;
});
MessagingCenter.Subscribe<Page, Xamarin.Forms.Internals.AlertArguments>(this, Page.AlertSignalName, (obj, obj1) => {
int aa = 0;
});
}
}
}

Answer 2

In Xamarin source test code (namespace Xamarin.Forms.Core.UnitTests), it's used in a strongly typed way like that:
[Test]
public void DisplayAlert ()
{
var page = new ContentPage ();
AlertArguments args = null;
MessagingCenter.Subscribe (this, Page.AlertSignalName, (Page sender, AlertArguments e) => args = e);
var task = page.DisplayAlert ("Title", "Message", "Accept", "Cancel");
Assert.AreEqual ("Title", args.Title);
Assert.AreEqual ("Message", args.Message);
Assert.AreEqual ("Accept", args.Accept);
Assert.AreEqual ("Cancel", args.Cancel);
bool completed = false;
var continueTask = task.ContinueWith (t => completed = true);
args.SetResult (true);
continueTask.Wait ();
Assert.True (completed);
}
So this should do it:
MessagingCenter.Subscribe(this, Page.AlertSignalName, (Page sender, AlertArguments args) =>
{
Console.WriteLine("Message received: " + args.Message);
});