A first implementation of alert dialog support in Xamarin + MVVMCross

Update: You can use this MvvmCross plugin: https://github.com/brianchance/MvvmCross-UserInteraction
              At this moment however, the plugin doesn't have an implementation for Windows Phone and Windows Store, it's only for Android and iOS.

The implementation steps:

#1. In the Core PCL project, have the dialog interface declared in IDialogService.cs.

namespace MyApp.Core.Services
{
    public interface IDialogService
    {
        Task<bool?> ShowAsync(string message, string title, string OKButtonContent, string CancelButtonContent);
    }
}

#2. In the platform specific project, implement the dialog support. Example for Android:

namespace MyApp.Droid.Services
{
    public class DialogService : IDialogService
    {
        public Task<bool?> ShowAsync(string message, string title, string OKButtonContent, string CancelButtonContent)
        {
            var tcs = new TaskCompletionSource<bool?>();

            var mvxTopActivity = Mvx.Resolve<IMvxAndroidCurrentTopActivity>();
            AlertDialog.Builder builder = new AlertDialog.Builder(mvxTopActivity.Activity);
            builder.SetTitle(title)
                   .SetMessage(message)
                   .SetCancelable(false)
                   .SetPositiveButton(OKButtonContent, (s, args) =>
                    {
                        tcs.SetResult(true);
                    })
                   .SetNegativeButton(CancelButtonContent, (s, args) =>
                   {
                       tcs.SetResult(false);
                   });

            builder.Create().Show();
            return tcs.Task;
        }
    }
}

#3. Still in the platform specific project, register the service:

namespace MyApp.Droid
{
    public class Setup : MvxAndroidSetup
    {
        public Setup(Context applicationContext) : base(applicationContext)
        {
        }

        protected override void InitializeLastChance()
        {
            Mvx.RegisterSingleton<IDialogService>(new DialogService());
            base.InitializeLastChance();
        }
    }
}

#4. In the view models, here is how I can call the display of the dialog:

void async DeleteUser()
{
    var result = await Mvx.Resolve<IDialogService>().Show("Are you sure you want to delete selected user?", "Confirmation", "OK", "Cancel");
    if(result == true)
    {
        // delete user...
    }
}

This is a first implementation of showing dialogs. There’s a lot of possible parameterization / customization, but it needs to take into account the capabilities and behavior of all platforms.
In tests, the IDialogService implementation obviously does not call showing a dialog, it does nothing.

One interesting discussion I had with Greg Shackles is about using a different approach than what I showed here: have the view full responsibility of displaying the dialog and communicate with view model using commands \ methods, etc. I understand the idea but I don't have a very clear way of doing this so until I see all the issues I think I will use this solution.