Show AlertDialog from ViewModel using MvvmCross

Question

I am using MvvmCross for creation my Android-app and I faced with the following problem:

When I'm trying to show AlertDialog, that was created in ViewModel, the

"Unhandled Exception: Android.Views.WindowManagerBadTokenException" appears.

public class MyViewModel : MvxViewModel
{
    public ICommand ShowAlertCommand { get; private set; }

    public AuthorizationViewModel()
    {
        ShowAlertCommand = new MvxCommand(() =>
            {
                var adb = new AlertDialog.Builder(Application.Context); 
                adb.SetTitle("Title here");
                adb.SetMessage("Message here");  
                adb.SetIcon(Resource.Drawable.Icon);
                adb.SetPositiveButton("OK", (sender, args) => { /* some logic */});   
                adb.SetNegativeButton("Cancel", (sender, args) => { /* close alertDialog */});

                adb.Create().Show();
            });
    }
}

When I was researching I have found that it happens because of transmission of the reference to the Context but not on the Activity in the AlertDialog.Builder.

In this topic I found the following decision: Receive references to the current Activity through the use of GetService(), but I didn't found mvvmcross plugins for work with IMvxServiceConsumer, IMvxAndroidCurrentTopActivity interfaces.

My question is can I show AlertDialog from ViewModel? And how can I get the reference to Activity, but not to the Application.Context? And what is the correct way to close AlertDialog that the user would stay on the current View?

Answer 1

In general, you should try not to put this type of code into ViewModels

• because ViewModels should stay platform independent
• because ViewModels should be unit testable - and it's hard to unit test when the code shows a dialog

I'd also recommend you don't put code like this inside a ViewModel Constructor - these constructors are generally called during navigations and displaying a Dialog during a transition is likely to be problematic.

---

With those things said, if you do want to get hold of the current top Activity within any code, then you can do this using the IMvxAndroidCurrentTopActivity

public interface IMvxAndroidCurrentTopActivity
{
    Activity Activity { get; }
}

Using this, any code can get the current Activity using:

var top = Mvx.Resolve<IMvxAndroidCurrentTopActivity>();
var act = top.Activity;
if (act == null)
{
   // this can happen during transitions
   // - you need to be sure that this won't happen for your code
   throw new MvxException("Cannot get current top activity");
}

var dlg = new AlertDialog.Builder(act); 
//...
dlg.Create().Show();

---

The use of IMvxAndroidCurrentTopActivity is discussed in MvvmCross: How to pass Android context down to MvxCommand?

The approach taken in that question/answer is also one of the ways I would generally approach showing dialogs from a ViewModel:

• I would create an IFooDialog interface
• Ideally I would probably make this interface asynchronous - e.g. using async or using an Action<DialogResult> callback parameter
• on each platform I would implement that in the UI project
• the ViewModels can then use IFooDialog when a dialog is needed and each platform can respond with an appropriate UI action

This 'Dialog Service' type of approach is common in Mvvm - e.g. see articles like http://www.codeproject.com/Articles/36745/Showing-Dialogs-When-Using-the-MVVM-Pattern (although that article is very Windows specific!)

There are also a few other questions on here about MvvmCross and dialogs - although they may contain reference to older v1 or vNext code - e.g. Alerts or Popups in MvvmCross and Unable run ProgressDialog - BadTokenException while showind