UI Doesnt update as expected in WPF MVVM application with Caliburn Micro and BusyIndicator control

Question

I'm writing an application in WPF using Caliburn Micro. Following some tutorials on their website I wanted to implement a BusyIndicator control from the Xceed.Wpf.Toolkit. I'm using coroutines and return IEnumerable from my method to do 3 things: show the busy indicator, switch screens, hide the busy indicator. Seems simple enough, but whats happening is, the BusyIndicator never shows up. I think there's something I don't understand about the way WPF renders its controls. Here's some code.

This is my Loader class for displaying my BusyIndicator control on the ShellView.xaml

public class Loader : IResult
{


    private readonly String _message;
    private readonly bool _hide;
    private readonly IShell _shell;

    public Loader(IShell shell, String message)
    {
        _message = message;
        _shell = shell;
    }

    public Loader(IShell shell, bool hide)
    {
        _hide = hide;
        _shell = shell;
    }

    public void Execute(CoroutineExecutionContext context)
    {
        var view = _shell.View as ShellView;
        if (view == null)
            return;

        if (_hide)
        {
            view.BusyIndicator.IsBusy = false;
        }
        else
        {
            view.BusyIndicator.BusyContent = _message;
            view.BusyIndicator.IsBusy = true;
            // I WOULD ASSUME THIS WOULD IMMEDIATELY UPDATE THE BusyIndicator CONTROL TO SHOW BUT IT DOESNT
        }
        Completed(this, new ResultCompletionEventArgs());
    }


    public event EventHandler<ResultCompletionEventArgs> Completed = delegate { };

    public static IResult Show(String message = null)
    {
        return new Loader(IoC.Get<IShell>(), message);
    }

    public static IResult Hide()
    {
        return new Loader(IoC.Get<IShell>(), true);
    }

}

This is my ShowScreen class that navigates to the next screen by getting the IShell and calling ActivateItem. Nothing fancy here.

public class ShowScreen : IResult
{

private readonly Type _screenType;

    public ShowScreen(Type screenType)
    {
        _screenType = screenType;
    }

    public void Execute(CoroutineExecutionContext context)
    {
        var screen = IoC.GetInstance(_screenType, null);
        shell.ActivateItem(screen);
        Completed(this, new ResultCompletionEventArgs());
    }

    public event EventHandler<ResultCompletionEventArgs> Completed;

    public static ShowScreen Of<T>()
    {
        return new ShowScreen(typeof(T));
    }

}

Both of these on their own work with no problems, its when I chain them together in a coroutine like this is when it doesnt work the way I'd expect:

    public class HomeViewModel : Screen
{

    public IEnumerable<IResult> OpenFirstPage()
    {
        yield return Loader.Show("Please Wait");
        yield return ShowScreen.Of<FirstPageViewModel>();
        yield return Loader.Hide();
    }

}

I almost feel like I need to tell WPF to explicitly show my BusyIndicator somehow. Like it doesn't instantly show the BusyIndicator when I tell it to. When I take out the last Loader.Hide() command, it navigates to the next screen THEN shoes the BusyIndicator. This is driving me insane.

Answer 1

After messing with this stupid thing all night I've finally found a solution. In my ShowScreen class I needed to wrap the showing of the screen in a Task.Factory.StartNew() like this

    public void Execute(CoroutineExecutionContext context)
    {
        Task.Factory.StartNew(() =>
        {
            object screen = null;
            var shell = IoC.Get<IShell>();
            if (_viewModel != null)
            {
                screen = _viewModel;
            }
            else
            {
                screen = !String.IsNullOrEmpty(_name)
                    ? IoC.Get<object>(_name)
                    : IoC.GetInstance(_screenType, null);
            }
            shell.ActivateItem(screen);
            Completed(this, new ResultCompletionEventArgs());
        });
    }

Now everything executes in the order I want it to execute. Thanks @pushpraj for the ideas.