Show/Hide Windows the MVVM way

Introduction

Some idea to handle Window and Dialog the MVVM way.

Background

Recently. In my take over the world app, I started to write Window and Dialog (aka Modal Window) ViewModel. I wanted to be able to handle the Window through data binding. I came up with this solution.

Using the code

In a MVVM it is customary to handle most, if not all, app state in your view model. Leaving the UserControl to be empty shell for loading XAML UI definition. As I started to add dialog & window to my app I hit a limitation of the MVVM tool I am familiar with.

I was trying to write

myDialogViewModel.Show = true; // must show window, somehow...

I came up with this utility class hosting Window visibility as AttachedProperties

public class Dialog
{
 public static bool GetShowDialog(Window obj) { return (bool)obj.GetValue(ShowDialogProperty); }

 public static void SetShowDialog(Window obj, bool value) { obj.SetValue(ShowDialogProperty, value); }

 public static readonly DependencyProperty ShowDialogProperty = ...;

 public static bool GetShowWindow(Window obj) { return (bool)obj.GetValue(ShowWindowProperty); }

 public static void SetShowWindow(Window obj, bool value) { obj.SetValue(ShowWindowProperty, value); }

 public static readonly DependencyProperty ShowWindowProperty = ...;
}

I can then use it in XAML as follow:

<MyDialog
 xmlns:gx="clr-namespace:MyApp"
 Title="My Dialog"
 gx:Dialog.ShowDialog="{Binding Show}"
 >
....
</MyDialog>

And voila! Full MVVM dialog and Windows.

Full Source Code

Here is the complete implementation of the Dialog class.

public class Dialog
{
 public static bool GetShowDialog(Window obj) { return (bool)obj.GetValue(ShowDialogProperty); }

 public static void SetShowDialog(Window obj, bool value) { obj.SetValue(ShowDialogProperty, value); }

 public static readonly DependencyProperty ShowDialogProperty = DependencyProperty.RegisterAttached("ShowDialog", typeof(bool), typeof(Dialog)
  , new FrameworkPropertyMetadata(false, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, (o, e) => OnShowDialogChanged(o, (bool)e.NewValue)));

 private static void OnShowDialogChanged(DependencyObject sender, bool state)
 {
  var w = sender as Window;
  if (w != null)
  {
   EventHandler closed = (o, e) => { SetShowDialog(w, false); };
   if (state)
   {
    w.Closed += closed;
    // WARNING: this is blocking => screw 2-way binding
    // => call in a Dispatcher
    w.Dispatcher.BeginInvoke(new Action(delegate { if (GetShowDialog(w)) w.ShowDialog(); }));
   }
   else
   {
    w.Closed -= closed;
    w.Close();
   }
  }
 }

 public static bool GetShowWindow(Window obj) { return (bool)obj.GetValue(ShowWindowProperty); }

 public static void SetShowWindow(Window obj, bool value) { obj.SetValue(ShowWindowProperty, value); }

 public static readonly DependencyProperty ShowWindowProperty = DependencyProperty.RegisterAttached("ShowWindow", typeof(bool), typeof(Dialog)
  , new FrameworkPropertyMetadata(false, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, (o, e) => OnShowWindowChanged(o, (bool)e.NewValue)));

 private static void OnShowWindowChanged(DependencyObject sender, bool state)
 {
  var w = sender as Window;
  if (w != null)
  {
   EventHandler closed = (o, e) => { SetShowWindow(w, false); };
   if (state)
   {
    w.Closed += closed;
    w.Show();
   }
   else
   {
    w.Closed -= closed;
    w.Close();
   }
  }
 }
}

Points of Interest

Ever more MVVM goodness! I didn't see many solution for hiding/showing Windows. I hope this will help!

History

Initial release on 18/01/2015