Wandering through the Wilderness - Using MVVM with Menus in WPF

Tuesday, April 21, 2009

One question I've fielded a couple of times is how to manage menus, primarily context menus, with the MVVM pattern. It turns out to be pretty easy once you know the "trick". The key thing to keep in mind is that menus are just ItemsControls - they support data templating and binding like any other ItemsControl. However, the part where people get lost is in hooking up the commands. Here's the trick:

Step 1: Define some code behind construct to manage each menu item in a hiearchial fashion. Here's one I've used:

public class MenuItem
{
   public string Text { get; set; }
   public List<MenuItem> Children { get; private set; }
   public ICommand Command { get; set; }

   public MenuItem(string item)
   {
       Text = item;
       Children = new List<MenuItem>();
   }
}

Step 2: Create your menu structure using the above container.

This involves just creating a List<MenuItem> which holds the root nodes and using a DelegatingCommand to wire up some code behind method. Here's an example:

public List<MenuItem> MenuOptions
{
get {

       menu.Add(new MenuItem("Close _All") { Command = new DelegatingCommand(OnCloseAll, () => FileList.Count > 0)});
       return menu;
    }

}

Note the use of an anonymous method to suck the correct file format into the command handler. A nice trick so the command is executed with some contextual information of what you clicked on.

Finally, you need to setup the context menu style:

<Style x:Key="ContextMenuItemStyle">
   <Setter Property="MenuItem.Header" Value="{Binding Text}"/>
   <Setter Property="MenuItem.ItemsSource" Value="{Binding Children}"/>
   <Setter Property="MenuItem.Command" Value="{Binding Command}" />
</Style>

And then use the style where you want the menu to appear:

</HierarchicalDataTemplate>

Now the context menu is populated and properly dispatches to your ViewModel commands!

Edit: added intermediate object in LINQ query to fix deferred query issue pointed out in comments.

Tuesday, April 21, 2009 5:35:59 PM (Central Daylight Time, UTC-05:00)

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