Accessing controls from within commands in WPF

Question

I've got a WPF app who's menu is using the commanding capabilities. Everything is wired up just fine and when I click the buttons in the menu, the commands run. However I'm having trouble getting the button's IsEnabled status to respect the CanExecute part of my commands.

One challenge is the commands need to see what you're doing in the UI. For instance, one command should only be available when certain items in a ListBox are selected so I need to get a reference to the ListBox... but since the command is exposed in my view model (MVVM pattern), it doesn't have access to the UI (BTW, the menu is in one user control [parent=mainwindow] while the ListBox is in another user control [parent=mainwindow]).

In addition, even when I hard code the command's CanExecute method to return false, the Enabled property of the button doesn't change... the command won't fire, but it won't change... frustrating. I assume I need to modify/raise the CanExecuteChanged event, but I'm not sure when I should be doing that (can't find a good sample).

Feedback?

Answer 1

Normally, you would bind the Command Property of the MenuItem/Button whatever - that means you still have the CommandParameter to work with - this can then be bound to some other control. However, when the two views do not know each-other, you need som kind of mediator between them (ie. a viewmodel that both views can access - have the listbox SelectedItem/SelectedItems bound to a property two-ways - and let the CommandParameter bind to the same property one-way).

As for when to fire the CanExecuteChanged event - you should fire that whenever there is a change in the logic contained in the CanExecute-method. If it is always false, you should never fire the event, as it will read the initalvalue when the Command-parameter is set. '

If your button is behaving oddly - check to see if the IsEnabled property is influenced by Styles or set directly.

Hope this helps.

Answer 2

Try the Messenger class from MVVMLight. It helps in communicating between ViewModels.

And give this a try:

Have a SelectedItem property in your ListBox's ViewModel. Broadcast messages while the property changes. Register for this message in the menu's ViewModel. Use the SelectedItem property in your CanExecute method for your logic.