I have a custom control (MyControl) that exposes a custom command. I want the parent Window to be able to invoke this command, and all MyControls should react to it.
I have added the command to MyControl's CommandBindings collection, which also provides a CanExecute callback that always returns true.
My problem is that a menu item that invokes this command never gets enabled. I'm presuming this is because the menu is above the MyControls in the visual tree, but to be honest I'm a little fuzzy on how RoutedUICommand's scope works exactly.
Can someone clarify what I'm doing wrong, or if this is even possible?
Yes, the point is that the CommandBinding for your custom command is at a lower that your menu item in the visual tree. From msdn
When the CanExecute method on a RoutedCommand is called, the PreviewCanExecute event is raised on the command target. If the event is not handled, the CanExecute event is raised. If the command target has a CommandBinding for the command, the CanExecute handler for that CommandBinding is called. If the command target does not have a CommandBinding for the command, the CanExecute event bubbles up the element tree searching for an element that has a CommandBinding associated with the command.
A very good article about commands in at this link.
Anyway you can use CommandTarget the command from your menu item such as
<MenuItem Header="Click me" Command="local:CommandClass.MyCustomCommand" CommandTarget="{Binding ElementName=myCustomCtrl}" />
In this way a particular command source (the menu item) directs the command to the specific target (instance of the custom control).