What is the use of tunneling the CommandBinding.previewcanexecute event ??

Question

What is the use of tunneling the CommandBinding.previewcanexecute event ?

and bubbling the CommandBinding.canexecute event ?? How is it useful

Answer 1

Tunnelling and bubbling events allow handlers to be shared among several elements, and intercepted by controls higher in the hierarchy. For example, given the following Xaml:

<StackPanel>
   <Button Command="MyCommand" />
   <Button Command="MyCommand" />
   <Button Command="MyCommand" />
   <Button Command="MyCommand" />
</StackPanel>

If I want to define a single CommandBinding for MyCommand that will affect all of the buttons, I can define it on the StackPanel. When each button tries to execute its command, the event will bubble upwards to the StackPanel - meaning that we need only one handler for four buttons. If I placed a handler at the root Window level, it would apply to every button in my application.

Conversely, tunnelling an event allows you to intercept it before it gets to the control that fired it. So, for example, if I want to disable a command for every button across my entire Window, I could do this with a tunnelling event and the buttons would always find that CanExecute returns false.