Simple Event Handling in MVVM

Question

Just wondering what people had for ideas on how best to handle events in a ViewModel from controls on a View ... in the most lightweight way possible.

Example:

<MediaElement
     MediaOpened={Binding SomeEventHandler} />

In this case we want to handle the MediaOpened event in a ViewModel. Without a framework like Prism, how would one bind this to a ViewModel?

Answer 1

Commanding - your 'SomeEventHandler' needs to be a class that implements ICommand... there's a heap of literature available online...

Also - I would consider getting a free, lightweight 'mini' MVVM framework, such as MvvmFoundation, which provides the RelayCommand for just such a purpose (without the complexity/overhead of learning PRISM)

EDIT:

Have a look at this blog for attaching command to any event... It is incredibly powerful, as I mentioned, but I guess you do need to make a judgement call if this is what you want, compared with something like attaching an old-fashioned event, and using a super-slim event handler in your code behind that simply invokes some method on your ViewModel, something like:

public void SomeEventHandler(object sender, SomeEventArgs e)
{
    MyViewModel vm = (MyViewModel)this.DataContext;
    vm.HandleLoadEvent( );
}

pragmatic vs Best-practise... I'll leave it with you ;)

Answer 2

MediaOpened is an event and does not support command binding.

In order to bind to the event a helper object may be used which attaches to the event and executes a command.

To bind to the view model add a property which implements ICommand. Figure 3 in this MSDN magazine article shows the RelayCommand which is a useful implementation of ICommand. RelayCommand is initialized with a delegate to connect to your view model.

http://msdn.microsoft.com/en-us/magazine/dd419663.aspx

Answer 3

Have a look at Marlon Grech's Attached Command Behaviors. It makes it easy to bind events to ViewModel commands