Calling method of UserControl from ViewModel with Caliburn.Micro

Question

I'm writing a ViewModel-first MVVM application using Caliburn.Micro My View contains a 3rd party UserControl that implements a Method I want/need to call from the associated ViewModel. How do I do that while still upholding the MVVM principles?

There exists an old thread here on SO where a similar question is asked in a more specific context. I would appreciate it if someone could flesh-out the approaches suggested there a bit.

Approach one suggests that the View could subscribe to an IEventAggregator message. But wouldn't I have to use the code behind file to do that? (I thought that was a big no no in MVVM)

Regarding approach two, I have no idea how to do that. And regarding approach three, thats what I tried first but somehow I didn't quite get it to work.

score 9 · Accepted Answer · answered Oct 31 '13 at 22:48

Let me clarify your understanding:

Yes code in the code-behind is generally avoided, but only because MVVM makes it so easy to bind to viewmodel properties and commands in order to wire up your visual element with the functionality behind the scenes

Code that is view-specific in the code-behind of the view is perfectly acceptable assuming it doesn't cross the boundary of concern. For instance, I have a view in my application that does some visual processing of the page, and to do so I require that there is code in the view. This code may also interact with the viewmodel layer, but it will not directly reference the viewmodel, therefore keeping my components loosely coupled

If you have controls that need particular methods calling, then creating an event aggregator message to propagate the notification to the view is perfectly fine since you are still maintaining the separation of concern between the viewmodel and view (and the application components remain encapsulated and testable)

Example View (I've left all event aggregator wire up code and potential dependency injection stuff out for clarity):

public class MyView : IHandle<SomeNotificationMessageType>
{
    // Handler for event aggregator messages of type SomeNotificationMessageType
    public void Handle(SomeNotificationMessageType message)
    {
        // Call a method on one of the page controls
        SomePageControl.SomeMethod();
    }
}

Obviously, what you wouldn't do is something like this in the ViewModel:

public class MyViewModel : IViewAware
{
    public void DoSomethingThatAffectsView()
    {
        var view = this.GetView() as MyView;

        view.SomePageControl.SomeMethod();
    }
}

Which violates the MVVM principles since you are tightly coupling MyViewModel and MyView.

What if you wanted to use the Context property in caliburn micro which allows multiple views over the same view model? The code above would break - even if you checked the View type, you would still end up with spaghetti code e.g.

public class MyViewModel : IViewAware
{
    public void DoSomethingThatAffectsView()
    {
        var myview = this.GetView() as MyView;

        if(myview != null)        
            myview.SomePageControl.SomeMethod();

        var myotherview = this.GetView() as MyOtherView;

        if(myotherview != null)        
            myotherview.SomePageControl.SomeMethod();

        // ad infinitum...
    }
}

Of course this is subjective: it may be that your usercontrol affects the viewmodel and the view in a complex way, in which case you might want to consider looking at the architecture and working out how that usercontrol can better fit

Have you got any background on what the UC is and what the method on it does?

Thank you for your explanation. I will go with the EventAggregator idea. The UC I'm talking about is AvalonDock and what I want to do is save/load the layout. In version 1.* there was a SaveLayout() method; now since version 2.0 one apparently has to pass the entire UC DockingManager to a special Serializer. — PeterE, Nov 01 '13 at 09:05
"left all ... dependency injection stuff out for clarity" and now it is not clear. :) The only way I can think of, to get Event Aggregator, is to use `IoC.Get()` in the parameterless constructor of my View. Is there a better way to inject dependency rather than just pulling the object directly from the container? — Anton Kedrov, Jan 18 '16 at 14:47
Your views participate in dependency injection too - just add it to the constructor. — Charleh, Jan 18 '16 at 14:58
If I do so, then I will get runtime exception: "No parameterless constructor defined for this object." Or do I miss something? So far I ended up calling `eventAggregator.Subscribe(view)` in the ViewModel, inside of `OnViewAttached()` method, which I think is better than using `IoC.Get()` in the View constructor. — Anton Kedrov, Jan 18 '16 at 21:49

Calling method of UserControl from ViewModel with Caliburn.Micro

1 Answers1