View knowledge about ViewModel

Question

I need to connect methods in View (WPF window) to events in ViewModel. Is it violation of MVVM pattern to DirectCast Object DataContext in view to concrete VM type and connect its events? If yes, is there better way to do it?

Answer 1

First look at what the methods in the view do. If they manipulate the view, consider adding properties to the viewmodel that you change in the events in the viewmodel and bind the view to. This way, by binding the view to properties you eliminate the need for code in the view.

If the methods contain other logic consider moving that logic to the viewmodel.

In other cases casting a DataContext to a viewmodel or interface can be a valid option and is not a violation of the MVVM pattern.

When adding code to a view, do consider testing. Automated/unit testing a view is harder than testing a viewmodel.

Answer 2

It is not a violation of the MVVM pattern, but the more abstract the better, of course (not because of MVVM but as a general good practice).

If you're setting your DataContext on XAML, you may be able to keep it abstract by using Interactivity EventTrigger and CallMethodAction... Maybe. But if you're setting it on code-behind (via injection or whatever), you're left with either casting the DataContext to a known type, or using Reflection (I wouldn't >_>).

Generally, creating an interface for your ViewModel, so you keep a decent level of abstraction and only expose what the view needs to know instead of its whole implementation, is good enough for most scenarios.

public interface IMyViewModel
{
    event EventHandler MyEvent;
}

public class MyViewModel : IMyViewModel
{
    public event EventHandler MyEvent;

    // More viewmodel related stuff

    protected virtual void OnMyEvent(EventArgs e)
    {
        if (MyEvent != null)
            MyEvent(this, e);
    }
}

public class MyWindow : Window
{
    public MyWindow(IMyViewModel viewModel)
    {
        this.DataContext = viewModel;
        InitializeComponent();
        (this.DataContext as IViewModel).MyEvent += MyEventHandler;
    }

    private void MyEventHandler(object sender, EventArgs e)
    {
        // Do view related stuff
    }

}