Bubbling RoutedEvent or RoutedCommand to a ViewModel

Question

I've got a collection of ViewModels that are rendered as tabs using a style to pull out the relevant content to display on the tab:

public class TabViewModel : DependencyObject
{
      public object Content
      {
          get { return (object)GetValue(ContentProperty); }
          set
          {
              SetValue(ContentProperty, value);
          }
      }

}

Here's the TabControl:

<TabControl 
     ItemsSource={Binding MyCollectionOfTabViewModels}" 
     ItemContainerStyle="{StaticResource TabItemStyle}" />

And here's the style

<Style TargetType="TabItem" x:Key="TabItemStyle">
     <Setter Property="Content" Value="{Binding Content}"/>
</Style>

We are creating an instance of a usercontrol and setting the "Content" property of the TabViewModel to that so that the usercontrol gets displayed in the TabItem's Content area.

MyCollectionOfViewModels.Add(new TabViewModel() 
{ 
     Content = new MyUserControl();
});

My question is, I would like to allow a MyUserControl (or any of its sub controls) added to the TabViewModel's Content property to be allowed to raise an event that the TabViewModel handles.

Anyone know how I would do that?

We've experimented using RoutedEvents and RoutedCommands, but haven't been able to get anything to work 100% and have it be compatible with MVVM. I really think that this could be done with a RoutedEvent or RoutedCommand, but I don't seem to be able to get this to work.

Note: I've removed some of the relevant Prism-specific code, but if you are wondering why we do something so silly, it is because we are trying to stay control agnostic by using Prism's RegionManager.

Answer 1

You could add a State property to your TabViewModel, and check the DependencyPropertyChanged events.

So imagine the following enum:

public enum TabViewModelState
{
    True,
    False,
    FileNotFound
}

Then add a State property to your TabViewModel of this enum:

public static readonly DependencyProperty StateProperty =
    DependencyProperty.Register("State", typeof(TabViewModelState), typeof(TabViewModel), new PropertyMetadata(OnStateChanged));

private static void OnStateChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
{
    TabViewModel viewModel= (TabViewModel)obj;

    //Do stuff with your viewModel
}

Use a two-way binding to this property in your control:

<CheckBox Checked="{Binding Path=State, Converter={StaticResource StateToBooleanConverter}, Mode=TwoWay}" />

And last but not least implement the converter that will convert to and from the original value needed for the control.. (in my example boolean <--> TabViewModelState):

public class StateToBooleanConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        TabViewModelState state = (TabViewModelState) value;
        return state == TabViewModelState.True;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        bool result = (bool) value;
        return result ? TabViewModelState.True : TabViewModelState.False;
    }
}

So now you have a State property that is managed by the UI, and throws changed events when you need to respond..

Hope this helps!

Answer 2

If you put a command on your ViewModel and just bind to that from your UserControl it will fire anyway. You don't have to bubble it, the UserControl will just find the command and use it.

If you had a delegate command 'GoCommand' on your ViewModel, your UserControls button binding would just look like this:

<Button Command="{Binding GoCommand}" >Go</Button>

I went through the same thought process thinking that the UserControl needs to bubble the command, but it dosn't - the binding will just hook itself up when it finds the command on the ViewModel.

Hope this helps and I havn't missed the point! ;)