binding a command to a list

Question

Hello

I have the same command that I want to use for two controls on a dialog type window. As potentially interesting background, I'm using Josh Smith's ViewModel / RelayCommand ideas, since I am new to WPF and it's the first thing I've seen that I can actually understand from a big picture point of view.

So the command is a property of a ViewModel, and with the Button's built-in support, it is trivial and painless to bind to the command in the XAML:

<Button ... Command="{Binding Path=PickCommand}"  Content="_Ok"></Button>

Now in a ListView, the only way I have gotten to use the same command hooked up to trigger on a double click is by using an event handler:

<ListView ...
              ItemsSource="{Binding Path=AvailableProjects}" 
              SelectedItem="{Binding Path=SelectedProject, Mode=TwoWay}"
              MouseDoubleClick="OnProjectListingMouseDoubleClick"
              >

private void OnProjectListingMouseDoubleClick(object sender, MouseButtonEventArgs e) {
        var vm = (ProjectSelectionViewModel) DataContext;
        vm.Pick(); // execute the pick command
    }

Is there a way to do this by binding the way the button does it?

Cheers,
Berryl

<------- implementation - is there a better way? --->

Your SelctionBehavior class was spot on, but I was confused at your xaml code. By setting the "Style" on the listViewItem I was getting the children of the DataContext where the command I want to execute lives. So I attached the behavior to the ListView itself:

<ListView ...Style="{StaticResource _attachedPickCommand}" >

And put the style in a resource dictionary:

<Style x:Key="_attachedPickCommand" TargetType="ListView">
    <Setter Property="behaviors:SelectionBehavior.DoubleClickCommand" Value="{Binding Path=PickCommand}" />
</Style>

It works! But it 'feels' awkward setting the style property of the list view. Is this just because I am not comfortable with style as more than something visual in wpf or is there a better way to do this?

Cheers, and thanks!
Berryl

Answer 1

Yes there is! You can use attached behaviors and bind the command to that behavior.

  public class SelectionBehavior {
public static readonly DependencyProperty CommandParameterProperty=
  DependencyProperty.RegisterAttached("CommandParameter", typeof(object), typeof(SelectionBehavior));

public static readonly DependencyProperty DoubleClickCommandProperty=
  DependencyProperty.RegisterAttached("DoubleClickCommand", typeof(ICommand), typeof(SelectionBehavior),
                                      new PropertyMetadata(OnDoubleClickAttached));

private static void OnDoubleClickAttached(DependencyObject d, DependencyPropertyChangedEventArgs e) {
  var fe=(FrameworkElement)d;

  if(e.NewValue!=null && e.OldValue==null) {
    fe.PreviewMouseDown+=fe_MouseDown;
  } else if(e.NewValue==null && e.OldValue!=null) {
    fe.PreviewMouseDown-=fe_MouseDown;
  }
}

private static void fe_MouseDown(object sender, MouseButtonEventArgs e) {
  if(e.ClickCount==2) {
    var dep=(FrameworkElement)sender;

    var command=GetDoubleClickCommand(dep);

    if(command!=null) {
      var param=GetCommandParameter(dep);
      command.Execute(param);
    }
  }
}

public static ICommand GetDoubleClickCommand(FrameworkElement element) {
  return (ICommand)element.GetValue(DoubleClickCommandProperty);
}

public static void SetDoubleClickCommand(FrameworkElement element, ICommand value) {
  element.SetValue(DoubleClickCommandProperty, value);
}

public static object GetCommandParameter(DependencyObject element) {
  return element.GetValue(CommandParameterProperty);
}

public static void SetCommandParameter(DependencyObject element, object value) {
  element.SetValue(CommandParameterProperty, value);
}

}

and in the xaml you would need to set a style for a ListViewItem which represents your data in the ListView. Example

        <ListView>
        <ListView.ItemContainerStyle>
            <Style TargetType="{x:Type ListViewItem}">
                <Setter Property="local:SelectionBehavior.DoubleClickCommand" Value="{Binding Path=DataContext.PickCommand}"/>
                <Setter Property="local:SelectionBehavior.CommandParameter" Value="{Binding Path=DataContext}"/>
            </Style>
        </ListView.ItemContainerStyle>
    </ListView>

Here is some more information about the Attached Behavior pattern