Attached Behavior to execute command for ListViewI

问题:

I am trying to use an attached behavior to execute a command in my ViewModel when the user Double Clicks on the list item.

I have reviewed a number of articles on the subject, and tried creating a simple test application but am still having problems eg. Firing a double click event from a WPF ListView item using MVVM

My simple test ViewModel has 2 collections, one that returns a list of strings and the other that returns a List of ListViewItem types

public class ViewModel
{
    public ViewModel()
    {
        Stuff = new ObservableCollection<ListViewItem>
                    {
                        new ListViewItem { Content = "item 1" },
                        new ListViewItem { Content = "item 2" }
                    };

        StringStuff = new ObservableCollection<string> { "item 1", "item 2" };
    }

    public ObservableCollection<ListViewItem> Stuff { get; set; }

    public ObservableCollection<string> StringStuff { get; set; }

    public ICommand Foo
    {
        get
        {
            return new DelegateCommand(this.DoSomeAction);
        }
    }

    private void DoSomeAction()
    {
        MessageBox.Show("Command Triggered");
    }
}

Here is the attached property which is like may other examples you see:

public class ClickBehavior
{
    public static DependencyProperty DoubleClickCommandProperty = DependencyProperty.RegisterAttached("DoubleClick",
               typeof(ICommand),
               typeof(ClickBehavior),
               new FrameworkPropertyMetadata(null, new PropertyChangedCallback(ClickBehavior.DoubleClickChanged)));

    public static void SetDoubleClick(DependencyObject target, ICommand value)
    {
        target.SetValue(ClickBehavior.DoubleClickCommandProperty, value);
    }

    public static ICommand GetDoubleClick(DependencyObject target)
    {
        return (ICommand)target.GetValue(DoubleClickCommandProperty);
    }

    private static void DoubleClickChanged(DependencyObject target, DependencyPropertyChangedEventArgs e)
    {
        ListViewItem element = target as ListViewItem;
        if (element != null)
        {
            if ((e.NewValue != null) && (e.OldValue == null))
            {
                element.MouseDoubleClick += element_MouseDoubleClick;
            }
            else if ((e.NewValue == null) && (e.OldValue != null))
            {
                element.MouseDoubleClick -= element_MouseDoubleClick;
            }
        }
    }

    static void element_MouseDoubleClick(object sender, MouseButtonEventArgs e)
    {
        UIElement element = (UIElement)sender;
        ICommand command = (ICommand)element.GetValue(ClickBehavior.DoubleClickCommandProperty);
        command.Execute(null);
    }
}

In my main window, I have defined the style which sets the attached behaviour and binds to the Foo command

<Window.Resources>
    <Style x:Key="listViewItemStyle" TargetType="{x:Type ListViewItem}">
        <Setter Property="local:ClickBehavior.DoubleClick" Value="{Binding Foo}"/>                 
    </Style>
</Window.Resources>

Works fine when ListViewItems are defined:

<!-- Works -->
<Label Grid.Row="2" Content="DoubleClick click behaviour:"/>        
<ListView Grid.Row="2" Grid.Column="1" ItemContainerStyle="{StaticResource listViewItemStyle}">
    <ListViewItem Content="Item 3" />
    <ListViewItem Content="Item 4" />
</ListView>

This works too, when bound to the list of type ListViewItem:

<!-- Works when items bound are of type ListViewItem -->
<Label Grid.Row="3" Content="DoubleClick when bound to ListViewItem:"/>        
  <ListView Grid.Row="3" Grid.Column="1" ItemContainerStyle="{StaticResource listViewItemStyle}" ItemsSource="{Binding Stuff}">        
 </ListView>

But this doesn't:

<!-- Does not work when items bound are not ListViewItem -->
<Label Grid.Row="4" Content="DoubleClick when bound to string list:"/>
  <ListView Grid.Row="4" Grid.Column="1" ItemContainerStyle="{StaticResource listViewItemStyle}" ItemsSource="{Binding StringStuff}">
</ListView>

In the output window you see the error, but finding it difficult to understand what is wrong.
System.Windows.Data Error: 39 : BindingExpression path error: 'Foo' property not found on 'object' ''String' (HashCode=785742638)'. BindingExpression:Path=Foo; DataItem='String' (HashCode=785742638); target element is 'ListViewItem' (Name=''); target property is 'DoubleClick' (type 'ICommand')

So my quesion is: How can you get the Command wired up correctly to each ListViewItem when you bind your ListView to a list of Model objects?

Thanks.

回答1:

The problem is that the DataContext for the Binding is the string. Since there is no Foo property of the string class, you are getting an error. This doesn't happen in the other cases because they inherit their DataContext from the parent (this doesn't happen for automatically generated containers for data items - their DataContext is the data item).

If you change your binding to use the parent ListView's DataContext, it should work fine:

Value="{Binding DataContext.Foo, RelativeSource={RelativeSource AncestorType={x:Type ListView}}}"