WPF behaviors and events - attaching to proper events - wpf

Question

I have here a behavior handling drag & drop, and I attach the mouse events through the view. The item's viewmodel inherit from IDraggable. I use Caliburn.Micro as MVVM framework.
<ItemsControl x:Name="Items">
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<Canvas />
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ItemsControl.ItemContainerStyle>
<Style TargetType="ContentPresenter">
<Setter Property="Canvas.Left" Value="{Binding Path=X}" />
<Setter Property="Canvas.Top" Value="{Binding Path=Y}" />
<Setter Property="Width" Value="{Binding Path=Width}" />
<Setter Property="Height" Value="{Binding Path=Height}" />
</Style>
</ItemsControl.ItemContainerStyle>
<ItemsControl.ItemTemplate>
<DataTemplate>
<Border>
<!-- item contents -->
<i:Interaction.Behaviors>
<behaviors:DragOnCanvasBehavior DraggableItem="{Binding}">
<i:Interaction.Triggers>
<i:EventTrigger EventName="PreviewMouseLeftButtonDown">
<i:InvokeCommandAction CommandName="StartDrag" />
</i:EventTrigger>
<i:EventTrigger EventName="PreviewMouseLeftButtonUp">
<i:InvokeCommandAction CommandName="StopDrag" />
</i:EventTrigger>
<i:EventTrigger EventName="PreviewMouseMove">
<i:InvokeCommandAction CommandName="Dragging" />
</i:EventTrigger>
</i:Interaction.Triggers>
</behaviors:DragOnCanvasBehavior>
</i:Interaction.Behaviors>
</Border>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
Then the behavior attaches the mouse events to the element's mouse handlers :
public class DragOnCanvasBehavior : Behavior<DependencyObject>
{
public static readonly DependencyProperty DraggableItemProperty =
DependencyProperty.RegisterAttached(
"DraggableItem",
typeof(IDraggable),
typeof(DragOnCanvasBehavior),
new PropertyMetadata(new PropertyChangedCallback((d, e) =>
{
((DragOnCanvasBehavior)d).draggable = (IDraggable)e.NewValue;
})));
private IDraggable draggable;
public DragOnCanvasBehavior()
{
this.StartDrag = new RelayCommand((o) =>
{
((UIElement)this.AssociatedObject).MouseLeftButtonDown += this.ElementOnMouseLeftButtonDown;
});
this.StopDrag = new RelayCommand((o) =>
{
((UIElement)this.AssociatedObject).MouseLeftButtonUp += this.ElementOnMouseLeftButtonUp;
});
this.Dragging = new RelayCommand((o) =>
{
((UIElement)this.AssociatedObject).MouseMove += this.ElementOnMouseMove;
});
}
public IDraggable DraggableItem
{
get { return (IDraggable)this.GetValue(DraggableItemProperty); }
set { this.SetValue(DraggableItemProperty, value); }
}
public ICommand Dragging { get; private set; }
public ICommand StartDrag { get; private set; }
public ICommand StopDrag { get; private set; }
// these handle the drag through the IDraggable properties
// and the mouse event args
private void ElementOnMouseLeftButtonDown(object sender, MouseButtonEventArgs e) {}
private void ElementOnMouseLeftButtonUp(object sender, MouseButtonEventArgs e) {}
private void ElementOnMouseMove(object sender, MouseEventArgs e) {}
}
It works, but I'm pretty sure I'm doing something wrong here. The mouse events are attached in the behavior constructor, but they are "hard-linked" to the events (meaning I can't change the triggers to something else than mousedown/up/move).
I must, however, have access to the mouse position in the ElementOnMousexxx methods, so I'm not sure how to do it properly.

Answer 1

OK so it turns out that you don't need the MouseEventArgs to get the mouse cursor position, you can just use System.Windows.Input.Mouse. You also don't need the "sender" object, when using Behavior<T> you can just use this.AssociatedObject
public DragOnCanvasBehavior()
{
this.StartDrag = new RelayCommand((o) =>
{
this.OnStartDrag();
});
}
private void OnStartDrag()
{
// get mouse position
this.mouseStartPosition = Mouse.GetPosition(Application.Current.MainWindow);
// access control properties
((UIElement)this.AssociatedObject).CaptureMouse();
}