WP7 - drag/drop of objects created from a DataTemplate within Canvas - silverlight

Question

At a glance:
My app displays an ItemsControl containing a Canvas as its ItemsPanel. The ItemsControl is bound to a collection of objects, each having Left/Top/Width/Height properties. A DataTemplate is used to generate rectangles that are rendered in the Canvas and positioned correctly (binding on the Left and Top properties).
How can I implement drag/drop to move these rectangles around the Canvas?
Background for my question:
My WP7 app displays a "CanvasItemsControl" defined as follows:
public class CanvasItemsControl : ItemsControl
{
public string XBindingPath { get; set; }
public string YBindingPath { get; set; }
protected override void PrepareContainerForItemOverride(DependencyObject element, object item)
{
FrameworkElement contentitem = element as FrameworkElement;
if (XBindingPath != null && YBindingPath != null)
{
Binding xBinding = new Binding(XBindingPath);
Binding yBinding = new Binding(YBindingPath);
if (contentitem != null)
{
contentitem.SetBinding(Canvas.LeftProperty, xBinding);
contentitem.SetBinding(Canvas.TopProperty, yBinding);
}
}
base.PrepareContainerForItemOverride(element, item);
}
}
and used in XAML as follows:
<hh:CanvasItemsControl Grid.Row="1" x:Name="TheItemsControl"
Style="{StaticResource CanvasItemsControlStyle}"
ItemsSource="{Binding AllObjects}"
XBindingPath="Left" YBindingPath="Top" />
This is the style for the CanvasItemsControl:
<Style x:Key="CanvasItemsControlStyle" TargetType="local:CanvasItemsControl">
<Setter Property="ItemTemplate" Value="{StaticResource ObjectTemplate}"/>
<Setter Property="ItemsPanel">
<Setter.Value>
<ItemsPanelTemplate>
<Canvas/>
</ItemsPanelTemplate>
</Setter.Value>
</Setter>
</Style>
And this is the DataTemplate I use to render my class:
<DataTemplate x:Key="ObjectTemplate" >
<Border Background="{Binding Brush}"
Width="{Binding Width}"
Height="{Binding Height}">
<TextBlock Text="{Binding Description}"/>
</Border>
</DataTemplate>
The source of the CanvasItemsControl is a collection of objects that have the properties Left, Top, Width, Height, Brush, etc.
My question
As you can see, the end result is, as you add items to the AllObjects collection, each object gets rendered and positioned correctly in the canvas. Now I need to drag/drop/move these objects around the canvas. What approach would you advise me to use to implement drag/drop? Can you please guide me through the process?
Thank you

Answer 1

Here's the solution to my question (at least the best one in my opinion):
1) Use of a regular Canvas as opposed of a custom control inherited from Canvas.
2) Use of a user control taking the data context (the instance of my business entity) via constructor
3) The binding between the Left/Top properties of my business class and the Canvas.Left/Top is declared at the UserControl level.
4) Use of a custom behavior inheriting from System.Windows.Interactivity.Behavior. This behavior is attached to the User Control.
I would like to acknowlege Calvin Schrotenboer and Joe Gershgorin for their immense help.
<!--____ The UserControl ____-->
<UserControl... Canvas.Left={Binding Left}" Canvas.Top={Binding Top}">
<Grid.... layout of the UserControl instead of using a DataTemplate/>
<i:Interaction.Behaviors>
<MyExample:MyMouseDragElementBehavior/>
</i:Interaction.Behaviors>
</UserControl>
The custom behavior:
public class MyMouseDragElementBehavior : Behavior<FrameworkElement>
{
public event MouseEventHandler DragBegun;
public event MouseEventHandler DragFinished;
public event MouseEventHandler Dragging;
private Point relativePosition;
public static readonly DependencyProperty IsEnabledProperty = DependencyProperty.Register("IsEnabled", typeof(bool), typeof(MyMouseDragElementBehavior), new PropertyMetadata(true));
public bool IsEnabled
{
get
{
return (bool)GetValue(IsEnabledProperty);
}
set
{
SetValue(IsEnabledProperty, value);
}
}
protected override void OnAttached()
{
AssociatedObject.AddHandler(
UIElement.MouseLeftButtonDownEvent, new MouseButtonEventHandler(OnMouseLeftButtonDown), false);
base.OnAttached();
}
protected override void OnDetaching()
{
AssociatedObject.RemoveHandler(
UIElement.MouseLeftButtonDownEvent, new MouseButtonEventHandler(OnMouseLeftButtonDown));
base.OnDetaching();
}
private static int zIndex = 0;
private void OnMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
if (!IsEnabled)
{
return;
}
zIndex++;
Canvas.SetZIndex(AssociatedObject, zIndex);
StartDrag(e.GetPosition(AssociatedObject));
if (DragBegun != null)
{
DragBegun(this, e);
}
}
private void OnMouseLeftButtonUp(object sender, MouseButtonEventArgs e)
{
AssociatedObject.ReleaseMouseCapture();
}
private void OnMouseMove(object sender, MouseEventArgs e)
{
HandleDrag(e.GetPosition(AssociatedObject));
if (Dragging != null)
{
Dragging(this, e);
}
}
internal void HandleDrag(Point newPositionInElementCoordinates)
{
double x = newPositionInElementCoordinates.X - relativePosition.X;
double y = newPositionInElementCoordinates.Y - relativePosition.Y;
if (AssociatedObject != null)
{
var currentLeft = Canvas.GetLeft(AssociatedObject);
var currentTop = Canvas.GetTop(AssociatedObject);
Canvas.SetLeft(AssociatedObject, currentLeft + x);
Canvas.SetTop(AssociatedObject, currentTop + y);
}
}
internal void StartDrag(Point positionInElementCoordinates)
{
relativePosition = positionInElementCoordinates;
AssociatedObject.CaptureMouse();
AssociatedObject.MouseMove += OnMouseMove;
AssociatedObject.LostMouseCapture += OnLostMouseCapture;
AssociatedObject.AddHandler(UIElement.MouseLeftButtonUpEvent, new MouseButtonEventHandler(OnMouseLeftButtonUp), false);
}
internal void EndDrag()
{
AssociatedObject.MouseMove -= OnMouseMove;
AssociatedObject.LostMouseCapture -= OnLostMouseCapture;
AssociatedObject.RemoveHandler(
UIElement.MouseLeftButtonUpEvent, new MouseButtonEventHandler(OnMouseLeftButtonUp));
}
private void OnLostMouseCapture(object sender, MouseEventArgs e)
{
EndDrag();
if (DragFinished != null)
{
DragFinished(this, e);
}
}
}