Dragging Shapes in Wpf

One of the projects I have in mind as my side-projects needs ability to drag-move shapes on a Canvas. I thought it would idea to explore the possibilities of achieving it via test projects first before starting the real one. So the goal of this article would be
* Create a Rectangle in a Canvas
* Support ability to Drag-move it within the Canvas boundary

Let’s begin by defining the basic UI including our Canvas and Rectangle. We would be using Caliburn Micro for supporting MVVM in the example.

<Canvas Grid.Row="2" Background="AliceBlue" >
<Rectangle Height="100" Width="100" Fill="Red" Canvas.Left="{Binding Left}" Canvas.Top="{Binding Top}" />
</Canvas>

Our ViewModel, at the moment looks like,

public class ShellViewModel:PropertyChangedBase
{
public double Left { get; set; } = 10;
public double Top { get; set; } = 10;
}

The Left and Top are double properties which will have the position of the Rectange with respect to the Canvas.

The next step is to capture the Mouse Position each time the User holds the Left Mouse Button down and drags the object. For this there are few things needs to be done. We need to detect when the Left Mouse Button is pressed down and released. Also, we need to trace the Mouse Position when the User holds the Left Mouse Down within the rectangle.

To detect the mouse position, we will write a Behavior.

public class MouseMoveBehavior:Behavior<Canvas>
{
public double MouseTop
{
get { return (double)GetValue(MouseTopProperty); }
set { SetValue(MouseTopProperty, value); }
}

public static readonly DependencyProperty MouseTopProperty =
DependencyProperty.Register("MouseTop", typeof(double), typeof(MouseMoveBehavior), new PropertyMetadata(0d));

public double MouseLeft
{
get { return (double)GetValue(MouseLeftProperty); }
set { SetValue(MouseLeftProperty, value); }
}

public static readonly DependencyProperty MouseLeftProperty =
DependencyProperty.Register("MouseLeft", typeof(double), typeof(MouseMoveBehavior), new PropertyMetadata(0d));

protected override void OnAttached()
{
AssociatedObject.MouseMove += AssociatedObject_MouseMove;
}

private void AssociatedObject_MouseMove(object sender, System.Windows.Input.MouseEventArgs e)
{
var currentPosition = e.GetPosition(AssociatedObject);
MouseLeft = currentPosition.X;
MouseTop = currentPosition.Y;
}

protected override void OnDetaching()
{
AssociatedObject.MouseMove -= AssociatedObject_MouseMove;
}
}

The Behavior MouseMoveBehavior defines two Dependency Properties named MouseTop and MouseLeft indicating the X-Y cordinates of the mouse position. The behavior intends to capture the Mouse position using the MouseEventArgs.GetPosition and update the dependency properties with the Position.X and Position.Y values.

So, we have our behavior in place. Now is the time to update our Xaml to use our Behavior.

<Canvas Grid.Row="2" Background="AliceBlue" cal:Message.Attach="[Event MouseMove]=[Action MouseMove];
[Event PreviewMouseLeftButtonUp]=[Action MouseUp];[Event MouseLeave]=[Action MouseUp]" >
<i:Interaction.Behaviors>
<behaviors:MouseMoveBehavior MouseLeft="{Binding Left,Mode=TwoWay}" MouseTop="{Binding Top,Mode=TwoWay}"/>
</i:Interaction.Behaviors>
<Rectangle Height="100" Width="100" Fill="Red" Canvas.Left="{Binding Left}" Canvas.Top="{Binding Top}" cal:Message.Attach="[Event PreviewMouseLeftButtonDown]=[Action MouseDown]" />
</Canvas>

As you can observe, we are updating the Left and Top properties by capturing them in Behavior and assigning the the values of current mouse position. This would ensure the the shape would move along the mouse pointer.

But you want to update the Left and Top position only when the Mouse Left Button is pressed and dragged, not when it is freely moved. To help us achieve this, we will introduce three additional properties in our ViewModel. Let us rewrite our ViewModel. This would also explain why we are capturing the Mouse Events in the XAML, which was intensionally not explained till this point.

public class ShellViewModel:PropertyChangedBase
{
public double Left { get; set; } = 10;
public double Top { get; set; } = 10;

public double CurrentMouseX { get; set; }
public double CurrentMouseY { get; set; }

public bool IsShapeCaptured { get; set; }

public void MouseDown()
{
IsShapeCaptured = true;
NotifyOfPropertyChange(nameof(IsShapeCaptured));
}

public void MouseMove()
{
if (!IsShapeCaptured) return;
Left = CurrentMouseX;
Top = CurrentMouseY;
NotifyOfPropertyChange(nameof(Left));
NotifyOfPropertyChange(nameof(Top));
}

public void MouseUp()
{
IsShapeCaptured = false;
NotifyOfPropertyChange(nameof(IsShapeCaptured));
}
}

The IsShapeCaptured is enabled only when the Mouse Left Button is pressed. As soon as the Mouse Button is released, you are resetting the IsShapeCaptured flag. The MouseMove method ensures that the Left & Top properties (which are bound to the position of Shape) are updated only when the IsShapeCaptured is set.

I guesss that bit of code is quite self explanatory. This is one way to achieve the drag/drop functionality, but obviously there could be others. Will provide another solution in coming days.

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