WPF中的行为

using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Interactivity;
using System.Windows.Media;

namespace CustomBehaviorsLibrary {
    public class DragInCanvasBehavior:Behavior<UIElement>
    {
        private Canvas canvas;

        protected override void OnAttached()
        {
            base.OnAttached();
            this.AssociatedObject.MouseLeftButtonDown += AssociatedObject_MouseLeftButtonDown;
            this.AssociatedObject.MouseMove += AssociatedObject_MouseMove;
            this.AssociatedObject.MouseLeftButtonUp += AssociatedObject_MouseLeftButtonUp;
        }

        protected override void OnDetaching()
        {
            base.OnDetaching();
            this.AssociatedObject.MouseLeftButtonDown -= AssociatedObject_MouseLeftButtonDown;
            this.AssociatedObject.MouseMove -= AssociatedObject_MouseMove;
            this.AssociatedObject.MouseLeftButtonUp -= AssociatedObject_MouseLeftButtonUp;
        }

        private bool isDragging = false;
        private Point mouseOffset;

        private void AssociatedObject_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
        {
            if(canvas==null) canvas=VisualTreeHelper.GetParent(this.AssociatedObject) as Canvas;
            isDragging = true;
            mouseOffset = e.GetPosition(AssociatedObject);
            AssociatedObject.CaptureMouse();
        }

        private void AssociatedObject_MouseMove(object sender, MouseEventArgs e)
        {
            if (isDragging)
            {
                Point point = e.GetPosition(canvas);
                AssociatedObject.SetValue(Canvas.TopProperty,point.Y-mouseOffset.Y);
                AssociatedObject.SetValue(Canvas.LeftProperty,point.X-mouseOffset.X);
            }
        }

        private void AssociatedObject_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            if (isDragging)
            {
                AssociatedObject.ReleaseMouseCapture();
                isDragging = false;
            }
        }
    }
}