通过设置光标形状实现拖拽控件时跟随一张透明图片的效果

以前做过一些winform的项目，实现标题描述的效果通常是在鼠标拖拽控件的时候，通常是在拖动过程中生成一个透明的form，把控件放进去，form的位置跟随鼠标一起改变。前几天和几位同事讨论winform的一些技巧和方法，正好又谈到这个，有同事提供了一种比较新颖的思路：就是生成一张透明的图片，设置成光标的形状。正好周末有时间试了一下。

    using System.Collections.Generic;
    using System.Drawing;
    using System.Windows.Forms;

    delegate void DropCallBack<in Source, in Target>(Source source, Target target)
        where Source : Control
        where Target : Control;

    interface IDragDropController<in Source, in Target>
        where Source : Control
        where Target : Control
    {
        void EnableDragDrop();
        void DisableDragDrop();
    }

    sealed class DragDropController<Source, Target> : IDragDropController<Source, Target>
        where Source : Control
        where Target : Control
    {
        private IEnumerable<Source> sources;
        private IEnumerable<Target> targets;
        private bool isMovePrepared;
        private Bitmap cursorImage;
        private Cursor cursor;
        private Cursor defaultCursor;
        private DropCallBack<Source, Target> dropCallBack;

        public DragDropController(IEnumerable<Source> sources, IEnumerable<Target> targets, DropCallBack<Source, Target> dropCallBack)
        {
            this.sources = sources;
            this.targets = targets;
            this.dropCallBack = dropCallBack;
        }

        public void EnableDragDrop()
        {
            this.DisableDragDrop();
            foreach (var source in this.sources)
            {
                source.MouseDown += this.OnSourceMouseDown;
                source.MouseMove += this.OnSourceMouseMove;
                source.MouseUp += this.OnSourceMouseUp;                
            }
        }

        public void DisableDragDrop()
        {
            foreach (var source in this.sources)
            {
                source.MouseDown -= this.OnSourceMouseDown;
                source.MouseMove -= this.OnSourceMouseMove;
                source.MouseUp -= this.OnSourceMouseUp;
            }
        }

        private void OnSourceMouseDown(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                var source = sender as Source;
                if (source != null)
                {
                    source.Capture = true;
                    this.isMovePrepared = true;
                }
            }
        }

        private void OnSourceMouseMove(object sender, MouseEventArgs e)
        {
            if (this.isMovePrepared)
            {
                var source = sender as Source;
                if (source == null)
                {
                    return;
                }

                if (this.cursor == null)
                {
                    this.defaultCursor = source.Cursor;
                    // Generate an image for cursor
                    int cursorWidth = source.Width >= 2 * e.X ? 2 * (source.Width - e.X) : 2 * e.X;
                    int cursorHeight = source.Height >= 2 * e.Y ? 2 * (source.Height - e.Y) : 2 * e.Y;
                    this.cursorImage = new Bitmap(cursorWidth, cursorHeight);
                    Rectangle bounds = new Rectangle(source.Width >= 2 * e.X ? cursorWidth - source.Width : 0, source.Height >= 2 * e.Y ? cursorHeight - source.Height : 0, source.Width, source.Height);
                    source.DrawToBitmap(cursorImage, bounds);
                    this.defaultCursor.Draw(Graphics.FromImage(this.cursorImage), new Rectangle(bounds.X + e.X, bounds.Y + e.Y, this.defaultCursor.Size.Width, this.defaultCursor.Size.Height));
                    this.ChangeAlpha(cursorImage, bounds);
                    this.cursor = new Cursor(this.cursorImage.GetHicon());
                    source.Cursor = this.cursor;
                }
            }
        }

        private void OnSourceMouseUp(object sender, MouseEventArgs e)
        {
            if (this.isMovePrepared)
            {
                var source = sender as Source;
                if (source == null)
                {
                    return;
                }

                if (this.cursor != null)
                {
                    source.Cursor = defaultCursor;
                    this.cursorImage.Dispose();
                    this.cursorImage = null;
                    this.cursor.Dispose();
                    this.cursor = null;
                }

                this.isMovePrepared = false;
                source.Capture = false;
                // find the target control where the source control is dropped by the mouse up location.
                Target target = null;
                var mouseLocation = source.PointToScreen(e.Location);
                foreach (var item in this.targets)
                {
                    Point offset = item.PointToScreen(new Point(0, 0));
                    Rectangle bounds = new Rectangle(offset, item.Size);
                    if (bounds.Contains(mouseLocation))
                    {
                        target = item;
                        break;
                    }
                }

                if (target != null && this.dropCallBack != null)
                {
                    this.dropCallBack(source, target);
                }
            }
        }

        private void ChangeAlpha(Bitmap img, Rectangle bounds)
        {
            for (int x = bounds.Left; x < bounds.Right; x++)
            {
                for (int y = bounds.Top; y < bounds.Bottom; y++)
                {
                    Color orgColor = img.GetPixel(x, y);
                    img.SetPixel(x, y, Color.FromArgb(150, orgColor.R, orgColor.G, orgColor.B));
                }
            }
        }
    }

这个类可以实现将拖拽source到targets中任意一个控件过程中，鼠标跟随一张透明的source控件的图片。下面是使用该类的方法：

    public partial class Form1 : Form
    {
        private DragDropController<PictureBox, PictureBox> dragDropTest;

        public Form1()
        {
            InitializeComponent();
            this.dragDropTest = new DragDropController<PictureBox, PictureBox>(new List<PictureBox>() { this.pictureBox1, this.pictureBox2 }, new List<PictureBox>() { this.pictureBox3, this.pictureBox4 }, this.DropCallBack);
            this.dragDropTest.EnableDragDrop();
        }

        private void DropCallBack(PictureBox source, PictureBox target)
        {
            target.Image = source.Image;
        }
    }

该方法实现从pictureBox1拖拽图片到pictureBox2或pictureBox3中的效果。

主要发现一个问题：在开始拖拽的时候，Cursor图片的生成比较复杂，性能似乎比较低，不知是否有优化之法。