拖动时使用鼠标光标显示图像缩略图

Question

我有一个小的 WPF 应用程序，它有一个带有图像控件的窗口。图像控件显示来自文件系统的图像。我希望用户能够将图像拖放到桌面或任何地方进行保存。它工作正常。

但是我想在用户拖动鼠标光标时显示小图像缩略图。就像我们将图像从 Windows 文件资源管理器拖到其他地方一样。如何实现？

当前拖放行为

期望的行为

这是我的 XAML 代码

<Grid>
   <Image x:Name="img" Height="100" Width="100" Margin="100,30,0,0"/>
</Grid>

这是 C# 代码

   public partial class MainWindow : Window
    {
        string imgPath;
        Point start;
        bool dragStart = false;

        public MainWindow()
        {
            InitializeComponent();
            imgPath = "C:\\Pictures\\flower.jpg";

            ImageSource imageSource = new BitmapImage(new Uri(imgPath));
            img.Source = imageSource;
            window.PreviewMouseMove += Window_PreviewMouseMove;
            window.PreviewMouseUp += Window_PreviewMouseUp;
            window.Closing += Window_Closing;
            img.PreviewMouseLeftButtonDown += Img_PreviewMouseLeftButtonDown;
        }

        private void Window_Closing(object sender, System.ComponentModel.CancelEventArgs e)
        {
            window.PreviewMouseMove -= Window_PreviewMouseMove;
            window.PreviewMouseUp -= Window_PreviewMouseUp;
            window.Closing -= Window_Closing;
            img.PreviewMouseLeftButtonDown -= Img_PreviewMouseLeftButtonDown;
        }


        private void Window_PreviewMouseMove(object sender, MouseEventArgs e)
        {
            if (!dragStart) return;
            if (e.LeftButton != MouseButtonState.Pressed)
            {
                dragStart = false; return;
            }

            Point mpos = e.GetPosition(null);
            Vector diff = this.start - mpos;

            if (Math.Abs(diff.X) > SystemParameters.MinimumHorizontalDragDistance &&
                Math.Abs(diff.Y) > SystemParameters.MinimumVerticalDragDistance)
            {
                string[] file = { imgPath };
                DataObject d = new DataObject();
                d.SetData(DataFormats.Text, file[0]);
                d.SetData(DataFormats.FileDrop, file);
                DragDrop.DoDragDrop(this, d, DragDropEffects.Copy);
            }
        }

        private void Img_PreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            this.start = e.GetPosition(null);
            dragStart = true;
        }

        private void Window_PreviewMouseUp(object sender, MouseButtonEventArgs e)
        {
            dragStart = false;
        }
    }

Answer 1

正式地，您应该使用IDragSourceHelper接口将预览位图添加到拖放操作中。

不幸的是，该接口使用IDataObject::SetData 方法，该方法并未由 .NET DataObject 类在 COM 级别实现，而仅在 .NET 级别实现。

解决方案是使用SHCreateItemFromParsingName 函数和 IShellItem::BindToHandler 方法，重新使用 Shell 提供的 IDataObject，而不是任何 Shell Item（此处为文件）。

请注意，这些函数会自动添加剪贴板格式（如 FileDrop），但我们仍然必须使用 IDragSourceHelper 来添加预览图像。

您可以这样使用它：

...
// get IDataObject from the Shell so it can handle more formats
var dataObject = DataObjectUtilities.GetFileDataObject(imgPath);

// add the thumbnail to the data object
DataObjectUtilities.AddPreviewImage(dataObject, imgPath);

// start d&d
DragDrop.DoDragDrop(this, dataObject, DragDropEffects.All);
...

这是代码：

public static class DataObjectUtilities
{
    public static void AddPreviewImage(System.Runtime.InteropServices.ComTypes.IDataObject dataObject, string imgPath)
    {
        if (dataObject == null)
            throw new ArgumentNullException(nameof(dataObject));

        var ddh = (IDragSourceHelper)new DragDropHelper();
        var dragImage = new SHDRAGIMAGE();

        // note you should use a thumbnail here, not a full-sized image
        var thumbnail = new System.Drawing.Bitmap(imgPath);
        dragImage.sizeDragImage.cx = thumbnail.Width;
        dragImage.sizeDragImage.cy = thumbnail.Height;
        dragImage.hbmpDragImage = thumbnail.GetHbitmap();
        Marshal.ThrowExceptionForHR(ddh.InitializeFromBitmap(ref dragImage, dataObject));
    }

    public static System.Runtime.InteropServices.ComTypes.IDataObject GetFileDataObject(string filePath)
    {
        if (filePath == null)
            throw new ArgumentNullException(nameof(filePath));

        Marshal.ThrowExceptionForHR(SHCreateItemFromParsingName(filePath, null, typeof(IShellItem).GUID, out var item));
        Marshal.ThrowExceptionForHR(item.BindToHandler(null, BHID_DataObject, typeof(System.Runtime.InteropServices.ComTypes.IDataObject).GUID, out var dataObject));
        return (System.Runtime.InteropServices.ComTypes.IDataObject)dataObject;
    }

    private static readonly Guid BHID_DataObject = new Guid("b8c0bd9f-ed24-455c-83e6-d5390c4fe8c4");

    [DllImport("shell32", CharSet = CharSet.Unicode)]
    private static extern int SHCreateItemFromParsingName(string path, System.Runtime.InteropServices.ComTypes.IBindCtx pbc, [MarshalAs(UnmanagedType.LPStruct)] Guid riid, out IShellItem ppv);

    [Guid("43826d1e-e718-42ee-bc55-a1e261c37bfe"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    private interface IShellItem
    {
        [PreserveSig]
        int BindToHandler(System.Runtime.InteropServices.ComTypes.IBindCtx pbc, [MarshalAs(UnmanagedType.LPStruct)] Guid bhid, [MarshalAs(UnmanagedType.LPStruct)] Guid riid, [MarshalAs(UnmanagedType.IUnknown)] out object ppv);

        // other methods are not defined, we don't need them
    }

    [ComImport, Guid("4657278a-411b-11d2-839a-00c04fd918d0")] // CLSID_DragDropHelper
    private class DragDropHelper
    {
    }

    [StructLayout(LayoutKind.Sequential)]
    private struct POINT
    {
        public int x;
        public int y;
    }

    [StructLayout(LayoutKind.Sequential)]
    private struct SIZE
    {
        public int cx;
        public int cy;
    }

    // https://learn.microsoft.com/en-us/windows/win32/api/shobjidl_core/ns-shobjidl_core-shdragimage
    [StructLayout(LayoutKind.Sequential)]
    private struct SHDRAGIMAGE
    {
        public SIZE sizeDragImage;
        public POINT ptOffset;
        public IntPtr hbmpDragImage;
        public int crColorKey;
    }

    // https://learn.microsoft.com/en-us/windows/win32/api/shobjidl_core/nn-shobjidl_core-idragsourcehelper
    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("DE5BF786-477A-11D2-839D-00C04FD918D0")]
    private interface IDragSourceHelper
    {
        [PreserveSig]
        int InitializeFromBitmap(ref SHDRAGIMAGE pshdi, System.Runtime.InteropServices.ComTypes.IDataObject pDataObject);

        [PreserveSig]
        int InitializeFromWindow(IntPtr hwnd, ref POINT ppt, System.Runtime.InteropServices.ComTypes.IDataObject pDataObject);
    }
}