前言

做桌面截屏功能时需要放大镜，显示鼠标所在位置的放大图像，在wpf中使用Bursh的ViewBox属性可以实现图像放大，桌面的画面则需要截屏了，总的来说还是比较容易实现的。

一、如何实现？

1、制作无边框窗口

推荐使用WindowChrome

<Window Background="{x:Null}" ResizeMode="NoResize"  WindowStyle="None">

WindowChrome放在Window 标签内

<WindowChrome.WindowChrome>
        <WindowChrome GlassFrameThickness="-1"   CaptionHeight="0"   />
</WindowChrome.WindowChrome>

2、Viewbox放大

定义一个Ellipse 控件作为放大镜，Viewbox默认为相对单位，即范围时0-1，值越小放大越大

<Ellipse Stroke="LightBlue">
   <Ellipse.Fill>
       <ImageBrush x:Name="ib" Viewbox="0,0,0.5,0.5" />
   </Ellipse.Fill>
/Ellipse>

3、截屏显示

（1）、截屏

参考《C# wpf 使用GDI+实现截屏》里的简单截屏即完成。获取的数据类型为Bitmap。

（2）、转BitmapSource

参考《C# wpf Bitmap转换成WriteableBitmap（BitmapSource）的方法》将Bitmap转换为转换成wpf对象。

（3）、显示

获取到BitmapSource给控件赋值即可。

//显示到界面
ib.ImageSource = wb;

4、定时截屏

显示桌面必然需要实时的画面，所以需要定时截屏。

//启动定时器，截屏
var dispatcherTimer = new DispatcherTimer() { Interval = TimeSpan.FromMilliseconds(33), };
dispatcherTimer.Tick += (s, e) =>
{
   //截屏并显示
};
dispatcherTimer.Start();

二、完整代码

完整代码依赖System.Drawing，添加引用方法可以参考《C# wpf 使用GDI+实现截屏》。

MainWindow.xaml

<Window x:Class="WpfMagnifier.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfMagnifier"
        mc:Ignorable="d"
        Background="{x:Null}"
        ResizeMode="NoResize"
        WindowStyle="None"
        ShowInTaskbar="False"
        Topmost="True"
        Title="MainWindow" Height="200" Width="200" 
        MouseLeftButtonDown="Window_MouseDown">
    <WindowChrome.WindowChrome>
        <WindowChrome GlassFrameThickness="-1"   CaptionHeight="0"   />
    </WindowChrome.WindowChrome>
    <Ellipse Stroke="LightBlue">
        <Ellipse.Fill>
            <ImageBrush x:Name="ib" Viewbox="0,0,0.5,0.5" />
        </Ellipse.Fill>
    </Ellipse>
</Window>

MainWindow.xaml.cs

using System;
using System.Windows;
using System.Windows.Input;
using System.Windows.Media.Imaging;
using System.Windows.Threading;
namespace WpfMagnifier
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            //启动定时器，截屏
            var dispatcherTimer = new DispatcherTimer() { Interval = TimeSpan.FromMilliseconds(33), };
            dispatcherTimer.Tick += (s, e) =>
            {
                //gdi+截屏，截取窗口左边的区域（可根据具体使用场景调整截屏位置）,使用PointToScreen消除dpi影响
                var leftTop = PointToScreen(new Point(-Width, 0));
                var rightBottom = PointToScreen(new Point(0, Height));
                var bm = Snapshot((int)leftTop.X, (int)leftTop.Y, (int)(rightBottom.X - leftTop.X), (int)(rightBottom.Y - leftTop.Y));
                var wb = BitmapToWriteableBitmap(bm);
                //显示到界面
                ib.ImageSource = wb;
            };
            dispatcherTimer.Start();
        }

        private void Window_MouseDown(object sender, MouseButtonEventArgs e)
        {
            DragMove();
        }
        /// <summary>
        /// 截取一帧图片
        /// </summary>
        /// <param name="x">x坐标</param>
        /// <param name="y">y坐标</param>
        /// <param name="width">宽</param>
        /// <param name="height">高</param>
        /// <returns>截屏后的位图对象，需要调用Dispose手动释放资源。</returns>
        public static System.Drawing.Bitmap Snapshot(int x, int y, int width, int height)
        {
            System.Drawing.Bitmap bitmap = new System.Drawing.Bitmap(width, height, System.Drawing.Imaging.PixelFormat.Format32bppArgb);
            using (System.Drawing.Graphics graphics = System.Drawing.Graphics.FromImage(bitmap))
            {
                graphics.CopyFromScreen(x, y, 0, 0, new System.Drawing.Size(width, height), System.Drawing.CopyPixelOperation.SourceCopy);
            }
            return bitmap;
        }
        //将Bitmap 转换成WriteableBitmap 
        public static WriteableBitmap BitmapToWriteableBitmap(System.Drawing.Bitmap src)
        {
            var wb = CreateCompatibleWriteableBitmap(src);
            System.Drawing.Imaging.PixelFormat format = src.PixelFormat;
            if (wb == null)
            {
                wb = new WriteableBitmap(src.Width, src.Height, 0, 0, System.Windows.Media.PixelFormats.Bgra32, null);
                format = System.Drawing.Imaging.PixelFormat.Format32bppArgb;
            }
            BitmapCopyToWriteableBitmap(src, wb, new System.Drawing.Rectangle(0, 0, src.Width, src.Height), 0, 0, format);
            return wb;
        }
        //创建尺寸和格式与Bitmap兼容的WriteableBitmap
        public static WriteableBitmap CreateCompatibleWriteableBitmap(System.Drawing.Bitmap src)
        {
            System.Windows.Media.PixelFormat format;
            switch (src.PixelFormat)
            {
                case System.Drawing.Imaging.PixelFormat.Format16bppRgb555:
                    format = System.Windows.Media.PixelFormats.Bgr555;
                    break;
                case System.Drawing.Imaging.PixelFormat.Format16bppRgb565:
                    format = System.Windows.Media.PixelFormats.Bgr565;
                    break;
                case System.Drawing.Imaging.PixelFormat.Format24bppRgb:
                    format = System.Windows.Media.PixelFormats.Bgr24;
                    break;
                case System.Drawing.Imaging.PixelFormat.Format32bppRgb:
                    format = System.Windows.Media.PixelFormats.Bgr32;
                    break;
                case System.Drawing.Imaging.PixelFormat.Format32bppPArgb:
                    format = System.Windows.Media.PixelFormats.Pbgra32;
                    break;
                case System.Drawing.Imaging.PixelFormat.Format32bppArgb:
                    format = System.Windows.Media.PixelFormats.Bgra32;
                    break;
                default:
                    return null;
            }
            return new WriteableBitmap(src.Width, src.Height, 0, 0, format, null);
        }
        //将Bitmap数据写入WriteableBitmap中
        public static void BitmapCopyToWriteableBitmap(System.Drawing.Bitmap src, WriteableBitmap dst, System.Drawing.Rectangle srcRect, int destinationX, int destinationY, System.Drawing.Imaging.PixelFormat srcPixelFormat)
        {
            var data = src.LockBits(new System.Drawing.Rectangle(new System.Drawing.Point(0, 0), src.Size), System.Drawing.Imaging.ImageLockMode.ReadOnly, srcPixelFormat);
            dst.WritePixels(new Int32Rect(srcRect.X, srcRect.Y, srcRect.Width, srcRect.Height), data.Scan0, data.Height * data.Stride, data.Stride, destinationX, destinationY);
            src.UnlockBits(data);
        }
    }
}