xna中的截屏操作处理

游戏中常会用到截图，如果是用默认的截屏键PrintScreen的话，用户又要在另外的地方粘贴才可以。

这样做的话比较自由：

 1 /// <summary>
 2         /// Saves a screenshot to disk with running number.
 3         /// Does not overwrite existing screenshots.
 4         /// </summary>
 5         public void SaveScreenshot() {
 6             // Find a free name
 7             int number = 0;
 8             string filename = String.Format("screenshot{0:00}.png", number);
 9             while (System.IO.File.Exists(filename)) {
                 filename = String.Format("screenshot{0:00}.png", ++number);
             }
 
             // Take the screenshot
             GraphicsDevice device = graphics.GraphicsDevice;
             int w = device.PresentationParameters.BackBufferWidth;
             int h = device.PresentationParameters.BackBufferHeight;
             using (ResolveTexture2D screenshot = new ResolveTexture2D(device, w, h, 1, SurfaceFormat.Color)) {
                 // Grab the screenshot
                 device.ResolveBackBuffer(screenshot);
 
                 // Set the alpha to full
                 Color[] data = new Color[screenshot.Width * screenshot.Height];
                 screenshot.GetData<Color>(data);
                 int pos = 0;
                 foreach (Color c in data) {
                     data[pos++] = new Color(c.R, c.G, c.B, 255);
                 }
 
                 // Write to disk
                 screenshot.SetData<Color>(data);
                 screenshot.Save(filename, ImageFileFormat.Png);
                 screenshot.Dispose();
             }
         }