c# - 异步等待线程完成

Question

嗯，我有一个从电脑上截取屏幕截图的函数，但不幸的是它阻塞了主 UI，所以我决定对其进行异步[线程调用]；但是，我发现在返回之前等待线程结果时遇到麻烦位图。

这是我的代码:

/// <summary>
/// Asynchronously uses the snapshot method to get a shot from the screen.
/// </summary>
/// <returns> A snapshot from the screen.</returns>
private Bitmap SnapshotAsync()
{
    Bitmap image = null;
    new Thread(() => image = Snapshot()).Start();

    while (image == null)
    {
        new Thread(() => Thread.Sleep(500)).Start(); //Here i create new thread to wait but i don't think this is a good way at all.
    }
    return image;
}

/// <summary>
/// Takes a screen shots from the computer.
/// </summary>
/// <returns> A snapshot from the screen.</returns>
private Bitmap Snapshot()
{
    var sx = Screen.PrimaryScreen.Bounds.Width;
    var sy = Screen.PrimaryScreen.Bounds.Height;
    var shot = new Bitmap(sx, sy, PixelFormat.Format32bppArgb);
    var gfx = Graphics.FromImage(shot);
    gfx.CopyFromScreen(0, 0, 0, 0, new Size(sx, sy));
    return shot;
}

虽然上面的方法是按照我想要的方式异步工作的，但我相信它可以改进。特别是我执行数百个线程来等待结果的方式，我确信这种方式不好。

所以我真的需要任何人看看代码并告诉我如何改进它。

[注意我正在使用 .NET 3.5]

提前致谢。

在 Eve 和 SiLo 的帮助下解决的问题是最好的 2 个答案

• 1 :

>     private void TakeScreenshot_Click(object sender, EventArgs e)
>     {
>       TakeScreenshotAsync(OnScreenshotTaken);
>     }
>     
>     private static void OnScreenshotTaken(Bitmap screenshot)
>     {
>       using (screenshot)
>         screenshot.Save("screenshot.png", ImageFormat.Png);
>     }
>     
>     private static void TakeScreenshotAsync(Action<Bitmap> callback)
>     {
>       var screenRect = Screen.PrimaryScreen.Bounds;
>       TakeScreenshotAsync(screenRect, callback);
>     }
>     
>     private static void TakeScreenshotAsync(Rectangle bounds, Action<Bitmap> callback)
>     {
>       var screenshot = new Bitmap(bounds.Width, bounds.Height,
>                                   PixelFormat.Format32bppArgb);
>     
>       ThreadPool.QueueUserWorkItem((state) =>
>       {
>         using (var g = Graphics.FromImage(screenshot))
>           g.CopyFromScreen(bounds.X, bounds.Y, 0, 0, bounds.Size);
>     
>         if (callback != null)
>           callback(screenshot);
>       });
>     }

• 2 :

>     void SnapshotAsync(Action<Bitmap> callback)
>     {
>         new Thread(Snapshot) {IsBackground = true}.Start(callback);
>     }

>     void Snapshot(object callback)
>     {
>         var action = callback as Action<Bitmap>;
>         var sx = Screen.PrimaryScreen.Bounds.Width;
>         var sy = Screen.PrimaryScreen.Bounds.Height;
>         var shot = new Bitmap(sx, sy, PixelFormat.Format32bppArgb);
>         var gfx = Graphics.FromImage(shot);
>         gfx.CopyFromScreen(0, 0, 0, 0, new Size(sx, sy));
>         action(shot);
>     }

Usage, for example, through a button's click:

void button1_Click(object sender, EventArgs e)
{
    SnapshotAsync(bitmap => MessageBox.Show("Copy successful!"));
}

Answer 1

async/await 关键字完全符合您的要求，非常优雅。

以下是我如何将您的方法转换为正确的模式:

private static async Task<Bitmap> TakeScreenshotAsync()
{
  var screenRect = Screen.PrimaryScreen.Bounds;
  return await TakeScreenshotAsync(screenRect);
}

private static async Task<Bitmap> TakeScreenshotAsync(Rectangle bounds)
{
  var screenShot = new Bitmap(bounds.Width, bounds.Height, 
                              PixelFormat.Format32bppArgb);

  // This executes on a ThreadPool thread asynchronously!
  await Task.Run(() =>
  {
    using (var g = Graphics.FromImage(screenShot))
      g.CopyFromScreen(bounds.X, bounds.Y, 0, 0, bounds.Size);

  });

  return screenShot;
}

然后你会做这样的事情:

private async void TakeScreenshot_Click(object sender, EventArgs e)
{
  var button = sender as Button;
  if(button == null) return;

  button.Enabled = false;
  button.Text = "Screenshoting...";

  var bitmap = await TakeScreenshotAsync();
  bitmap.Save("screenshot.png", ImageFormat.Png);

  button.Text = "Take Screenshot";
  button.Enabled = true;
}