WebClient.DownloadProgressChanged：Console.WriteLine（）是阻塞UI線程

Question

我有以下簡單的代碼：

private void btn_download_Click(object sender, EventArgs e){ 

    WebClient client = new WebClient(); 
    client.DownloadProgressChanged += client_DownloadProgressChanged; 
    client.DownloadFileAsync(new Uri("http://.../file.zip"), "file.zip"); 

} 

void client_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e){ 
    //Prints: "Downloaded 3mb of 61.46mb (4%)" 

    Console.WriteLine("Downloaded " 
     + ((e.BytesReceived/1024f)/1024f).ToString("#0.##") + "mb" 
     + " of " 
     + ((e.TotalBytesToReceive/1024f)/1024f).ToString("#0.##") + "mb" 
     + " (" + e.ProgressPercentage + "%)" 
    ); 
} 

這是爲什麼呢阻塞UI線程？當我用代碼替換Console.WriteLine()來更新我的進度條（不在代碼中顯示）時，它可以工作。用戶界面很敏感。

Answer 1

你這樣做的方式似乎是MSDN shows in its examples。我也試過了，得到了同樣的結果。在單獨的線程中運行某些內容時，您會看到類似的行爲，然後這些線程會過快地回調主UI線程，並通過更新來加重它。 UI線程得到備份並有效凍結。

DownloadProgressChangedDownloadProgressChanged事件觸發得很快......似乎是每秒數百次，這意味着它試圖快速寫入控制檯。

你可以限制你多久寫到控制檯，這將解決這個問題（我測試了它，試圖下載一個4GB的ISO，並同時使UI響應寫信給控制檯）：

// define a class-level field variable 
private int counter; 

private void client_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e) 
{ 
    counter++; 

    // Only print to the console once every 500 times the event fires, 
    // which was about once per second when I tested it 
    if (counter % 500 == 0) 
    { 
     //Prints: "Downloaded 3mb of 61.46mb (4%)" 
     Console.WriteLine("Downloaded " 
          + ((e.BytesReceived/1024f)/1024f).ToString("#0.##") + "mb" 
          + " of " 
          + ((e.TotalBytesToReceive/1024f)/1024f).ToString("#0.##") + "mb" 
          + " (" + e.ProgressPercentage + "%)" 
      ); 
    } 
}