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

Question

我有以下简单的代码：

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 + "%)" 
    ); 
} 

这是为什么呢阻塞UI线程？当我用代码替换Console.WriteLine()来更新我的进度条（不在代码中显示）时，它可以工作。用户界面很敏感。

Answer 1

你这样做的方式似乎是MSDN shows in its examples。我也试过了，得到了同样的结果。在单独的线程中运行某些内容时，您会看到类似的行为，然后这些线程会过快地回调主UI线程，并通过更新来加重它。 UI线程得到备份并有效冻结。

DownloadProgressChangedDownloadProgressChanged事件触发得很快......似乎是每秒数百次，这意味着它试图快速写入控制台。

你可以限制你多久写到控制台，这将解决这个问题（我测试了它，试图下载一个4GB的ISO，并同时使UI响应写信给控制台）：

// 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 + "%)" 
      ); 
    } 
}