初学者线程或调度程序方向

Question

我希望有人能指出我正确的方向。我想做一个简单的 WPF 应用程序，它有一个按钮和一个文本框。我单击按钮，它开始循环下载一堆文件。我似乎无法弄清楚如何不让下载阻止 UI 更新。据我所知，我可能不得不使用一些线程代码；但到目前为止，我发现并尝试过的所有示例都不适合我。关于我应该在哪里寻找和学习的任何帮助或方向都会很棒。我似乎无法弄清楚如何在每个文件下载周围输出那些 textbox.text 消息。

foreach (var ticker in tickers)
    {
        var url = string.Format(urlPrototype, ticker, startMonth, startDay, startYear, finishMonth, finishDay, finishYear, "d");
        var csvfile = directory + "\\" + ticker.ToUpper() + ".csv";

        tbOutput.Text += "Starting Download of : " + ticker + "\n";

        webClient.DownloadFile(url, csvfile);

        tbOutput.Text += "End Download of : " + ticker + "\n";
        numStocks++;
    }
    tbOutput.Text += "Total stocks downloaded = " + numStocks + "\n";

Answer 1

如果您将方法标记为异步，则可以使用DownloadFileTaskAsync 方法

await webClient.DownloadFileTaskAsync(url, csvfile)

Answer 2

如果您选择使用BackgroundWorker，它允许您在每次下载文件时将这些消息输出到文本框。这是一个适合您要求的粗略示例。

1) 在类级别，创建 BackgroundWorker 类的实例，并将事件处理程序添加到 BackgroundWorker 实例的事件中：

BackgroundWorker workerDownload = new BackgroundWorker();
workerDownload.WorkerReportsProgress = true;
workerDownload.DoWork += workerDownload_DoWork;
workerDownload.ProgressChanged += workerDownload_ProgressChanged;
workerDownload.RunWorkerCompleted += workerDownload_RunWorkerCompleted;

2) 为后台工作人员的 DoWork 事件创建事件处理程序：

DoWork 事件处理程序是您运行耗时操作的地方 在后台线程上。传递给后台的任何值 操作在 DoWorkEventArgs 的 Argument 属性中传递 传递给事件处理程序的对象。

private void workerDownload_DoWork(object sender, DoWorkEventArgs e)
{
    foreach (var ticker in tickers)
    {
        // you can pass the required info as argument:
        string[] arrArg = (string[])e.Argument;
        string theUrl = arrArg[0];
        string directory = arrArg[1];

        var url = string.Format(theUrl, ticker);
        var csvfile = directory + "\\" + ticker.ToUpper() + ".csv";

        // perform the download operation and report progress:
        workerDownload.ReportProgress(0, "Starting Download of : " + ticker + "\n");
        webClient.DownloadFile(url, csvfile);
        workerDownload.ReportProgress(100, "End Download of : " + ticker + "\n");
        numStocks++;
    }
}

3) 为后台工作人员的 ProgressChanged 事件创建事件处理程序：

在 ProgressChanged 事件处理程序中，添加代码以指示 进度，例如更新用户界面。

private void workerDownload_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
    tbOutput.Text += e.UserState.ToString();
}

4) 为 RunWorkerCompleted 事件创建事件处理程序：

RunWorkerCompleted 事件在后台工作人员完成时引发 完成。

private void workerDownload_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
    tbOutput.Text += "Total stocks downloaded = " + numStocks + "\n";
}

5) 通过调用 RunWorkerAsync 方法开始运行后台操作：

int numStocks = 0;
string strDirectory = "<a_directory>";
string strUrl = string.Format(urlPrototype, startMonth, startDay, startYear, finishMonth, finishDay, finishYear, "d");
string[] args = new string[2] { strUrl, strDirectory };
workerDownload.RunWorkerAsync(args);

Answer 3

有很多方法可以实现它。例如：

1) 如果您在 .Net Framework 4.5 中编程，请使用 async/await。比BackgroundWorker简单

https://msdn.microsoft.com/en-us/library/hh191443.aspx

private async void button_Click(object sender, RoutedEventArgs e)
{
    Uri someUrl=new Uri(@"http://dotnetperls.com");
    WebClient webClient=new WebClient();
    await webClient.DownloadFileTaskAsync(someUrl, csvFile);
}

2) 背景工作者。此类实际上是为了进行异步操作以避免冻结 UI。 见http://www.wpf-tutorial.com/misc/multi-threading-with-the-backgroundworker

public partial class MainWindow : Window
{
    BackgroundWorker bw;
    public MainWindow()
    {
        InitializeComponent();
        bw = new BackgroundWorker();
        bw.DoWork += bw_DoWok;
        bw.RunWorkerCompleted += bw_RunWorkerCompleted;
    }              
}


void bw_RunWorkerComleted(object sender, RunWorkerCompletedEventAgs e)
{
    MessageBox.Show("The result is " + e.Result.ToString());
}

void bw_DoWork(object sender, DoWorkEventArgs e)
{
    foreach (var ticker in tickers)
    {
        var url = string.Format(urlPrototype, ticker, startMonth, startDay, startYear, finishMonth, finishDay, finishYear, "d");
        var csvfile = directory + "\\" + ticker.ToUpper() + ".csv";
        webClient.DownloadFile(url, csvfile);    
        numStocks++;
     }
    e.Result = "End Of Download ";
}

private void button_Click(object sender, RoutedEventArgs e)
{
    bw.RunWorkerAsync();
    tbOutput.Text += "Starting Download of : " + ticker + "\n";
}

3) 使用 Thread 类并使用 Dispatcher 类进行更新：

ThreadStart job = new ThreadStart(() =>
{
    foreach (var ticker in tickers)
    {
       var url = string.Format(urlPrototype, ticker, startMonth, startDay, startYear, finishMonth, finishDay, finishYear, "d");
       var csvfile = directory + "\\" + ticker.ToUpper() + ".csv";
       webClient.DownloadFile(url, csvfile);    
       numStocks++;
    }
    Dispatcher.BeginInvoke((Action)(()=> tbOutput.Text += "End Download of : " + ticker + "\n";}));

});

Thread thread = new Thread(job);
thread.Start();

http://www.beingdeveloper.com/use-dispatcher-in-wpf-to-build-responsive-applications