VB.Net一个循环内的异步后台工作

Question

我使用Vb.net visual studio 2008.我有一个进程（system.diagnostics.process）在后台运行，它更新了Ui线程进度条和一个标签，我已经使用backgroundworker.runworkerAsync工作。现在的问题是我必须用不同的输入工作多次。

的代码块：

Private Sub fnStartEvent(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnStart.click 

Dim inputFolder = Directory.GetFiles(textboxSourceFolder.text) 
Dim currentNumberOfFile As Integer=1 
For each Files in inputFolder 
    Dim arguments As Object = Files 
    updateProgressStatus(0, currentNumberOfFile) 
    BackgroundWorker1.WorkerReportsProgress = True 
    BackgroundWorker1.RunWorkerAsync(New Object() {arguments}) 
    currentNumberOfFile += 1 
    Next  
End Sub 

    Private Sub BackgroundWorker1_DoWork(ByVal sender As System.Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork 
    'uses the arguments 
     Dim Bg_process As System.Diagnostics.Process = New System.Diagnostics.Process 
      With Bg_process.StartInfo 
       .Arguments = str_arguments 
       .FileName = ffmpegPath 
       .CreateNoWindow = True 
       .UseShellExecute = False 
       .RedirectStandardOutput = True 
       .RedirectStandardError = True 
      End With 

      Bg_process.Start() 
      Dim outputReader As StreamReader = Bg_process.StandardError 
      Dim output As String 

      While Not Bg_process.HasExited 
       output = outputReader.ReadLine() 
       BackgroundWorker1.ReportProgress(0, output) 
       Threading.Thread.Sleep(500) 
      End While 
     End Sub 

Private Sub BackgroundWorker1_ProgressChanged(ByVal sender As Object, ByVal e As System.ComponentModel.ProgressChangedEventArgs) Handles BackgroundWorker1.ProgressChanged 
    ' process args 

updateProgressStatus(args(0), args(1)) 
End Sub 

Private Function BackgroundWorker1_RunWorkerCompleted(ByVal sender As Object, ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles BackgroundWorker1.RunWorkerCompleted 
      Messagebox.Show(e.error.ToString) 
End Try 

    Sub updateProgressStatus(ByVal progressValue As Integer, ByVal progressStatus As String) 
progressBar.value = progressValue 
lblprogressStatus.Text = progressStatus 
     End Sub 

在这里的问题是fnStartEvent方法一旦被启动，它会调用BackgroundWorker1.runWorkerAsync进程，在一个单独的线程中运行，并且不等待线程完成然后它移动到下一行代码，即循环到下一个项目，并返回到相同的BackgroundWorker1.runWorkerAsync行，并引发它已在运行的异常。

试图 1.

Private Sub fnStartEvent(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnStart.click 
    Dim inputFolder = Directory.GetFiles(textboxSourceFolder.text) 
    Dim currentNumberOfFile As Integer=1 
    For each Files in inputFolder 

     Dim arguments As Object = Files 
     updateProgressStatus(0, currentNumberOfFile) 
     BackgroundWorker1.WorkerReportsProgress = True 
     BackgroundWorker1.RunWorkerAsync(New Object() {arguments}) 
    '' 
     Do Until BackgroundWorker1.IsBusy 
     Thread.Sleep(500) 
     Loop 

    currentNumberOfFile += 1 
    Next  
End Sub 

但这并不更新用来表示进度UI线程。

2.在DoWork线程中放入整个进程，并为其中的每个文件循环，但要在进度条更新中返回交叉线程错误。

什么是标准程序在为后台工程人员执行循环以等待完成以及更新UI而不会阻止它。

Answer 1

每当你使用Thread.Sleep，你实际上正在睡眠当前挂钩到你的启动类（即表单）的线程。所以，如果你睡觉你的主线程，不会发生UI更新。

根据关于BackgroundWorker.IsBusy的MSDN文章，您需要抛出一个Application.DoEvents。下面的代码从上面链接的文章复制而来。

Private Sub downloadButton_Click(_ 
    ByVal sender As Object, _ 
    ByVal e As EventArgs) _ 
    Handles downloadButton.Click 

    ' Start the download operation in the background. 
    Me.backgroundWorker1.RunWorkerAsync() 

    ' Disable the button for the duration of the download. 
    Me.downloadButton.Enabled = False 

    ' Once you have started the background thread you 
    ' can exit the handler and the application will 
    ' wait until the RunWorkerCompleted event is raised. 

    ' If you want to do something else in the main thread, 
    ' such as update a progress bar, you can do so in a loop 
    ' while checking IsBusy to see if the background task is 
    ' still running. 
    While Me.backgroundWorker1.IsBusy 
     progressBar1.Increment(1) 
     ' Keep UI messages moving, so the form remains 
     ' responsive during the asynchronous operation. 
     Application.DoEvents() 
    End While 
End Sub