VB.Net 循环内的异步后台工作者

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Ui线程进度条和标签，我已经使用backgroundworker.runworkerAsync来工作。现在的问题是我必须使用不同的输入多次执行相同的流程。

代码块是：

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

这里的问题是 fnStartEvent 方法一旦启动，它就会调用 BackgroundWorker1.runWorkerAsync 进程，并且在单独的线程中运行，不会等待线程完成，而是移动到下一行代码，即循环到下一项，它返回到相同的 BackgroundWorker1.runWorkerAsync 行并抛出它已经在运行的异常。

尝试过 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

但这不会更新用于表示进度的 Ui 线程。

2.将整个过程放入DoWork线程中，并为其中的每个文件循环，但这会在进度条更新中返回跨线程错误。

执行后台工作循环等待完成以及更新 UI 而不阻塞它的标准程序是什么。

Answer 1

每当您使用 Thread.Sleep 时，您实际上都在休眠挂接到启动类（即 Form）的当前线程。因此，如果您休眠主线程，则不会发生 UI 更新。

根据有关 的 MSDN 文章BackgroundWorker.IsBusy，需要抛出一个Application.DoEvents。下面的代码是从上面链接的文章复制的。

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