演练：多线程处理

更新：2007 年 11 月

该演练说明如何创建一个多线程应用程序，该程序在文本文件中搜索字词的匹配项。它演示：

• 用可以由 BackgroundWorker 组件调用的方法定义一个类。

• 处理由 BackgroundWorker 组件引发的事件。

• 启动 BackgroundWorker 组件以运行方法。

• 实现停止 BackgroundWorker 组件的 Cancel 按钮。

生成此主题的代码示例

1. 打开新的 Visual Basic Windows 应用程序项目，并创建一个名为 Form1 的窗体。

2. 向 Form1 中添加两个按钮和四个文本框。

3. 如下表所示命名对象。

   对象	属性	设置
   第一个按钮	Name, Text	Start, Start
   第二个按钮	Name, Text	Cancel, Cancel
   第一个文本框	Name, Text	SourceFile, ""
   第二个文本框	Name, Text	CompareString, ""
   第三个文本框	Name, Text	WordsCounted, "0"
   第四个文本框	Name, Text	LinesCounted, "0"

4. 近邻每个文本框添加一个标签。为每个标签设置 Text 属性，如下表所示。

   对象	属性	设置
   第一个标签	Text	Source File
   第二个标签	Text	Compare String
   第三个标签	Text	Matching Words
   第四个标签	Text	Lines Counted

5. 将“工具箱”的“组件”部分中的 BackgroundWorker 组件添加到窗体。它将出现在窗体的组件栏中。

6. 为 BackgroundWorker1 对象设置下列属性。

   属性	设置
   WorkerReportsProgress	True
   WorkerSupportsCancellation	True

定义将在单独的线程上运行的方法

1. 从“项目”菜单中选择“添加类”，在项目中添加类。显示“添加新项”对话框。

2. 从模板窗口选择“类”，并在名称字段内输入 Words.vb。

3. 单击“添加”。显示 Words 类。

4. 将 Option Compare 语句添加到 Words 类的顶部（Class 语句的上方）：

   Option Compare Text ' Case insensitive search.
   ' Use Option Compare Binary for case sensitive search.
   

5. 将下面的代码添加到 Words 类中：

   Public Class Words
       ' Object to store the current state, for passing to the caller.
       Public Class CurrentState
           Public LinesCounted As Integer
           Public WordsMatched As Integer
       End Class
   
       Public SourceFile As String
       Public CompareString As String
       Private WordCount As Integer = 0
       Private LinesCounted As Integer = 0
   
       Public Sub CountWords( _
           ByVal worker As System.ComponentModel.BackgroundWorker, _
           ByVal e As System.ComponentModel.DoWorkEventArgs _
       )
           ' Initialize the variables.
           Dim state As New CurrentState
           Dim myStream As System.IO.StreamReader = Nothing
           Dim line = ""
           Dim elapsedTime = 20
           Dim lastReportDateTime = Now
   
           If CompareString Is Nothing Or _
                   CompareString = System.String.Empty Then
               Throw New Exception("CompareString not specified.")
           End If
   
           Try
               ' Open a new stream.
               myStream = My.Computer.FileSystem.OpenTextFileReader(SourceFile)
               ' Do while there are lines remaining in the file to be read.
               Do While Not myStream.EndOfStream
                   If worker.CancellationPending Then
                       e.Cancel = True
                       Exit Do
                   Else
                       line = myStream.ReadLine
                       WordCount += CountInString(line, CompareString)
                       LinesCounted += 1
   
                       ' Raise an event so the form can monitor progress.
                       If Now > lastReportDateTime.AddMilliseconds(elapsedTime) Then
                           state.LinesCounted = LinesCounted
                           state.WordsMatched = WordCount
                           worker.ReportProgress(0, state)
                           lastReportDateTime = Now.AddMilliseconds(elapsedTime)
                       End If
                   End If
               Loop
   
               ' Report the final count values.
               state.LinesCounted = LinesCounted
               state.WordsMatched = WordCount
               worker.ReportProgress(0, state)
           Finally
               If myStream IsNot Nothing Then
                   ' Close the file.
                   myStream.Close()
               End If
           End Try
       End Sub
   
       Private Function CountInString( _
           ByVal SourceString As String, _
           ByVal CompareString As String _
       ) As Integer
           ' This function counts the number of times
           ' a word is found in a line.
           If SourceString Is Nothing Then
               Return 0
           End If
   
           Dim regex As New System.Text.RegularExpressions.Regex( _
               System.Text.RegularExpressions.Regex.Escape(CompareString))
           Dim matches As System.Text.RegularExpressions.MatchCollection
           matches = regex.Matches(SourceString)
           Return matches.Count
       End Function
   End Class
   

处理线程中的事件

• 将下面的事件处理程序添加到主窗体中：

  Private Sub BackgroundWorker1_RunWorkerCompleted( _
      ByVal sender As Object, _
      ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) _
      Handles BackgroundWorker1.RunWorkerCompleted
  
      ' This event handler is called when the background thread finishes.
      ' This method runs on the main thread.
  
      If e.Error IsNot Nothing Then
          MsgBox("Error: " & e.Error.Message)
      ElseIf e.Cancelled Then
          MsgBox("Word counting canceled.")
      Else
          MsgBox("Finished counting words.")
      End If
  End Sub
  
  Private Sub BackgroundWorker1_ProgressChanged( _
      ByVal sender As Object, _
      ByVal e As System.ComponentModel.ProgressChangedEventArgs) _
      Handles BackgroundWorker1.ProgressChanged
  
      ' This event handler is called after the background thread
      ' reads a line from the source file.
      ' This method runs on the main thread.
  
      Dim state As Words.CurrentState = _
          CType(e.UserState, Words.CurrentState)
      Me.LinesCounted.Text = state.LinesCounted.ToString
      Me.WordsCounted.Text = state.WordsMatched.ToString
  End Sub
  

启动并调用运行 WordCount 方法的新线程

1. 将下面的过程添加到程序中：

   Private Sub BackgroundWorker1_DoWork( _
       ByVal sender As Object, _
       ByVal e As System.ComponentModel.DoWorkEventArgs) _
       Handles BackgroundWorker1.DoWork
   
       ' This event handler is where the actual work is done.
       ' This method runs on the background thread.
   
       ' Get the BackgroundWorker object that raised this event.
       Dim worker As System.ComponentModel.BackgroundWorker
       worker = CType(sender, System.ComponentModel.BackgroundWorker)
   
       ' Get the Works object and call the main method.
       Dim WC As Words = CType(e.Argument, Words)
       WC.CountWords(worker, e)
   End Sub
   
   Sub StartThread()
       ' This method runs on the main thread.
   
       Me.WordsCounted.Text = "0"
   
       ' Initialize the object that the background worker calls.
       Dim WC As New Words
       WC.CompareString = Me.CompareString.Text
       WC.SourceFile = Me.SourceFile.Text
   
       ' Start the asynchronous operation.
       BackgroundWorker1.RunWorkerAsync(WC)
   End Sub
   

2. 通过窗体上的 Start 按钮调用 StartThread 方法：

   Private Sub Start_Click( _
       ByVal sender As System.Object, _
       ByVal e As System.EventArgs) _
       Handles Start.Click
   
       StartThread()
   End Sub
   

实现停止线程的 Cancel 按钮

• 从 Cancel 按钮的 Click 事件处理程序中调用 StopThread 过程。

  Private Sub Cancel_Click( _
      ByVal sender As System.Object, _
      ByVal e As System.EventArgs) _
      Handles Cancel.Click
  
      ' Cancel the asynchronous operation.
      Me.BackgroundWorker1.CancelAsync()
  End Sub
  

测试

现在可以测试该应用程序以确保其正确运行。

测试应用程序

1. 按 F5 运行该应用程序。

2. 显示窗体时，在 sourceFile 框中输入要测试的文件的路径。例如，假定测试文件名为 Test.txt，则输入 C:\Test.txt。

3. 在第二个文本框中，输入要让该应用程序在文本文件中搜索的字词或短语。

4. 单击 Start 按钮。LinesCounted 按钮应当立即开始递增。在任务完成后，应用程序将显示“计数完成”(Finished Counting) 消息。

测试 Cancel 按钮

1. 按 F5 以启动该应用程序，按照前面过程中所述内容输入文件名和搜索字词。确保所选择的文件足够大，以确保在搜索结束之前有时间取消该过程。

2. 单击Start按钮启动该应用程序。

3. 单击 Cancel 按钮。应用程序应该立即停止计数。

后续步骤

此应用程序包含一些基本错误处理。它检测空白搜索字符串。可以通过处理其他错误（如超过可以计数的最大字词数或行数）使该程序更加可靠。

请参见

任务

演练：用 Visual Basic 创作简单的多线程组件

其他资源

多线程处理 (Visual Basic)

修订记录