演练：在后台运行操作

  • 项目

如果某项操作需要很长时间才能完成，而你不希望造成用户界面的延迟，则可以使用 BackgroundWorker 类在另一个线程上运行此操作。

有关此示例中使用的代码的完整列表，请参阅如何：在后台运行操作。

在后台运行操作

  1. 当窗体在 Visual Studio 中的 Windows 窗体设计器中处于活动状态时，将两个 Button 控件从“工具箱”拖动到窗体中，然后根据下表设置按钮的 Name 和 Text 属性。

    Button 名称 文本
    button1 startBtn 开始
    button2 cancelBtn Cancel

  2. 打开“工具箱”，单击“组件”选项卡，然后将 BackgroundWorker 组件拖动到窗体上。

    backgroundWorker1 组件显示在“组件栏”中。

  3. 在“属性” 窗口中，将 WorkerSupportsCancellation 属性设置为 true。

  4. 在“属性”窗口中，单击“事件”按钮，然后双击 DoWork 和 RunWorkerCompleted 事件以创建事件处理程序。

  5. 将耗时的代码插入 DoWork 事件处理程序。

  6. 从参数 DoWorkEventArgs 的 Argument 属性中提取操作所需的任何参数。

  7. 将计算结果分配给 DoWorkEventArgs 的 Result 属性。

    此结果将可供 RunWorkerCompleted 事件处理程序使用。

    private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
    // Do not access the form's BackgroundWorker reference directly.
    // Instead, use the reference provided by the sender parameter.
    BackgroundWorker bw = sender as BackgroundWorker;

    // Extract the argument.
    int arg = (int)e.Argument;

    // Start the time-consuming operation.
    e.Result = TimeConsumingOperation(bw, arg);

    // If the operation was canceled by the user,
    // set the DoWorkEventArgs.Cancel property to true.
    if (bw.CancellationPending)
    {
        e.Cancel = true;
    }
}

    Private Sub backgroundWorker1_DoWork( _
sender As Object, e As DoWorkEventArgs) _
Handles backgroundWorker1.DoWork

   ' Do not access the form's BackgroundWorker reference directly.
   ' Instead, use the reference provided by the sender parameter.
   Dim bw As BackgroundWorker = CType( sender, BackgroundWorker )
   
   ' Extract the argument.
   Dim arg As Integer = Fix(e.Argument)
   
   ' Start the time-consuming operation.
   e.Result = TimeConsumingOperation(bw, arg)
   
   ' If the operation was canceled by the user, 
   ' set the DoWorkEventArgs.Cancel property to true.
   If bw.CancellationPending Then
      e.Cancel = True
   End If

End Sub

  8. 在 RunWorkerCompleted 事件处理程序中插入用于检索操作结果的代码。

    // This event handler demonstrates how to interpret
// the outcome of the asynchronous operation implemented
// in the DoWork event handler.
private void backgroundWorker1_RunWorkerCompleted(
    object sender,
    RunWorkerCompletedEventArgs e)
{
    if (e.Cancelled)
    {
        // The user canceled the operation.
        MessageBox.Show("Operation was canceled");
    }
    else if (e.Error != null)
    {
        // There was an error during the operation.
        string msg = String.Format("An error occurred: {0}", e.Error.Message);
        MessageBox.Show(msg);
    }
    else
    {
        // The operation completed normally.
        string msg = String.Format("Result = {0}", e.Result);
        MessageBox.Show(msg);
    }
}

    ' This event handler demonstrates how to interpret 
' the outcome of the asynchronous operation implemented
' in the DoWork event handler.
Private Sub backgroundWorker1_RunWorkerCompleted( _
sender As Object, e As RunWorkerCompletedEventArgs) _
Handles backgroundWorker1.RunWorkerCompleted

   If e.Cancelled Then
      ' The user canceled the operation.
      MessageBox.Show("Operation was canceled")
   ElseIf (e.Error IsNot Nothing) Then
      ' There was an error during the operation.
      Dim msg As String = String.Format("An error occurred: {0}", e.Error.Message)
      MessageBox.Show(msg)
   Else
      ' The operation completed normally.
      Dim msg As String = String.Format("Result = {0}", e.Result)
      MessageBox.Show(msg)
   End If
End Sub

  9. 实现 TimeConsumingOperation 方法。

    // This method models an operation that may take a long time
// to run. It can be cancelled, it can raise an exception,
// or it can exit normally and return a result. These outcomes
// are chosen randomly.
private int TimeConsumingOperation(
    BackgroundWorker bw,
    int sleepPeriod )
{
    int result = 0;

    Random rand = new Random();

    while (!bw.CancellationPending)
    {
        bool exit = false;

        switch (rand.Next(3))
        {
            // Raise an exception.
            case 0:
            {
                throw new Exception("An error condition occurred.");
                break;
            }

            // Sleep for the number of milliseconds
            // specified by the sleepPeriod parameter.
            case 1:
            {
                Thread.Sleep(sleepPeriod);
                break;
            }

            // Exit and return normally.
            case 2:
            {
                result = 23;
                exit = true;
                break;
            }

            default:
            {
                break;
            }
        }

        if( exit )
        {
            break;
        }
    }

    return result;
}

    ' This method models an operation that may take a long time 
' to run. It can be cancelled, it can raise an exception,
' or it can exit normally and return a result. These outcomes
' are chosen randomly.
Private Function TimeConsumingOperation( _
bw As BackgroundWorker, _
sleepPeriod As Integer) As Integer

   Dim result As Integer = 0
   
   Dim rand As New Random()
   
     While Not bw.CancellationPending
         Dim [exit] As Boolean = False

         Select Case rand.Next(3)
             ' Raise an exception.
             Case 0
                 Throw New Exception("An error condition occurred.")
                 Exit While

                 ' Sleep for the number of milliseconds
                 ' specified by the sleepPeriod parameter.
             Case 1
                 Thread.Sleep(sleepPeriod)
                 Exit While

                 ' Exit and return normally.
             Case 2
                 result = 23
                 [exit] = True
                 Exit While

             Case Else
                 Exit While
         End Select

         If [exit] Then
             Exit While
         End If
     End While
   
   Return result
End Function

  10. 在 Windows 窗体设计器中，双击 startButton 以创建 Click 事件处理程序。

  11. 调用 startButton 的 Click 事件处理程序中的 RunWorkerAsync 方法。

    private void startBtn_Click(object sender, EventArgs e)
{
    this.backgroundWorker1.RunWorkerAsync(2000);
}

    Private Sub startButton_Click(ByVal sender As Object, ByVal e As EventArgs) Handles startBtn.Click
    Me.backgroundWorker1.RunWorkerAsync(2000)
End Sub

  12. 在 Windows 窗体设计器中，双击 cancelButton 以创建 Click 事件处理程序。

  13. 调用 cancelButton 的 Click 事件处理程序中的 CancelAsync 方法。

    private void cancelBtn_Click(object sender, EventArgs e)
{
    this.backgroundWorker1.CancelAsync();
}

    Private Sub cancelButton_Click(ByVal sender As Object, ByVal e As EventArgs) Handles cancelBtn.Click
    Me.backgroundWorker1.CancelAsync()
End Sub

  14. 在文件的顶部，导入 System.ComponentModel 和 System.Threading 命名空间。

    using System;
using System.ComponentModel;
using System.Drawing;
using System.Threading;
using System.Windows.Forms;

    Imports System.ComponentModel
Imports System.Drawing
Imports System.Threading
Imports System.Windows.Forms

  15. 按 F6 生成解决方案，然后按 Ctrl+F5 在调试程序外部运行应用程序。

    注意

    如果按 F5 在调试程序下运行应用程序，则调试程序将捕获并显示 TimeConsumingOperation 方法中引发的异常。 在调试程序外部运行应用程序时，BackgroundWorker 将处理异常并将其缓存到 RunWorkerCompletedEventArgs 的 Error 属性中。

  16. 单击“启动”按钮以运行异步操作，然后单击“取消”按钮以停止正在运行的异步操作。

    每个操作的结果显示在 MessageBox 中。

后续步骤

另请参阅