【IT168技术文档】

  许多经常执行的操作可能需要很长的执行时间。例如：
 
  图像下载
   Web 服务调用
  文件下载和上载（包括点对点应用程序）
  复杂的本地计算
  数据库事务
  本地磁盘访问（相对于内存访问来说其速度很慢）
  例子里是微软本身的例子和我的例子，一个窗体两个 BackgroundWorker，两个可以一起运行啊

  第一个是求斐波纳契数
  http://msdn.microsoft.com/zh-cn/library/system.componentmodel.backgroundworker%28VS.80%29.aspx
  第二个是不断地向richtextbox里添加行，类似于日志，完成一个后在里面显示一下。

  微软的东西都封装了哎。例子外有几个问题：
  （1）“您必须非常小心，确保在 DoWork 事件处理程序中不操作任何用户界面对象。而应该通过 ProgressChanged 和 RunWorkerCompleted 事件与用户界面进行通信。”
但是测试发现，放在ProgressChangedUI还是没有响应的。所以只能放在 DoWork里

  （2） 如果出现Cross-thread operation not valid: Control 'RichTextBox1' accessed from a thread other than the thread it was created on.
  在窗体的构造方法里加上这个即可。
  Control.CheckForIllegalCrossThreadCalls = False

  .vb

Imports System.ComponentModel
Public Class TestBackGroundWorkClass TestBackGroundWork
Private lStrValue = String.Empty
Private numberToCompute As Integer = 0
Private highestPercentageReached As Integer = 0
Sub New()Sub New()
' This call is required by the Windows Form Designer.
    InitializeComponent()
Me.BackgroundWorker1.WorkerSupportsCancellation = True
' Add any initialization after the InitializeComponent() call.
    'Control.CheckForIllegalCrossThreadCalls = False
  End Sub
msdn 例子#Region "msdn 例子"
Private Sub Button1_Click()Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
'befor backgroundwork
    ' Me.TextBox2.Text = mFnGetNum(CInt(TextBox1.Text))
    TextBox2.Text = String.Empty
numberToCompute = CInt(TextBox1.Text)
highestPercentageReached = 0
BackgroundWorker1.RunWorkerAsync(numberToCompute)
Me.Button1.Enabled = False
While Me.BackgroundWorker1.IsBusy
' Keep UI messages moving, so the form remains 
      ' responsive during the asynchronous operation.
      Application.DoEvents()
End While
Me.Button1.Enabled = True
End Sub
Private Sub Button2_Click()Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
Me.BackgroundWorker1.CancelAsync()
End Sub
Public Function mFnGetNum()Function mFnGetNum(ByVal Num As Integer, _
ByVal worker As BackgroundWorker, _
ByVal e As DoWorkEventArgs) As Long
Dim result As Long = 0
If worker.CancellationPending Then
e.Cancel = True
Else
If Num < 2 Then
result = 1
Else
result = mFnGetNum(Num - 1, worker, e) + _
mFnGetNum(Num - 2, worker, e)
End If
' Report progress as a percentage of the total task.
      Dim percentComplete As Integer = _
CSng(Num) / CSng(numberToCompute) * 100
If percentComplete > highestPercentageReached Then
highestPercentageReached = percentComplete
worker.ReportProgress(percentComplete)
End If
End If
Return result
End Function
Public Function mFnGetNum()Function mFnGetNum(ByVal Num As Integer) As Long
Dim result As Long = 0
If Num < 2 Then
result = 1
Else
result = mFnGetNum(Num - 1) + mFnGetNum(Num - 2)
End If
Return result
End Function
Private Sub BackgroundWorker1_DoWork()Sub BackgroundWorker1_DoWork(ByVal sender As System.Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
Dim worker As System.ComponentModel.BackgroundWorker = CType(sender, System.ComponentModel.BackgroundWorker)
e.Result = mFnGetNum(e.Argument, worker, e)
End Sub
Private Sub BackgroundWorker1_ProgressChanged()Sub BackgroundWorker1_ProgressChanged(ByVal sender As System.Object, ByVal e As System.ComponentModel.ProgressChangedEventArgs) Handles BackgroundWorker1.ProgressChanged
Me.ProgressBar1.Value = e.ProgressPercentage
End Sub
Private Sub BackgroundWorker1_RunWorkerCompleted()Sub BackgroundWorker1_RunWorkerCompleted(ByVal sender As System.Object, ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles BackgroundWorker1.RunWorkerCompleted
If Not (e.Error Is Nothing) Then
MessageBox.Show(e.Error.Message)
ElseIf e.Cancelled Then
' Next, handle the case where the user canceled the 
      ' operation.
      ' Note that due to a race condition in 
      ' the DoWork event handler, the Cancelled
      ' flag may not have been set, even though
      ' CancelAsync was called.
      TextBox2.Text = "Canceled"
    Else
' Finally, handle the case where the operation succeeded.
      TextBox2.Text = e.Result.ToString()
End If
End Sub
#End Region