本文介绍了一个使用VB.NET实现的后台文件搜索应用示例。该示例利用BackgroundWorker组件进行后台操作，避免了UI冻结的问题。文章详细展示了如何设置搜索路径、搜索模式，并通过事件处理实现了进度更新和搜索结果展示。
来源:Form: http://www.experts-exchange.com/Programming/Languages/.NET/Visual_Basic.NET/Q_22479449.html
This code sample assumes that the form has the following controls on it:（ 需要的控件：）TextBox called txtSearchPathTextBox called txtSearchPattern Label called lblMessageButton called btnStart 注意这个按钮的.text属性为:“Start”ListBox called lstFiles
Imports
System.IO
Public
Class SearchFilesFrm
Class SearchFilesFrm
' To set up for a background operation, add an event handler for the DoWork event. ' Call your time-consuming operation in this event handler. To start the operation, ' call RunWorkerAsync. To receive notifications of progress updates, handle the ' ProgressChanged event. To receive a notification when the operation is completed, ' handle the RunWorkerCompleted event. ' Note ' You must be careful not to manipulate any user-interface objects in your DoWork ' event handler. Instead, communicate to the user interface through the ' ProgressChanged and RunWorkerCompleted events. ' If your background operation requires a parameter, call RunWorkerAsync with your ' parameter. Inside the DoWork event handler, you can extract the parameter from ' the DoWorkEventArgs.Argument property. ' Store the results of the search Private Files As List(Of String) ' Used to prevent nested calls to ProgressChanged Private CallInProgress As Boolean ' The Worker thread Private WithEvents BgWorker As New System.ComponentModel.BackgroundWorker
Private Sub SearchFilesFrm_Load()Sub SearchFilesFrm_Load(ByVal sender As Object, _ ByVal e As System.EventArgs) Handles Me.Load ' Enable ProgressChanged and Cancellation mothed BgWorker.WorkerReportsProgress = True BgWorker.WorkerSupportsCancellation = True
End Sub ' Start and Stop the search Private Sub btnStart_Click()Sub btnStart_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles btnStart.Click If btnStart.Text = "Start" Then If Not Directory.Exists(txtSearchPath.Text.Trim()) Then MessageBox.Show("The Search Directory does not exist", _ "Path Error", MessageBoxButtons.OK, MessageBoxIcon.Error) Return End If If txtSearchPattern.Text.Trim() = "" Then txtSearchPattern.Text = "*" Dim arguments() As String = {txtSearchPath.Text.Trim(), _ txtSearchPattern.Text.Trim()} lstFiles.Items.Clear() ' Start the background worker thread. Thread runs in DoWork event. BgWorker.RunWorkerAsync(arguments) btnStart.Text = "Stop" Else BgWorker.CancelAsync() End If End Sub ' Start the Asynchronous file search. The background thread does it work from ' this event. Private Sub BgWorker_DoWork()Sub BgWorker_DoWork(ByVal sender As Object, _ ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BgWorker.DoWork 'Retrieve the search path which was requested Dim path As String = DirectCast(e.Argument, String())(0) Dim pattern As String = DirectCast(e.Argument, String())(1) ' Invoke the worker procedure Files = New List(Of String) SearchFiles(path, pattern) ' Return a result to the RunWorkerCompleted event Dim message As String = String.Format("Found {0} Files", Files.Count) e.Result = message End Sub ' Recursively search directory and sub directories Private Sub SearchFiles()Sub SearchFiles(ByVal path As String, ByVal pattern As String) ' Displat message Dim message As String = String.Format("Parsing Directory {0}", path) BgWorker.ReportProgress(0, message) 'Read the files and if the Stop button is pressed cancel the operation For Each fileName As String In Directory.GetFiles(path, pattern) If BgWorker.CancellationPending Then Return Files.Add(fileName) Next For Each dirName As String In Directory.GetDirectories(path) If BgWorker.CancellationPending Then Return SearchFiles(dirName, pattern) Next End Sub ' The bacground thread calls this event when you make a call to ReportProgress ' It is OK to access user controls in the UI from this event. ' If you use a Progress Bar or some other control to report the tasks progress ' you should avoid unnecessary calls to ReportProgress method because this causes ' a thread switch which is a relatively expensive in terms of processing time. Private Sub BgWorker_ProgressChanged()Sub BgWorker_ProgressChanged(ByVal sender As Object, _ ByVal e As System.ComponentModel.ProgressChangedEventArgs) _ Handles BgWorker.ProgressChanged ' Reject a nested call. If CallInProgress Then Return CallInProgress = True ' Display the message received in the UserState property lblMessage.Text = e.UserState.ToString() ' Display all files added since last call. For idx As Integer = lstFiles.Items.Count To Files.Count - 1 lstFiles.Items.Add(Files(idx)) Next ' If a Me.Refresh is in this code you will need to place a Application.DoEvents() ' otherwise the UI will not respond without them it works fine. ' Me.Refresh() ' Let the Windows OS process messages in the queue ' Application.DoEvents() CallInProgress = False End Sub ' The background thread calls this event just before it reaches the End Sub ' of the DoWork event. It is OK to access user controls in the UI from this event. Private Sub BgWorker_RunWorkerCompleted()Sub BgWorker_RunWorkerCompleted(ByVal sender As Object, _ ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) _ Handles BgWorker.RunWorkerCompleted ' Display the last message and reset the Start/Stop button text lblMessage.Text = e.Result.ToString() btnStart.Text = "Start" End Sub
End Class
在win2k+vb.net 2005 测试过。不错。
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