在一段時間後取消非同步工作 (Visual Basic)
如果不想等候作業完成,則可以使用 CancellationTokenSource.CancelAfter 方法,在一段時間之後取消非同步作業。 這個方法排定取消未在 CancelAfter
運算式所指定之2期間內完成的任何相關工作。
這個範例會新增至取消一項非同步工作或工作清單 (Visual Basic) 中所開發的程式碼來下載網站清單,以及顯示每個網站內容的長度。
注意
若要執行範例,您必須在電腦上安裝 Visual Studio 2012 或更新版本以及 .NET Framework 4.5 或更新版本。
下載範例
您可以從 Async Sample: Fine Tuning Your Application (非同步範例:微調應用程式) 下載完整 Windows Presentation Foundation (WPF) 專案,然後遵循下列步驟。
解壓縮您下載的檔案,然後啟動 Visual Studio。
在功能表列上,依序選擇 [檔案] 、[開啟舊檔] 及 [專案/方案] 。
在 [開啟專案] 對話方塊中,開啟保存已解壓縮之範例程式碼的資料夾,然後開啟 AsyncFineTuningVB 的方案 (.sln) 檔案。
在方案總管中,開啟 CancelAfterTime 專案的捷徑功能表,然後選擇 [設定為啟始專案]。
選擇 F5 鍵以執行專案。
選擇 CTRL+F5 鍵以執行專案,而不進行偵錯。
執行程式數次,確認輸出可能會顯示所有網站、沒有網站或某些網站的輸出。
如果您不想要下載專案,則可以檢閱本主題結尾的 MainWindow.xaml.vb 檔案。
建置範例
本主題中的範例會新增至取消一項非同步工作或工作清單 (Visual Basic) 中所開發的專案來取消工作清單。 雖然未明確地使用 [取消] 按鈕,但是此範例會使用相同的 UI。
若要自行逐步建置範例,請遵循<下載範例>一節中的指示,但選擇 [CancelAListOfTasks] 作為 [啟始專案]。 將本主題中的變更新增至該專案。
若要指定將工作標記為取消之前的最長時間,請將 CancelAfter
呼叫新增至 startButton_Click
,如下列範例所示。 新增的項目會以星號標記。
Private Async Sub startButton_Click(sender As Object, e As RoutedEventArgs)
' Instantiate the CancellationTokenSource.
cts = New CancellationTokenSource()
resultsTextBox.Clear()
Try
' ***Set up the CancellationTokenSource to cancel after 2.5 seconds. (You
' can adjust the time.)
cts.CancelAfter(2500)
Await AccessTheWebAsync(cts.Token)
resultsTextBox.Text &= vbCrLf & "Downloads complete."
Catch ex As OperationCanceledException
resultsTextBox.Text &= vbCrLf & "Downloads canceled." & vbCrLf
Catch ex As Exception
resultsTextBox.Text &= vbCrLf & "Downloads failed." & vbCrLf
End Try
' Set the CancellationTokenSource to Nothing when the download is complete.
cts = Nothing
End Sub
執行程式數次,確認輸出可能會顯示所有網站、沒有網站或某些網站的輸出。 下列輸出是範例:
Length of the downloaded string: 35990.
Length of the downloaded string: 407399.
Length of the downloaded string: 226091.
Downloads canceled.
完整範例
下列程式碼是範例的 MainWindow.xaml.vb 檔案的完整文字。 星號會標記已針對此範例新增的項目。
請注意,您必須新增 System.Net.Http 的參考。
您可以從 Async Sample: Fine Tuning Your Application (非同步範例:微調應用程式) 下載專案。
' Add an Imports directive and a reference for System.Net.Http.
Imports System.Net.Http
' Add the following Imports directive for System.Threading.
Imports System.Threading
Class MainWindow
' Declare a System.Threading.CancellationTokenSource.
Dim cts As CancellationTokenSource
Private Async Sub startButton_Click(sender As Object, e As RoutedEventArgs)
' Instantiate the CancellationTokenSource.
cts = New CancellationTokenSource()
resultsTextBox.Clear()
Try
' ***Set up the CancellationTokenSource to cancel after 2.5 seconds. (You
' can adjust the time.)
cts.CancelAfter(2500)
Await AccessTheWebAsync(cts.Token)
resultsTextBox.Text &= vbCrLf & "Downloads complete."
Catch ex As OperationCanceledException
resultsTextBox.Text &= vbCrLf & "Downloads canceled." & vbCrLf
Catch ex As Exception
resultsTextBox.Text &= vbCrLf & "Downloads failed." & vbCrLf
End Try
' Set the CancellationTokenSource to Nothing when the download is complete.
cts = Nothing
End Sub
' You can still include a Cancel button if you want to.
Private Sub cancelButton_Click(sender As Object, e As RoutedEventArgs)
If cts IsNot Nothing Then
cts.Cancel()
End If
End Sub
' Provide a parameter for the CancellationToken.
' Change the return type to Task because the method has no return statement.
Async Function AccessTheWebAsync(ct As CancellationToken) As Task
Dim client As HttpClient = New HttpClient()
' Call SetUpURLList to make a list of web addresses.
Dim urlList As List(Of String) = SetUpURLList()
' Process each element in the list of web addresses.
For Each url In urlList
' GetAsync returns a Task(Of HttpResponseMessage).
' Argument ct carries the message if the Cancel button is chosen.
' Note that the Cancel button can cancel all remaining downloads.
Dim response As HttpResponseMessage = Await client.GetAsync(url, ct)
' Retrieve the website contents from the HttpResponseMessage.
Dim urlContents As Byte() = Await response.Content.ReadAsByteArrayAsync()
resultsTextBox.Text &=
vbCrLf & $"Length of the downloaded string: {urlContents.Length}." & vbCrLf
Next
End Function
' Add a method that creates a list of web addresses.
Private Function SetUpURLList() As List(Of String)
Dim urls = New List(Of String) From
{
"https://msdn.microsoft.com",
"https://msdn.microsoft.com/library/hh290138.aspx",
"https://msdn.microsoft.com/library/hh290140.aspx",
"https://msdn.microsoft.com/library/dd470362.aspx",
"https://msdn.microsoft.com/library/aa578028.aspx",
"https://msdn.microsoft.com/library/ms404677.aspx",
"https://msdn.microsoft.com/library/ff730837.aspx"
}
Return urls
End Function
End Class
' Sample output:
' Length of the downloaded string: 35990.
' Length of the downloaded string: 407399.
' Length of the downloaded string: 226091.
' Downloads canceled.