取消一項非同步工作或工作清單 (Visual Basic)

發行項
05/10/2023

如果您不想要等候非同步應用程式完成，則可以設定可用來取消非同步應用程式的按鈕。遵循本主題中的範例，即可將取消按鈕新增至下載某個網站內容或網站清單的應用程式。

這些範例會使用微調非同步應用程式 (Visual Basic) 所描述的 UI。

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。

取消工作

第一個範例會建立 [取消] 按鈕與單一下載工作的關聯。如果您在應用程式下載內容時選擇該按鈕，則會取消下載。

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 CancelATask 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。

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

下列變更會將 [取消] 按鈕新增至下載網站的應用程式。如果您不想要下載或建置範例，則可以檢閱本主題結尾處的＜完整範例＞一節中的最終產品。星號會標記程式碼中的變更。

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 [StarterCode] 作為 [啟始專案]，而非 [CancelATask]。

然後將下列變更新增至該專案的 MainWindow.xaml.vb 檔案。

宣告 CancellationTokenSource 變數 cts，這是在存取它之所有方法的範圍內。

Class MainWindow

    ' ***Declare a System.Threading.CancellationTokenSource.
    Dim cts As CancellationTokenSource

針對 [取消] 按鈕新增下列事件處理常式。事件處理常式會使用 CancellationTokenSource.Cancel 方法，以在使用者要求取消時通知 cts。

' ***Add an event handler for the Cancel button.
Private Sub cancelButton_Click(sender As Object, e As RoutedEventArgs)

    If cts IsNot Nothing Then
        cts.Cancel()
    End If
End Sub

在 [開始] 按鈕 startButton_Click 的事件處理常式中進行下列變更。

具現化 CancellationTokenSource、cts。

' ***Instantiate the CancellationTokenSource.
cts = New CancellationTokenSource()

在下載所指定網站內容的 AccessTheWebAsync 呼叫中，傳送 cts 的 CancellationTokenSource.Token 屬性作為引數。如果要求取消，則 Token 屬性會傳播訊息。新增 catch 區塊，以在使用者選擇取消下載作業時顯示訊息。下列程式碼示範這些變更。

Try
    ' ***Send a token to carry the message if cancellation is requested.
    Dim contentLength As Integer = Await AccessTheWebAsync(cts.Token)

    resultsTextBox.Text &=
        vbCrLf & $"Length of the downloaded string: {contentLength}." & vbCrLf

    ' *** If cancellation is requested, an OperationCanceledException results.
Catch ex As OperationCanceledException
    resultsTextBox.Text &= vbCrLf & "Download canceled." & vbCrLf

Catch ex As Exception
    resultsTextBox.Text &= vbCrLf & "Download failed." & vbCrLf
End Try

在 AccessTheWebAsync 中，使用 HttpClient 型別中 GetAsync 方法的 HttpClient.GetAsync(String, CancellationToken) 多載來下載網站的內容。將 ct (AccessTheWebAsync 的 CancellationToken 參數) 傳遞為第二個引數。如果使用者選擇 [取消] 按鈕，則權杖會夾帶訊息。

下列程式碼示範 AccessTheWebAsync 中的變更。

' ***Provide a parameter for the CancellationToken.
Async Function AccessTheWebAsync(ct As CancellationToken) As Task(Of Integer)

    Dim client As HttpClient = New HttpClient()

    resultsTextBox.Text &= vbCrLf & "Ready to download." & vbCrLf

    ' You might need to slow things down to have a chance to cancel.
    Await Task.Delay(250)

    ' GetAsync returns a Task(Of HttpResponseMessage).
    ' ***The ct argument carries the message if the Cancel button is chosen.
    Dim response As HttpResponseMessage = Await client.GetAsync("https://msdn.microsoft.com/library/dd470362.aspx", ct)

    ' Retrieve the website contents from the HttpResponseMessage.
    Dim urlContents As Byte() = Await response.Content.ReadAsByteArrayAsync()

    ' The result of the method is the length of the downloaded website.
    Return urlContents.Length
End Function

如果您未取消程式，則會產生下列輸出：
```
Ready to download.
Length of the downloaded string: 158125.
```
如果您在程式完成下載內容之前選擇 [取消] 按鈕，程式會產生下列輸出：
```
Ready to download.
Download canceled.
```

取消工作清單

您可以將相同 CancellationTokenSource 執行個體與每項工作建立關聯，以擴充先前的範例來取消許多工作。如果您選擇 [取消] 按鈕，即會取消所有尚未完成的工作。

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 CancelAListOfTasks 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。

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

若要自行逐步擴充範例，請遵循＜下載範例＞一節中的指示，但選擇 [CancelATask] 作為 [啟始專案]。將下列變更新增至該專案。星號會標記程式中的變更。

新增方法以建立網址清單。

' ***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

在 AccessTheWebAsync 中呼叫方法。

' ***Call SetUpURLList to make a list of web addresses.
Dim urlList As List(Of String) = SetUpURLList()

在 AccessTheWebAsync 中新增下列迴圈，以處理清單中的每個網址。

' ***Add a loop to process 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

因為 AccessTheWebAsync 顯示長度，所以此方法不需要傳回任何項目。請移除 return 陳述式，並將方法的傳回型別變更為 Task，而非 Task<TResult>。
```
Async Function AccessTheWebAsync(ct As CancellationToken) As Task
```
使用陳述式，從 startButton_Click 呼叫方法，而不是使用運算式。
```
Await AccessTheWebAsync(cts.Token)
```

如果您未取消程式，則會產生下列輸出：

Length of the downloaded string: 35939.

Length of the downloaded string: 237682.

Length of the downloaded string: 128607.

Length of the downloaded string: 158124.

Length of the downloaded string: 204890.

Length of the downloaded string: 175488.

Length of the downloaded string: 145790.

Downloads complete.

如果您在下載完成之前選擇 [取消] 按鈕，則輸出會包含在取消之前完成之下載的長度。

Length of the downloaded string: 35939.

Length of the downloaded string: 237682.

Length of the downloaded string: 128607.

Downloads canceled.

完整範例

下列各節包含每個先前範例的程式碼。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。

取消工作範例

下列程式碼是取消單一工作之範例的完整 MainWindow.xaml.vb 檔案。

' 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
            ' ***Send a token to carry the message if cancellation is requested.
            Dim contentLength As Integer = Await AccessTheWebAsync(cts.Token)

            resultsTextBox.Text &=
                vbCrLf & $"Length of the downloaded string: {contentLength}." & vbCrLf

            ' *** If cancellation is requested, an OperationCanceledException results.
        Catch ex As OperationCanceledException
            resultsTextBox.Text &= vbCrLf & "Download canceled." & vbCrLf

        Catch ex As Exception
            resultsTextBox.Text &= vbCrLf & "Download failed." & vbCrLf
        End Try

        ' ***Set the CancellationTokenSource to Nothing when the download is complete.
        cts = Nothing
    End Sub

    ' ***Add an event handler for the Cancel button.
    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.
    Async Function AccessTheWebAsync(ct As CancellationToken) As Task(Of Integer)

        Dim client As HttpClient = New HttpClient()

        resultsTextBox.Text &=
            vbCrLf & "Ready to download." & vbCrLf

        ' You might need to slow things down to have a chance to cancel.
        Await Task.Delay(250)

        ' GetAsync returns a Task(Of HttpResponseMessage).
        ' ***The ct argument carries the message if the Cancel button is chosen.
        Dim response As HttpResponseMessage = Await client.GetAsync("https://msdn.microsoft.com/library/dd470362.aspx", ct)

        ' Retrieve the website contents from the HttpResponseMessage.
        Dim urlContents As Byte() = Await response.Content.ReadAsByteArrayAsync()

        ' The result of the method is the length of the downloaded website.
        Return urlContents.Length
    End Function
End Class

' Output for a successful download:

' Ready to download.

' Length of the downloaded string: 158125.

' Or, if you cancel:

' Ready to download.

' Download canceled.

取消工作清單範例

下列程式碼是取消工作清單之範例的完整 MainWindow.xaml.vb 檔案。

' 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
            ' ***AccessTheWebAsync returns a Task, not a Task(Of Integer).
            Await AccessTheWebAsync(cts.Token)
            '  ***Small change in the display lines.
            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

    ' Add an event handler for the Cancel button.
    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()

        ' ***Add a loop to process 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

' Output if you do not choose to cancel:

' Length of the downloaded string: 35939.

' Length of the downloaded string: 237682.

' Length of the downloaded string: 128607.

' Length of the downloaded string: 158124.

' Length of the downloaded string: 204890.

' Length of the downloaded string: 175488.

' Length of the downloaded string: 145790.

' Downloads complete.

'  Sample output if you choose to cancel:

' Length of the downloaded string: 35939.

' Length of the downloaded string: 237682.

' Length of the downloaded string: 128607.

' Downloads canceled.

另請參閱

CancellationTokenSource
CancellationToken
使用 Async 和 Await 進行非同步程式設計 (Visual Basic)
微調非同步應用程式 (Visual Basic)
Async Sample: Fine Tuning Your Application (非同步範例：微調應用程式)

共用方式為

取消一項非同步工作或工作清單 (Visual Basic)

取消工作

下載範例

建置範例

取消工作清單

下載範例

建置範例

完整範例

取消工作範例

取消工作清單範例

另請參閱

其他資源