Question

我正在尝试运行异步任务，取消它，然后再次运行它，但是当我第一次取消它时，我再也无法再次运行它了，我做错了什么？。

Private TypeTask As Threading.Tasks.Task
Private TypeTaskCTS As New Threading.CancellationTokenSource
Private TypeTaskCT As Threading.CancellationToken = TypeTaskCTS.Token
Private RequestCancel As Boolean = True

Private Sub TypeWritter(ByVal CancellationToken As Threading.CancellationToken,
                        ByVal [Text] As String,
                        ByVal TypeSpeed As Integer,
                        ByVal PauseSpeed As Integer)

    ' For each Character in text to type...
    For Each c As Char In [Text]

        ' If not want to cancel then...
        If Not CancellationToken.IsCancellationRequested Then

            ' Type the character.
            Console.Write(CStr(c))

            ' Type-Wait.
            Threading.Thread.Sleep(TypeSpeed)

            If ".,;:".Contains(c) Then
                ' Pause-Wait.
                Threading.Thread.Sleep(PauseSpeed)
            End If

        Else ' want to cancel.

            ' Reset the request cancellation.
            RequestCancel = False

            ' Exit iteration.
            Exit For

        End If ' CancellationToken.IsCancellationRequested

    Next c ' As Char In [Text]

End Sub

Public Sub TypeWritter(ByVal [Text] As String,
                       Optional ByVal TypeSpeed As Integer = 75,
                       Optional ByVal PauseSpeed As Integer = 400)

    ' Run the asynchronous Task.
    TypeTask = Threading.Tasks.
               Task.Factory.StartNew(Sub()
                                         TypeWritter(TypeTaskCT, [Text], TypeSpeed, PauseSpeed)
                                     End Sub, TypeTaskCT)

    ' Until Task is not completed or is not cancelled, do...
    Do Until TypeTask.IsCompleted OrElse TypeTask.IsCanceled

        If RequestCancel Then

            If Not TypeTaskCTS.IsCancellationRequested Then
                TypeTaskCTS.Cancel
            End If

            RequestCancel = False
            Exit Do

        End If

    Loop ' TypeTask.IsCompleted OrElse TypeTask.IsCanceled

End Sub

Public Sub TypeWritterLine(ByVal [Text] As String,
                           Optional ByVal TypeSpeed As Integer = 75,
                           Optional ByVal PauseSpeed As Integer = 400)

    TypeWritter([Text] & vbCrLf, TypeSpeed, PauseSpeed)
    Console.WriteLine()

End Sub

注意变量：

Private RequestCancel As Boolean = True

第一次使用时设置为True取消任务（只是为了更快地测试当我第二次调用Task时会发生什么，我期待错误）。

我正在尝试的用法是：

Sub Main()

    RequestCancel = True ' This should cancel this task:
    TypeWritterLine("Some text")

    ' And this task should run as normally, but it doesn't, I get an empty line:
    TypeWritterLine("Some other text")

End Sub

Answer 1

这是设计的。任务总是执行一次，即使之前没有开始取消。这意味着您必须注意不要多次调用任务的Start方法。要“重新运行”任务，您必须再次调用Task.Factory.StartNew方法。

无法重新运行已取消的任务

1 个答案: