Question

我正在尝试从一个文件夹复制到另一个文件夹并跟随msdn article我试图在我的WPF应用程序上实现asnyc调用。但我不确定我做错了它阻止UI线程，我不认为这第一个样本是asnyc。

这是我的代码

  Dim tasks = myImages.Select(Async Function(x)
                                                Dim result As Image
                                                Try

                                                    result = Await CopyImage(x, If(cts IsNot Nothing, cts.Token, Nothing))

                                                Catch ex2 As Exception

                                                End Try

                                                ProgressValue += 1
                                                CompletedText = ProgressValue.ToString() & " of " + MaxValue.ToString() & " images completed."
                                                Return result
                                            End Function)
                Dim results = Await Task.WhenAll(tasks)

  Public Async Function CopyImage(Image As Image, ct As CancellationToken) As Task(Of Image)
    If ct.IsCancellationRequested Then
        Return Nothing
    End If

    Await _mutex.WaitAsync()
    Dim SourceMainDirectory As String = "\\Server\Folder1"
    Dim DestinationMainDirectory As String = = "\\Server\Folder2"

    Dim Path = Image.Path.Replace("/", "\")
    Dim Data As String() = Path.Split("\")
    Dim Folder As String
    Dim ImageName As String
    If Data IsNot Nothing AndAlso Data.Length > 1 Then
        Folder = Data(0)
        ImageName = Data(1)      
    End If

    Dim ImgFullSource = SourceMainDirectory + Folder + "\" + ImageName
    Dim ImgFullDest = DestinationMainDirectory + Folder + "\" + ImageName

    Try

        Using SourceStream As FileStream = File.Open(ImgFullSource, FileMode.Open)
            Using DestinationStream As FileStream = File.Create(ImgFullDest)
                Await SourceStream.CopyToAsync(DestinationStream, 81920, ct)
                Return Image
            End Using
        End Using

    Catch ex As OperationCanceledException
        Return Nothing
    Catch ex As Exception
        Return Nothing
    Finally
        _mutex.Release()
    End Try

    Return Nothing
End Function

上面的ProgressValue被引发来更新我的进度条值。此代码在没有阻止UI线程的情况下正常工作，并且异步完美地更新进度如果我使用myHttpClient.GetAsync方法来验证Web上的相同图像，例如

  Dim tasks = myImages.Select(Async Function(x)
                                                Dim result As Image
                                                Try

  result = Await  testUrl_async_cancel(x, If(cts IsNot Nothing, cts.Token, Nothing))
                                                Catch ex2 As Exception

                                                End Try

                                                ProgressValue += 1
                                                CompletedText = ProgressValue.ToString() & " of " + MaxValue.ToString() & " images completed."
                                                Return result
                                            End Function)
                Dim results = Await Task.WhenAll(tasks)

  Async Function testUrl_async_cancel( ByVal myImage As Image, ByVal ct As CancellationToken) As Task(Of AEL)

        If ct.IsCancellationRequested Then
            Return Nothing
        End If

        Await _mutex.WaitAsync()

        Dim myHttpClient As New HttpClient()
        Dim myHttpResponse As HttpResponseMessage

 myHttpClient.BaseAddress = New Uri(imageUrlD)


        Try
            myHttpResponse = Await myHttpClient.GetAsync(myImageUrl, ct)
        Catch ex As OperationCanceledException
            myHttpResponse = Nothing
        Catch ex As Exception
            myHttpResponse = Nothing
        Finally
            _mutex.Release()
        End Try

        If myHttpResponse IsNot Nothing AndAlso myHttpResponse.IsSuccessStatusCode Then
            Return Nothing
        Else
            Return myImage
        End If


    End Function

因此它应该与CopyImage函数有关，并且分别使用源流来阻止UI线程，因为所有其他代码都是相同的。如何使此代码异步不会阻止WPF上的UI线程？

Answer 1

此行为是由于文件流中的奇怪。 File.Open和File.Create只能返回同步文件流。

要获得真正的异步文件流，您必须使用FileStream constructor并为true参数传递isAsync，或在FileOptions.Asynchronous标记中包含options标记{1}}参数。必须使用isAsync或options调用构造函数重载，否则文件流将是同步的。

Answer 2

基于Stephen Cleary的回答。我更改了文件流读写部分，如下所示，它在不阻塞UI线程的情况下工作异步。我希望这可以帮助其他人寻找相同的解决方案。

最后一个参数 True是斯蒂芬提到的异步参数。

 Using SourceStream As FileStream = New FileStream(ImgFullSource, FileMode.Open, FileAccess.Read, FileShare.Read, 81920, True)
                Using DestinationStream As FileStream = New FileStream(ImgFullDest, FileMode.Create, FileAccess.ReadWrite, FileShare.ReadWrite, 81920, True)
                    Await SourceStream.CopyToAsync(DestinationStream, 81920, ct)
                    Return Image
                End Using
            End Using

使用FileStream的异步文件I / O阻止了WPF上的UI线程

2 个答案: