Question

我正在尝试为我的程序创建一个更新程序，它会自动从网上下载我的程序的最新版本。现在我希望使用进度条完成此过程（因此，当下载进度为50％时，进度条会中途完成）。这是我的代码：

 Private Sub client_ProgressChanged(ByVal sender As Object, ByVal e As DownloadProgressChangedEventArgs)
    Dim bytesIn As Double = Double.Parse(e.BytesReceived.ToString())
    Dim totalBytes As Double = Double.Parse(e.TotalBytesToReceive.ToString())
    Dim percentage As Double = bytesIn / totalBytes * 100

    client.Value = Int32.Parse(Math.Truncate(percentage).ToString())
End Sub

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Dim url As String = "MY DOWNLOAD LINK"
    'Download
    Dim client As WebClient = New WebClient
    AddHandler client.DownloadProgressChanged, AddressOf client_ProgressChanged
    AddHandler client.DownloadFileCompleted, AddressOf client_DownloadCompleted
    client.DownloadFileAsync(New Uri(url), "C:\Users\User\Desktop\BACKUP\TESTING\New folder\1.exe")
End Sub
End Class

现在我知道保存文件的地方已由我手动输入，但我稍后会更改。我目前的问题是该文件未被下载。但是，当我将DownloadFileAsync方法更改为DownloadFile时，我的程序会下载该文件。但是，使用DownloadFile方法，我将无法使用进度条来跟踪下载进度。非常感谢任何帮助： - ）

Answer 1

当你说文件没有下载时，不知道你的意思吗？你得到一个错误/例外？什么都没发生？你有没有放置断点，debug.prints等？

使用VS2012和asyn / await，您可以将所有内容放在一个方法中，以保持“线性代码流”。导入System.Threading和System.Threading.Tasks

Private Async Function DownloadWithProgress(ByVal url As String, ByVal p As ProgressBar) As Task(Of Integer)

    Dim wc As Net.HttpWebRequest = DirectCast(Net.HttpWebRequest.Create(url), Net.HttpWebRequest)
    Dim resp = Await (New TaskFactory(Of Net.WebResponse)).StartNew(AddressOf wc.GetResponse)
    p.Value = 0
    p.Maximum = CInt(resp.ContentLength)
    Dim rqs = resp.GetResponseStream


    Dim bufsize As Integer = 1 << 16
    Dim buffer(bufsize) As Byte
    Dim got As Integer = 0
    Dim total As Integer = 0
    Do
        got = Await (New TaskFactory(Of Integer)).FromAsync(AddressOf rqs.BeginRead, AddressOf rqs.EndRead, buffer, 0, bufsize, Nothing)
        total += got
        Me.Label1.Text = "got: " & total.ToString
        p.Increment(got)
    Loop Until got = 0

    Return total

End Function

在此示例中，来自Web的数据被下载到数组中，但您当然也可以将其写入文件，或者根据数据执行任何操作。

使用示例：

Private running As Boolean = False
Private Async Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click

    If running Then
        MessageBox.Show("Cant you see, I'm working?")
        Exit Sub
    Else
        running = True
    End If

    Await DownloadWithProgress("http://download.thinkbroadband.com/5MB.zip", ProgressBar1)
    running = False

End Sub

如何使用进度条下载.exe文件 - VB 2012

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