WebClient 异步下载器无法正常工作

Question

所以，我一直在努力解决这个问题已经有一段时间了。我是一个业余程序员，所以我并不总是知道自己做错了什么。

总之，我最近项目的前提：

我和我的朋友们经常玩《我的世界》，但他们并不是很聪明，而且我们并不总是在附近获取模组并向他们发送链接等等。所以我想我应该编写一些程序来自动下拉模组，以便它们与服务器同步并同时获取服务器数据。

我正在使用免费的 FTP 主机，但我认为这不是问题所在，原因将变得很清楚。

基本上，我想使用进度条和理想的标签来指示整个数据块的进度（所有模块一起......不超过 1GB - 小得多）。但是，我似乎遇到了一些关于 Async 选项的问题：

• 它会随机选择不下载它应该下载的文件

• 在声称完整之前，它可能不会完整下载文件

• 当 msgbox 触发说它已完成下载所有项目时，进度条可能已满 50%。

但是，由于在同步使用 Webclient 时不存在进度报告事件，进度条不起作用，但当我在 BGworker 中运行同步时，它每次都能正确下载。但是，我失去了进度报告，这很重要....

所以，基本上：

• 有没有更好的方法来实现这一点？

这是我在准备好之前需要开始工作的最后一块，所以我真的很想尝试这样做。感谢您的帮助！

编辑：用代码更新：

Public Function GetDownloadSize(ByVal URL As String) As Long
    Dim request As Net.FtpWebRequest = DirectCast(Net.WebRequest.Create(URL), Net.FtpWebRequest)
    request.Method = Net.WebRequestMethods.Ftp.GetFileSize
    request.Credentials = New Net.NetworkCredential(dl_user, dl_pass)
    Dim response As Net.FtpWebResponse = DirectCast(request.GetResponse(), Net.FtpWebResponse)
    Dim fileSize As Long = response.ContentLength
    Return fileSize
End Function

Private Sub btn_sync_Click(sender As Object, e As EventArgs) Handles btn_sync.Click
    Dim cont As DialogResult = MsgBox("Continue? " + (total_dl_size / 1000).ToString("N0") + " KB remain to be downloaded.", MsgBoxStyle.YesNo, "CAUTION!")
    If cont = DialogResult.No Then
        tb_warnings.AppendText("-ERR: User declined to synchronize files. Restart the application to sync.")
        tb_warnings.AppendText(ControlChars.NewLine)
        Label3.BackColor = Color.Firebrick
        Return
    End If
    btn_sync.Enabled = False
    btn_scan.Enabled = false
    tb_warnings.AppendText("-Deleting outmoded/unused mods. Protected mods will be kept.")
    For Each i As fdata_obj In deleted_files
        My.Computer.FileSystem.DeleteFile(mc_dir + "\mods\" + i.name)
    Next
    tb_warnings.AppendText(ControlChars.NewLine)
    tb_warnings.AppendText("-Deleting mod subdirectories to ensure no conflicts.")
    tb_warnings.AppendText(ControlChars.NewLine)

    For Each d In My.Computer.FileSystem.GetDirectories(mc_dir + "\mods")
        My.Computer.FileSystem.DeleteDirectory(d, FileIO.DeleteDirectoryOption.DeleteAllContents)
    Next

    initialize_download()


End Sub

Private Sub initialize_download()

           Dim wc As New System.Net.WebClient() ' SORRY, ASSUME THIS IS A PUBLIC VAR SO IT CAN BE REFERENCED ACROSS ITS OTHER METHODS
    AddHandler wc.DownloadProgressChanged, AddressOf OnDownloadProgressChanged
    AddHandler wc, AddressOf OnFileDownloadCompleted

    Dim usr As String = "randouser"
    Dim pass As String = "randopass"
    For Each s In (From dl As fdata_obj In new_files Select dl_server + "/mods/" + mods_dir + "/" + dl.name).ToList
        downloads.Enqueue(s)
    Next
    wc.Credentials = New Net.NetworkCredential(usr, pass)

        Dim urix As String = downloads.Dequeue
        Try
            wc.DownloadFileasync(New Uri(urix), mc_dir + "\mods\" + IO.Path.GetFileName(urix))
        Catch ex As Exception
            MsgBox(ex.Message)
            If tb_warnings.InvokeRequired = True Then
                tb_warnings.Invoke(New tb_updater(AddressOf tb_update), "-ERR: Could not download file: " + urix, urix)
            Else
                tb_warnings.AppendText("-ERR: Could not download file: " + IO.Path.GetFileName(urix))
                tb_warnings.AppendText(ControlChars.NewLine)

            End If
    end try
End Sub
Private Sub OnDownloadProgressChanged(ByVal sender As Object, ByVal e As System.Net.DownloadProgressChangedEventArgs)
    MsgBox("This is happening!")
    total_dl = total_dl + e.BytesReceived
    Dim percentage As Integer = (CType((total_dl / total_dl_size), Integer) * 100)
    if percentage > 100 then
        percentage = 100
    endif
    prog_update(percentage)

End Sub

delegate sub progress_update(byval prog as integer)
' POTENTIAL ISSUES HERE???????
private sub prog_update(byval prog as integer)
    if progressbar1.invokerequired then
        progressbar1.invoke(new prog_update(addressof progress),prog)
    else
        progressbar1.value = prog


Private Sub OnFileDownloadCompleted(ByVal sender As Net.WebClient, ByVal e As System.ComponentModel.AsyncCompletedEventArgs)

    If e.Cancelled Then
        MsgBox(e.Cancelled)
    ElseIf Not e.Error Is Nothing Then
        MsgBox(e.Error.Message)
    Else
    if downloads.count > 0 then
                Dim urix As String = downloads.Dequeue
        Try
            wc.DownloadFileasync(New Uri(urix), mc_dir + "\mods\" + IO.Path.GetFileName(urix))
        Catch ex As Exception
            MsgBox(ex.Message)
            If tb_warnings.InvokeRequired = True Then
                tb_warnings.Invoke(New tb_updater(AddressOf tb_update), "-ERR: Could not download file: " + urix, urix)
            Else
                tb_warnings.AppendText("-ERR: Could not download file: " + IO.Path.GetFileName(urix))
                tb_warnings.AppendText(ControlChars.NewLine)

            End If
        End Try
    End If

End Sub

Answer 1

首先，您的进度条不起作用的主要原因是：

Dim percentage As Integer = (CType((total_dl / total_dl_size), Integer) * 100)

代码将首先计算total_dl / total_dl_size，说它的结果是 0.34，然后将其转换为整数，结果为 0（0.34 向下舍入为零，因为整数没有小数），最后相乘0 和 100（仍然是 0）。
你要做的就是先将被除数乘以 100，这样结果就会从 0-100 变为 0-1：(total_dl * 100) / total_dl_size。

至于线程安全（调用）我总是使用我创建的这个extension method：

Imports System.Runtime.CompilerServices

Public Module Extensions
    <Extension()> _
    Public Sub InvokeIfRequired(ByVal Control As Control, ByVal Method As [Delegate], ByVal ParamArray Parameters As Object())
        If Parameters Is Nothing OrElse _
            Parameters.Length = 0 Then Parameters = Nothing 'If Parameters is null or has a length of zero then no parameters should be passed.
        If Control.InvokeRequired = True Then
            Control.Invoke(Method, Parameters)
        Else
            Method.DynamicInvoke(Parameters)
        End If
    End Sub
End Module

（最好放在另一个文件中）

与Lambda expressions（在 Visual Studio 2010 中引入）一起，将极大地为您简化调用。这是因为不是将If InvokeRequired 模式放在任何地方：

If Me.InvokeRequired Then
    Me.Invoke(New Action(AddressOf SomeMethod), params)
Else
    SomeMethod()
End If

您只需输入：

Me.InvokeIfRequired(AddressOf SomeMethod, params)

扩展方法会为您完成剩下的工作。

如果您使用 lambda 表达式，您可以动态创建方法：

Me.InvokeIfRequired(Sub()
                        Label1.Text = "Hello world!"
                        ProgressBar1.Value += 1
                    End Sub)

---

现在，到你的代码。

我已经将您的代码分开了一些，以便更容易处理。对于初学者，我没有将下载代码复制粘贴到DownloadFileCompleted 事件处理程序，而是创建了一个更通用的方法，称为DownloadFile()。

''' <summary>
''' Downloads a file from the specified URL with the specified credentials.
''' </summary>
''' <param name="URL">The URL of the file.</param>
''' <param name="Username">The username which to login with.</param>
''' <param name="Password">The password which to login with.</param>
''' <remarks></remarks>
Private Sub DownloadFile(ByVal URL As String, ByVal Username As String, ByVal Password As String)
    If wc.IsBusy = True Then Throw New Exception("A download is already ongoing!")

    wc.Credentials = New NetworkCredential(dl_user, dl_pass)
    total_dl_size = GetDownloadSize(URL, Username, Password)

    Try
        Dim FileName As String = Path.GetFileName(URL)
        AppendWarning("Downloading " & FileName & "...")
        wc.DownloadFileAsync(New Uri(URL), Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.Desktop), FileName))
    Catch ex As Exception
        AppendWarning("-ERR: Could not download file: " & Path.GetFileName(URL))
    End Try
End Sub

如你所见，我还制作了一个通用方法来输出警告和错误消息：

''' <summary>
''' (Thread-safe) Appends a warning or status message to the "tb_warnings" text box.
''' </summary>
''' <param name="Text">The text to append.</param>
''' <remarks></remarks>
Private Sub AppendWarning(ByVal Text As String)
    Me.InvokeIfRequired(Sub() tb_warnings.AppendText(Text & Environment.NewLine))
End Sub

这是完整的代码，它对我来说可以正常工作：

Private dl_user As String = "someusername"
Private dl_pass As String = "somepassword"

Private dl_urls As String() = {"URL1", "URL2"} 'Temporary. Use your own code.
Private total_dl_size As Long = 0
Private total_dl As Long = 0

Dim WithEvents wc As New System.Net.WebClient()
Dim downloads As New Queue(Of String)

Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
    'Populate the download queue.
    downloads.Enqueue(dl_urls(0)) 'Temporary. Use your own code here.
    downloads.Enqueue(dl_urls(1))
End Sub

'The download button.
Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
    'Do your pre-download stuff here.

    DownloadFile(downloads.Dequeue(), dl_user, dl_pass) 'Download the first file.
End Sub

''' <summary>
''' Downloads a file from the specified URL with the specified credentials.
''' </summary>
''' <param name="URL">The URL of the file.</param>
''' <param name="Username">The username which to login with.</param>
''' <param name="Password">The password which to login with.</param>
''' <remarks></remarks>
Private Sub DownloadFile(ByVal URL As String, ByVal Username As String, ByVal Password As String)
    If wc.IsBusy = True Then Throw New Exception("A download is already ongoing!")

    wc.Credentials = New NetworkCredential(dl_user, dl_pass) 'Set the credentials.
    total_dl_size = GetDownloadSize(URL, Username, Password) 'Get the size of the current file.

    Try
        Dim FileName As String = Path.GetFileName(URL) 'Get the current file's name.
        AppendWarning("Downloading " & FileName & "...") 'Download notice.
        wc.DownloadFileAsync(New Uri(URL), Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.Desktop), FileName)) 'Download the file to the desktop (use your own path here).
    Catch ex As Exception
        AppendWarning("-ERR: Could not download file: " & Path.GetFileName(URL))
    End Try
End Sub

''' <summary>
''' (Thread-safe) Appends a warning or status message to the "tb_warnings" text box.
''' </summary>
''' <param name="Text">The text to append.</param>
''' <remarks></remarks>
Private Sub AppendWarning(ByVal Text As String)
    Me.InvokeIfRequired(Sub() tb_warnings.AppendText(Text & Environment.NewLine))
End Sub

Private Sub wc_DownloadProgressChanged(sender As Object, e As System.Net.DownloadProgressChangedEventArgs) Handles wc.DownloadProgressChanged
    Me.InvokeIfRequired(Sub()
                            Dim Progress As Integer = CType(Math.Round((e.BytesReceived * 100) / total_dl_size), Integer)
                            If Progress > 100 Then Progress = 100
                            If Progress < 0 Then Progress = 0
                            ProgressBar1.Value = Progress
                        End Sub)
End Sub

Private Sub wc_DownloadFileCompleted(sender As Object, e As System.ComponentModel.AsyncCompletedEventArgs) Handles wc.DownloadFileCompleted
    If e.Cancelled Then
        MessageBox.Show(e.Cancelled)

    ElseIf Not e.Error Is Nothing Then
        MessageBox.Show(e.Error.Message)

    Else
        If downloads.Count > 0 Then
            DownloadFile(downloads.Dequeue(), dl_user, dl_pass) 'Download the next file.
        Else
            AppendWarning("Download complete!")
        End If

    End If
End Sub

需要记住的其他一些事项：

• MsgBox() function 的存在纯粹是为了向后兼容。您应该改用 .NET 的标准 MessageBox.Show() method。

• 字符串连接应使用与号 (&) 而不是加号 (+)。 See why。

• 应始终使用Path.Combine() 连接路径，因为这将确保创建正确的路径。如果您输入任何无效的内容，它将引发异常。

  用法：

  Path.Combine(Path1, Path2, Path3, ...)
  Path.Combine("C:\", "Foo") 'Results in: C:\Foo
  Path.Combine("C:\", "Foo", "Bar", "Hello World.txt") 'Results in: C:\Foo\Bar\Hello World.txt
  

希望这会有所帮助！