WebClient 异步下载器无法正常工作
WebClient asynchronous downloader not working properly
所以，我已经为此苦苦思索了一段时间。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：
我的朋友和我经常玩 MineCraft，但他们不是很聪明，而且我们并不总是四处寻找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 mod，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，但我认为这不是这里的问题，原因会变得很清楚。
基本上，我想使用进度条，最好也使用标签来指示整个数据块的进度（所有 mod 加在一起……不超过 1GB - 小很多）。但是，我似乎 运行 关注有关异步选项的一些问题：
它会随机选择不下载它应该下载的文件
在声称完整之前可能没有完整下载文件
当消息框触发说它已完成所有项目的下载时，进度条可能已满 50%。
但是，由于在 Webclient 的同步使用中不存在进度报告事件，因此进度条无法正常工作，但当我 运行 BGworker 中的 syncro 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：
- 有没有更好的实现方式？
这是在它准备好之前我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：更新代码：
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
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，假设结果为 0.34，然后它将其转换为整数，结果为 0（0.34 向下舍入为零，因为整数没有小数），最后将 0 乘以 100（结果仍然是 0）。
你要做的是先将被除数乘以 100，这样结果将从 0-100 而不是 0- 1: (total_dl * 100) / total_dl_size.
至于thread-safety（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 年引入）一起，将大大 简化您的调用。这是因为没有将 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)
扩展方法将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。
我将你的代码分开了一些，这样更容易处理。对于初学者，我创建了一个更通用的方法，而不是 copy-pasting 下载代码到 DownloadFileCompleted 事件处理程序，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
如您所见，我还制作了一个用于输出警告和错误消息的通用方法：
''' <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，它适合我：
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
其他一些注意事项：
用MsgBox() function exists purely for backwards compatibility. You should use .NET's standard MessageBox.Show() method代替。
字符串连接应该使用和号 (&) 而不是加号 (+)。 See why.
连接路径应始终使用 Path.Combine() 完成，因为它将确保创建正确的路径。如果您输入任何无效内容，它会抛出异常。
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
希望对您有所帮助！
所以，我已经为此苦苦思索了一段时间。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：
我的朋友和我经常玩 MineCraft，但他们不是很聪明，而且我们并不总是四处寻找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 mod，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，但我认为这不是这里的问题，原因会变得很清楚。
基本上，我想使用进度条，最好也使用标签来指示整个数据块的进度（所有 mod 加在一起……不超过 1GB - 小很多）。但是，我似乎 运行 关注有关异步选项的一些问题：
它会随机选择不下载它应该下载的文件
在声称完整之前可能没有完整下载文件
当消息框触发说它已完成所有项目的下载时，进度条可能已满 50%。
但是，由于在 Webclient 的同步使用中不存在进度报告事件，因此进度条无法正常工作，但当我 运行 BGworker 中的 syncro 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：
- 有没有更好的实现方式？
这是在它准备好之前我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：更新代码：
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
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，假设结果为 0.34，然后它将其转换为整数，结果为 0（0.34 向下舍入为零，因为整数没有小数），最后将 0 乘以 100（结果仍然是 0）。
你要做的是先将被除数乘以 100，这样结果将从 0-100 而不是 0- 1: (total_dl * 100) / total_dl_size.
至于thread-safety（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 年引入）一起，将大大 简化您的调用。这是因为没有将 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)
扩展方法将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。
我将你的代码分开了一些，这样更容易处理。对于初学者，我创建了一个更通用的方法，而不是 copy-pasting 下载代码到 DownloadFileCompleted 事件处理程序，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
如您所见，我还制作了一个用于输出警告和错误消息的通用方法：
''' <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，它适合我：
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
其他一些注意事项：
用
MsgBox()function exists purely for backwards compatibility. You should use .NET's standardMessageBox.Show()method代替。字符串连接应该使用和号 (
&) 而不是加号 (+)。 See why.连接路径应始终使用
Path.Combine()完成，因为它将确保创建正确的路径。如果您输入任何无效内容，它会抛出异常。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
希望对您有所帮助！