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    VB6, XHTML & CSS hobbyist Merri's Avatar
    Join Date
    Oct 2002

    VB6 Download multiple files at once: no API, no extra dependency

    You can actually download files in VB without extra dependancy to anything. You don't even need a single API call. The only requirement is a user control.

    Below is a simple sample that has the ability to download multiple files at once, and your application remains responsive as well. It keeps track of the given keys (or "PropertyNames") so you won't run into an error condition if you try to download a file while it is already being downloaded. The control will cancel the existing download when that same file is being tried to download again.
    ' Downloader.ctl
    Option Explicit
    Public Event Complete(ByRef URL As String, ByRef Data As String, ByRef Key As String)
    Public Event Progress(ByRef URL As String, ByRef Key As String, ByVal BytesDone As Long, ByVal BytesTotal As Long, ByVal Status As AsyncStatusCodeConstants)
    Public Enum DownloaderCache
        [Always download] = vbAsyncReadForceUpdate
        [Get cache copy only] = vbAsyncReadOfflineOperation
        [Update cached copy only] = vbAsyncReadResynchronize
        [Use cache if no connection] = vbAsyncReadGetFromCacheIfNetFail
    End Enum
    Private m_Keys As String
    Private Function Private_AddKey(ByRef Key As String) As Boolean
        ' see if we do not have the key
        Private_AddKey = InStr(m_Keys, vbNullChar & Key & vbNullChar) = 0
        ' we can add it
        If Private_AddKey Then
            m_Keys = m_Keys & Key & vbNullChar
        End If
    End Function
    Private Sub Private_RemoveKey(ByRef Key As String)
        ' remove the key
        m_Keys = Replace(m_Keys, vbNullChar & Key & vbNullChar, vbNullChar)
    End Sub
    Public Sub Start(ByRef URL As String, Optional ByVal CacheMode As DownloaderCache = [Always download], Optional ByVal Key As String)
        ' use URL as key if no key is given
        If LenB(Key) = 0 Then Key = URL
        ' do we already have this key?
        If Not Private_AddKey(Key) Then
            ' cancel the old one
            CancelAsyncRead Key
        End If
        ' begin download process
        AsyncRead URL, vbAsyncTypeByteArray, Key, CacheMode
    End Sub
    Private Sub UserControl_AsyncReadComplete(AsyncProp As AsyncProperty)
        Dim strData As String
        ' get Variant byte array to byte string (needs StrConv to Unicode for displaying in a textbox)
        If AsyncProp.BytesRead Then strData = AsyncProp.Value Else strData = vbNullString
        ' redirect information
        RaiseEvent Complete(AsyncProp.Target, strData, AsyncProp.PropertyName)
        ' remove the key
        Private_RemoveKey AsyncProp.PropertyName
    End Sub
    Private Sub UserControl_AsyncReadProgress(AsyncProp As AsyncProperty)
        With AsyncProp
            ' redirect event information
            If LenB(.PropertyName) Then
                RaiseEvent Progress(.Target, .PropertyName, .BytesRead, .BytesMax, .StatusCode)
                RaiseEvent Progress(.Target, vbNullString, .BytesRead, .BytesMax, .StatusCode)
            End If
        End With
    End Sub
    Private Sub UserControl_Initialize()
        m_Keys = vbNullChar
    End Sub

    A sample application: you need two command buttons, one multiline textbox and a listbox. You need the user control as well.
    Option Explicit
    Private Sub Command1_Click()
        Downloader1.Start ""
    End Sub
    Private Sub Command2_Click()
        Downloader1.Start ""
    End Sub
    Private Sub Downloader1_Complete(URL As String, Data As String, Key As String)
        Me.Caption = URL
        Text1.Text = StrConv(Data, vbUnicode)
    End Sub
    Private Sub Downloader1_Progress(URL As String, Key As String, ByVal BytesDone As Long, ByVal BytesTotal As Long, ByVal Status As AsyncStatusCodeConstants)
        List1.AddItem URL & " @ " & (BytesDone \ 1024) & " kB, status code: " & Status, 0
    End Sub
    You can also download images directly as a Picture object. To see other possible values, open up Object Browser with F2 and type Async into the search field.

    Possible issues
    Showing a message box seems to entirely prevent events from triggering under the IDE. I haven't tested compiled, but I believe it works just fine there: timer events don't run in the IDE when a messagebox is shown, but compiled it works just fine.

    2008-09-14: changed Data to string from byte array and made a check for if any data was received to avoid an error.
    Last edited by Merri; Sep 14th, 2008 at 11:00 AM.

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