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Tip of the Day
Language: VB5,VB6
Expertise: Intermediate
Jan 1, 2000

Determine when a TextBox control is scrolled

When the user scrolls the contents of a TextBox control - either by clicking on the companion scrollbars or by typing new characters in the edit area - the TextBox control sends its parent form a WM_COMMAND message, and passes its own hWnd property in lParam and the value EN_HSCROLL or EN_VSCROLL in the high word of wParam.

Therefore, to determine when the contents of a TextBox control, you can subclass its parent form, using the following code (based on the MsgHook DLL component):


' REQUIRES THE MSGHOOK.DLL COMPONENT
'
' you can omit the following constant definitions, 
' because they are contained in the MsgHook type library
Const WM_COMMAND = &H111
Const EN_HSCROLL = &H601
Const EN_VSCROLL = &H602

Dim WithEvents FormHook As MsgHook

Private Sub Form_Load()
    ' start form subclassing 
    Set FormHook = New MsgHook
    FormHook.StartSubclass hWnd
End Sub

Private Sub FormHook_AfterMessage(ByVal uMsg As Long, ByVal wParam As Long, _
    ByVal lParam As Long, retValue As Long)
    If uMsg = WM_COMMAND Then
        ' The user selects a command item from a menu,
        ' or a control sends a notification message to its parent window,
        ' or an accelerator keystroke is translated.
        '
        ' If this is a notification from a control, lParam holds its handle.
        If lParam = Text1.hWnd Then
            ' In this case, the notification message is in the
            ' high-order word of wParam.
            ' NOTE: this msg doesn't arrive if the scroll bar indicator is 
            ' scrolled.
            Select Case (wParam \ &H10000)
                Case EN_HSCROLL
                    ' The TextBox control has been scrolled horizontally
                Case EN_VSCROLL
                    ' The TextBox control has been scrolled vertically
            End Select
        End If
    End If
End Sub

Note that the WM_COMMAND message is sent only when the TextBox scrolls because new chars have been entered, or because the user clicked on the scrollbar's arrow buttons. No messsage is sent when the scrollbar's thumb indicator is moved. Finally, notice that you can use the same code to determine scrolling activity for all the TextBox controls on the form, simply by comparing the value in lParam with the handle of all the TextBox control you want to monitor.
Francesco Balena
 
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