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

Track changes in system time and display resolution

Whenever the system time or the display resolution changes, Windows sends all the top-level windows (forms, in VB jargon) the WM_TIMECHANGE or WM_DISPLAYCHANGE message, respectively. Therefore, you can easily determine when these system-wide settings change by subclassing any form in your application, which is especially useful with the MsgHook DLL.

Using the same method you can also trap other system-wide messages, such as the WM_COMPACTING message, which Windows sends when it's short on memory:


' REQUIRES THE MSGHOOK.DLL COMPONENT

' you can omit the following constant definitions, 
' because they are defined in the MsgHook type library
Const WM_TIMECHANGE = &H1E
Const WM_DISPLAYCHANGE = &H7E
Const WM_COMPACTING = &H41

Dim WithEvents FormHook As MsgHook

Private Sub Form_Load()
    ' start form subclassing with events
    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)
    Select Case uMsg
        Case WM_DISPLAYCHANGE
            ' The screen resolution has changed.
            ' ...
            ' ... (add here your procedure to resize your forms) ...
            ' ...
        Case WM_TIMECHANGE
            ' System date/time has changed.
            ' ...
        Case WM_COMPACTING
            ' Windows is low in memory (release resources if possible).
            ' ...
            ' ... (release as many resources as possible) ...
            ' ...
    End Select
End Sub
Note that you can indirectly trap this messages also by using the SysInfo.Ocx control, which exposes them as events. Using the subclassing technique outlined here you don't need any additional component except MsgHook.Dll (which you can use for other subclassing jobs).
Francesco Balena
 
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