control subclassing switchboard
The Switchboard:A method for handling subclassing in ActiveX controls f you develop ActiveX controls and intend to subclass or hook a window, you'll very quickly discover a problem when you attempt to site multiple instances of your control. The subclassing, which worked fine with a single instance of your control, now no longer works and is, in fact, most likely is causing a GPF. Why is this happening? The AddressOf operator requires you to place the callback routine in a module. This module is shared between all instances of your control and the variables and subroutines that the module provide are not unique to each instance. The easiest way to visualize the problem is to imagine a shared phoneline (or a partyline as we hicks call it) where multiple parties are trying to dial a number, talk, and hangup, all at the same time. What's needed is an operator, a routine that controls the dialing (hooking), the talking (the callback routine), and who routes information to the instance of the control that requested it. The Switchboard subroutine (see below) and it's supporting code provides a method for subclassing from multiple instances of your ActiveX control. It is not memory intensive, nor is it slow. It's biggest weakness is that it is hardcoded to intercept particular messages (in this case, WM_SIZE, to trap resize events) and will require some minor modification on your part to use.
ملخص الذكاء الاصطناعي: This codebase represents a historical implementation of the logic described in the metadata. Our preservation engine analyzes the structure to provide context for modern developers.
' Place this code in the General Declarations area
Dim m_MyInstance as Integer
' Place this block of code in the user control's
' INITIALIZE event
Dim Instance_Scan As Integer
For Instance_Scan = MIN_INSTANCES To MAX_INSTANCES
If Instances(Instance_Scan).in_use = False Then
m_MyInstance = Instance_Scan
Instances(Instance_Scan).in_use = True
Instances(Instance_Scan).ClassAddr = ObjPtr(Me)
Exit For
End If
Next Instance_Scan
' Note the Friend keyword.
' If you plan on modifying wMsg, pass it ByRef...
Friend Sub ParentResized(ByVal wMsg As Long)
Static ParentWidth As Long
Static ParentHeight As Long
If wMsg = WM_CLOSE Then UnhookParent
If ParentWidth <> Usercontrol.Parent.Width Or _
ParentHeight <> Usercontrol.Parent.Height Then
Debug.Print m_MyInstance & ": Resize event"
End If
ParentWidth = TrueParentWidth
ParentHeight = TrueParentHeight
End Sub
Public Function SwitchBoard(ByVal hwnd As Long, ByVal MSG As Long, _
ByVal wParam As Long, ByVal lParam As Long) As Long
Dim instance_check As Integer
Dim cMyUC As MyUC
Dim PrevWndProc As Long
'Do this early as we may unhook
PrevWndProc = Is_Hooked(hwnd)
If MSG = WM_SIZE Or MSG = WM_CLOSE Then
For instance_check = MIN_INSTANCES To MAX_INSTANCES
If Instances(instance_check).hwnd = hwnd Then
On Error Resume Next
CopyMemory cMyUC, Instances(instance_check).ClassAddr, 4
cMyUC.ParentResized MSG
CopyMemory cMyUC, 0&, 4
End If
Next instance_check
End If
SwitchBoard = CallWindowProc(PrevWndProc, hwnd, MSG, wParam, lParam)
End Function
'Hooks a window or acts as if it does if the window is
'already hooked by a previous instance of myUC.
Public Sub Hook_Window(ByVal hwnd As Long, ByVal instance_ndx As Integer)
Instances(instance_ndx).PrevWndProc = Is_Hooked(hwnd)
If Instances(instance_ndx).PrevWndProc = 0& Then
Instances(instance_ndx).PrevWndProc = SetWindowLong(hwnd, _
GWL_WNDPROC, AddressOf SwitchBoard)
End If
Instances(instance_ndx).hwnd = hwnd
End Sub
' Unhooks only if no other instances need the hWnd
Public Sub UnHookWindow(ByVal instance_ndx As Integer)
If TimesHooked(Instances(instance_ndx).hwnd) = 1 Then
SetWindowLong Instances(instance_ndx).hwnd, GWL_WNDPROC, _
Instances(instance_ndx).PrevWndProc
End If
Instances(instance_ndx).hwnd = 0&
End Sub
'Determine if we have already hooked a window,
'and returns the PrevWndProc if true, 0& if false
Private Function Is_Hooked(ByVal hwnd As Long) As Long
Dim ndx As Integer
Is_Hooked = 0&
For ndx = MIN_INSTANCES To MAX_INSTANCES
If Instances(ndx).hwnd = hwnd Then
Is_Hooked = Instances(ndx).PrevWndProc
Exit For
End If
Next ndx
End Function
'Returns a count of the number of times a given
'window has been hooked by instances of myUC.
Private Function TimesHooked(ByVal hwnd As Long) As Long
Dim ndx As Integer
Dim cnt As Integer
For ndx = MIN_INSTANCES To MAX_INSTANCES
If Instances(ndx).hwnd = hwnd Then
cnt = cnt + 1
End If
Next ndx
TimesHooked = cnt
End Function