虽然Form有Activate与DeActivate的Events，但是这两个Events只对form的Focus是在
同一个Process不同Form之间的切换有效，如果我们在Form1，而Click其他的Process，
则Form1并不会产生DeActivate的Events，相同的，由其他的Process 回到Form1时，也
不会产生Activate的Events。唯一能得知的便是透过WM_ACTIVATE，其LowWord of wParam
有以下三个值：
WA_ACTIVE Activated by some method other than a mouse click
WA_CLICKACTIVE Activated by a mouse click.
WA_INACTIVE Deactivated
透过Subclassing的技巧便可来解决这个问题
'以下在.bas
Option Explicit
Declare Function SetWindowLong Lib "user32" Alias "SetWindowLongA" _
(ByVal hwnd As Long, ByVal nIndex As Long, ByVal dwNewLong As Long) As Long
Declare Function GetWindowLong Lib "user32" Alias "GetWindowLongA" _
(ByVal hwnd As Long, ByVal nIndex As Long) As Long
Declare Function CallWindowProc Lib "user32" Alias "CallWindowProcA" _
(ByVal lpPrevWndFunc As Long, ByVal hwnd As Long, ByVal Msg As Long, _
ByVal wParam As Long, ByVal lParam As Long) As Long
Public Const GWL_WNDPROC = (-4)
Public Const WM_ACTIVATE = &H6
Public Const WA_ACTIVE = 1
Public Const WA_CLICKACTIVE = 2
Public Const WA_INACTIVE = 0
Public preWinProc As Long
Public Function wndproc(ByVal hwnd As Long, ByVal Msg As Long, _
ByVal wParam As Long, ByVal lParam As Long) As Long
Dim fActive As Integer
If Msg = WM_ACTIVATE Then
'取得wParam的LowWord
fActive = CInt(wParam And &HFFFF)
If fActive = WA_INACTIVE Then
Debug.Print "InActive "
Else
Debug.Print "Active"
End If
End If
'将之送往原来的Window Procedure
wndproc = CallWindowProc(preWinProc, hwnd, Msg, wParam, lParam)
End Function
'以下在form
Option Explicit
Private Sub Form_Activate()
Debug.Print "Event Activate"
End Sub
Private Sub Form_Deactivate()
Debug.Print "Event DeActivate"
End Sub
Private Sub Form_Load()
Dim ret As Long
'记录原本的Window Procedure的位址
preWinProc = GetWindowLong(Me.hwnd, GWL_WNDPROC)
'设定Combo1的window Procedure到wndproc
ret = SetWindowLong(Me.hwnd, GWL_WNDPROC, AddressOf wndproc)
End Sub
Private Sub Form_Unload(Cancel As Integer)
Dim ret As Long
'取消Message的截取，而使之又只送往原来的Window Procedure
ret = SetWindowLong(Me.hwnd, GWL_WNDPROC, preWinProc)
End Sub