Windows提供的鼠标移出消息有时候很有用，但是VB6中没有把这个事件封装给我们。
但是我们仍然可以使用子类化技术实现他，下面的代码就是一个简单的例子来处理Windows的
WM_MOUSELEAVE消息的，我演示的是鼠标移出一个Button时的情形。

1.加入一个模块，专门用来处理子类函数：

Option Explicit

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'Copyright 2002 40Star, All Rights Reserved.
'
'E-Mail      ：40Star@163.com
'Distribution：你可以完全自由随便的使用这段代码，不管你用于任何目的
'              程序在于交流和学习
'              如有任何BUG请和我联系
'
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Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" _
    (ByVal hwnd As Long, ByVal wMsg As Long, _
    ByVal wParam As Long, lParam As String) As Long

Private Declare Function GetWindowLong Lib "user32" Alias _
    "GetWindowLongA" (ByVal hwnd As Long, _
    ByVal nIndex As Long) As Long

Private Declare Function SetWindowLong Lib "user32" Alias _
        "SetWindowLongA" (ByVal hwnd As Long, ByVal nIndex _
        As Long, ByVal dwNewLong As Long) As Long
       
Private 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
 
Const GWL_WNDPROC = (-4&)

Dim PrevWndProc&

Private Const WM_DESTROY = &H2

Public Declare Function TrackMouseEvent Lib "user32" (lpEventTrack As TRACKMOUSEEVENTTYPE) As Long

Public Const TME_CANCEL = &H80000000
Public Const TME_HOVER = &H1&
Public Const TME_LEAVE = &H2&
Public Const TME_NONCLIENT = &H10&
Public Const TME_QUERY = &H40000000

Private Const WM_MOUSELEAVE = &H2A3&

Public Type TRACKMOUSEEVENTTYPE
    cbSize As Long
    dwFlags As Long
    hwndTrack As Long
    dwHoverTime As Long
End Type

Public bTracking As Boolean
Dim evtTrack As TRACKMOUSEEVENTTYPE
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Private Function SubWndProc(ByVal hwnd As Long, ByVal Msg As Long, _
                            ByVal wParam As Long, ByVal lParam As Long) _
                            As Long

   If Msg = WM_DESTROY Then Terminate (hwnd)

   '处理鼠标移出消息
   If Msg = WM_MOUSELEAVE Then
      bTracking = False
      Form1.Print "The mouse left the form!"
   End If
   SubWndProc = CallWindowProc(PrevWndProc, hwnd, Msg, wParam, lParam)
End Function

Public Sub Init(hwnd As Long)
  PrevWndProc = SetWindowLong(hwnd, GWL_WNDPROC, AddressOf SubWndProc)
End Sub

Public Sub Terminate(hwnd As Long)
  Call SetWindowLong(hwnd, GWL_WNDPROC, PrevWndProc)
End Sub

' -- 模块结束 -- '

2. 窗体中处理需要加入的代码：

Option Explicit

Private Sub Command1_MouseMove(Button As Integer, Shift As Integer, X As Single, Y As Single)
If bTracking = False Then
   bTracking = True
    Dim ET As TRACKMOUSEEVENTTYPE
    'initialize structure
    ET.cbSize = Len(ET)
    ET.hwndTrack = Command1.hwnd
    ET.dwFlags = TME_LEAVE
    'start the tracking
    TrackMouseEvent ET
End If
End Sub

Private Sub Form_Load()
Call Init(Command1.hwnd)
End Sub

Private Sub Form_Unload(Cancel As Integer)
Call Terminate(Command1.hwnd)
End Sub

此例程在Win2000 + VB6中调试通过