撤消功能的實現

方法：利用VB的API函數SendMessage來實現．

函數用法：

　　聲明：Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, lParam As Any) As Long

　　說明：調用一個窗口的窗口函數，將一條消息發給那個窗口。除非消息處理完畢，否則該函數不會返回。SendMessageBynum， SendMessageByString是該函數的“類型安全”聲明形式
　　返回值：Long，由具體的消息決定
　　參數表：hwnd -----------  Long，要接收消息的那個窗口的句柄
　　　　　　wMsg -----------  Long，消息的標識符
　　　　　　wParam ---------  Long，具體取決於消息
　　　　　　lParam ---------  Any，具體取決於消息

例如：
　　　
'　聲明API函數
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, lParam As Any) As Long

'　調用API函數
Private Sub MnuChe_Click()
Dim ls
ls = SendMessage(Txt.hwnd, &H304, 0, 0)
End Sub