当用户从菜单中选择了一项，或者子控件发出一个通知消息，或者转换了一个加速键的击键事件，框架就调用这个成员函数。
The framework calls this member function when the user selects an item from a menu, when a child control sends a notification message, or when an accelerator keystroke is translated.
当用户滚动鼠标滚轮并遇到滚轮的下一个刻度时，框架就调用这个成员函数。
The framework calls this member function as a user rotates the mouse wheel and encounters the wheel's next notch.
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