系统注册热键,当用户按住热键的时候，它就会产生WM_HOTKEY消息（也就是说时间到了，该执行代码了），那么我们的还要处理这个消息来为自己所用，假如用户按住热键我打算让他弹出个对话框，我们可以这样处理这个消息-Registration system hotkey, when the user press a hotkey, it will produce WM_HOTKEY message (ie time to the execution code), then we have to deal with this message for their own use, if the user holds down the hotkey I intend to pop up a dialog box to let him, we can handle the message