Windows应用程序是基于消息驱动的，任何线程只要注册窗口类都会有一个消息队列用于接收用户输入的消息和系统消息。为了拦截消息，Windows提出了钩子的概念。钩子（Hook）是Windows消息处理机制中的一个监视点，钩子提供一个回调函数。当在某个程序中安装钩子后，它将监视该程序的消息，在指定消息还没到达窗口之前钩子程序先捕获这个消息。这样就有机会对此消息进行过滤，或者对Windows消息实现监控。-Windows application is based on the message-driven, any thread as long as the registered window class will have a message queue for receiving user input and system messages. In order to intercept the message, Windows made ​ ​ a hook concept. Hook (Hook) is a Windows message handling mechanism in a watch point, hook a callback function. When a program is installed in the hook, it will monitor the program s message, the message has not reached the specified window procedure first before capturing the news hook. This will have the opportunity to filter this information, or to achieve control of Windows messages.