C语言设计模式：命令模式 //tbd

Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

Four terms always associated with the command pattern are command, receiver, invoker and client. A concrete command object has a receiver object and invokes a method of the receiver in a way that is specific to that receiver's class. The receiver then does the work. A concrete command object is separately passed to an invoker object, which invokes the command, and optionally does bookkeeping about the command execution. Any concrete command object can be passed to the same invoker object. Both an invoker object and several concrete command objects are held by a client object. The client contains the decision making about which commands to execute at which points. To execute a command, it passes the command object to the invoker object. 

将方法（请求）定义为对象，这些对象即为命令。支持命令的排队、记录以及undo功能。

代码：TBD