3.4 面向对象分析（一）面向对象设计（初步）

一般原则

　　 An object-oriented system is composed of objects sending messages to other objects

　　 The quality of the overall design depends on which object is doing what

　　　　 比喻：“人尽其责、各有所长”

　　 That is, the quality depends on how we assign responsibilities to the objects

　　 Problem:

　　　　 Define “good quality”？
职责驱动的设计 (RDD)

　　 RDD，Responsibility Driven Design

　　　　 Pioneered by Wirfs-Brock in early 90s

　　　　 Think of objects in terms of what they do or know (the human worker metaphor!)

　　　　　　设计时考虑对象做什么、或者知道什么
　　　　 An object’s obligation or contract that it offers to other objects

　　　　　　一个对象对其他对象承担的义务或者合约

　　　　 A responsibility is really a behavior the other classes depend on

　　　　　　职责是一个对象的行为，而其他的对象依赖这种行为 
职责的定义 Responsibilities

　　 这里定义为两类Knowing、Doing
　　 认知职责 Knowing

　　　　 about private encapsulated data

　　　　 about related objects

　　　　 about things it can derive or calculate
　　 行为职责 Doing

　　　　 doing something itself

　　　　 initiating action in other objects

　　　　 controlling and coordinating activities in other objects