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