3、GRASP原则三: 低耦合 Low Coupling
How to support low dependency, low change impact and increased reuse?
如何保证设计方案支持低的依赖性、低的变化影响度、增加可重用性？
3.2 耦合的定义Coupling
耦合: 一个元素与其它元素的联接、感知以及依赖程度的度量
Measure of how strongly one element is: connected to、 has knowledge of、relies on another element 比较
Cohesion，内聚: 模块内的操作之间联系紧密的程度
Coupling，耦合: 两个子模块之间联系的强度
高耦合带来的问题 Problems with High Coupling
“牵一发动全身”Forced local changes because of changes in related class
A依赖B，A与B之间有耦合，一旦B变化了，A就会受影响
元素孤立是无法理解 Harder to understand in isolation
元素很难重用 Harder to reuse — drags in more classes
3.3 GRASP rule3： Low Coupling
Name: Low Coupling
Problem:
How to support low dependency, low change impact and increased reuse?
Solution:
Assign responsibility so coupling remains low.
Use this principle to evaluate alternatives
All other things being equal prefer the low coupling solution
Note: Information Expert encourages Low Coupling
Why most secret service (spy) is one-way contact? 为什么特 务工作都是单线联系？
3.4 Discuss： Low Coupling
低耦合是所有设计决策时自然而然要考虑的原则
Low Coupling is a principle to keep in mind during all design decisions; it is an underlying goal to continually consider
低耦合是一种评估原则，是设计师用来对设计方案进行评价的一种指标
It is an evaluative principle that a designer applies while evaluating all design decisions
X与Y存在耦合的情况，例如
X has an attribute that refers to Y
X calls on services of Y
X has a method that references Y (parameter, local variable, or return value)
X is a direct or indirect subclass of Y
Y is an interface, and X implements that interface
低耦合支持类的设计相对独立，减少了变化带来的相互影响
Low Coupling supports the design of classes that are more independent, which reduces the impact of change
低耦合与其他的原则，如信息专家、高内聚必须综合考虑
不能单独考虑低耦合
“是药，三分毒”
继承关系中，子类与父类的耦合非常紧密 A subclass is strongly coupled to its superclass.
所以， 能用组合的地方不要用继承
There is no absolute measure of when coupling is too high
低耦合不具备可操作性！类似“只可意会，不可言传”
极端情况，类之间没有耦合
不希望这种情况出现
because a central metaphor of object technology is a system of connected objects that communicate via messages
Too little coupling means we aren’t a “collaborating community of objects”
这样会形成很差的设计
产生少量的不内聚、臃肿、行为复杂的对象，承担了全部的工作，这些类 可能独立工作，成为
一 个个简单的数据仓库
类之间存在适度的耦合是正常的、必须的
这样才能产生面向对象系统，其任务是由相互连接的对象通过协作来完成
to create an object-oriented system in which tasks are fulfilled by a collaboration
between connected objects
3.4 Discuss：Low Coupling— When Not To
“背靠大树好乘凉”
因为大树不会倒、不常变、稳定
High coupling to stable elements and to pervasive elements is seldom a problem
e.g. language libraries For example, a Java J2EE application can safely couple itself to the J
ava libraries (java.util, and so on) because they are stable and widespread