其他面向对象设计原则1: 开-闭原则OCP
Open-Closed Principle (OCP)
5.1 设计变坏的前兆 Signs of Rotting Design
僵硬性 Rigidity
难以更改代码 code difficult to change
从管理角度，拒绝任何的变化成为一种制度
management reluctance(拒绝) to change anything becomes policy
易碎性 Fragility
即使是小小的改动也会导致级联性的后果的 even small changes can cause cascading effects
代码在意想不到的地方终止 code breaks in unexpected places
固定性 Immobility
代码纠缠在一起根本不可能重用 code is so tangled(纠结) that it's impossible to reuse anything
黏滞性 Viscosity
宁愿重新编写也不愿意修改 much easier to hack(乱砍) than to preserve original design
5.2 设计变坏的原因 Causes of Rotting Design
需求不停地变化 Changing Requirements
这是不可避免的 is inevitable
I. Jacobson, OOSE, 1992说，“所有的系统在其生命周期都在发生变化, 如果拟开发的系统生命期多
于一个版本的话，就必须记住这一点”
All systems change during their life-cycles. This must be borne in mind when
developing systems expected to last longer than the first version"
设计的问题：“依赖管理”失衡
导致高耦合、低内聚
5.3 开闭原则 Open-Closed Principle (OCP)
软件系统在其生命周期都在发生变化
无论是好的设计还是坏的设计，都面临着这个问题 both better designs and poor designs have to
face the changes
但好的设计是稳定的 good designs are stable
开-闭原则
软件系统应当允许功能扩展(即开放性）Software entities should be open for extension,
但是不允许修改原有的代码（即关闭性）but closed for modification (OCP)
遵循开-闭原则的模块符合下列准则
Open for Extension – 可以扩展行为以满足新的需求
CLosed for Modification – 但不允许修改模块的源代码
the source code of the module is not allowed to change
5.5 OCP的启发 Heuristics
修改公有的数据，经常冒着“打开”模块的风险
Changes to public data are always at risk to “open” the module
它们常常会引起涟漪效应，导致许多地方连锁修改
They may have a rippling effect requiring changes at many unexpected locations
很难找全出错的地方并修改，一处修改会导致多处又出问题
Errors can be difficult to completely find and fix. Fixes may cause errors elsewhere
1）定义所有的对象 - 数据为私有的 Make all object-data private
2）不要使用全局变量 No Global Variables!
5.7 小结
OCP解决软件的僵硬性和易碎性 attacks software rigidity and fragility!
When one change causes a cascade of changes
OCP 宣言
我们应当尝试设计永远不需要修改的模块
we should attempt to design modules that never need to be changed
系统行为的扩展只需要增加新的代码，不能修改已有的代码
extend the behavior of the system by adding new code. We do not modify old code
模块不允许修改已有的代码，而这种修改会影响客户端
the module is closed to modification in ways that affect clients