GRASP原则六: 多态 Polymorphism
How to handle alternative behaviors based on type 如何处理依据类型不同而有 不同行为的一类需求 ？
比如，开餐馆
苏州人喜欢甜、四川人喜欢麻、湖南人喜欢辣，咋处理 ？
1.1 9条GRASP原则
Information Expert
responsibilities should be assigned to objects that contain relevant information
Creator
the creator of an object is usually an object that contains, or aggregates it
High Cohesion
responsibilities of a certain class must be highly related
Low Coupling
interdependency between classes should remain low
Controller
class which handles external system events
Polymorphism 多态原则
Indirection 间接原则
Pure Fabrication 纯虚构原则
Protected Variations 隔离变化
1.2 Iteration 2 More Requirement
第一次迭代结束时，完成了
当前软件功能测试：单元测试、用户可接受测试、负载测试、可使用性测试等
必须有客户加入，并给出反馈 Customers engaged in and feedback
把基线稳定下来，发布内部版本 stabilized baseline internal release
第二次迭代时，要考虑加入新的功能
需求变化、业务规则细化、考虑更多的用例
制定本次迭代的计划活动等
Ex, Monopoly game
When a player lands on the Go square, the player receives $200
When a player lands on the Go-To-Jail square, they move to the Jail square
When a player lands on the Income-Tax square, the player pays the minimum of $200 or 10%
of their worth
1.2 Iteration 2 More Requirement
例如POS系统，第二次迭代时增加另外的需求
支持多种第三方服务的接口 Support for variations in third-party external services
计算税费、信用卡授权认证等
复杂的定价机制 Complex pricing rules
可插拔的业务规则 Pluggable business rules
GUI窗口在信息发生变化时得到更新 GUI window updates when information changes
这些功能点，可能属于前轮迭代同样的用例，但更多的是讨论非功能性需求
这些需求对领域模型的影响较小
同样一项功能，原来一种处理方法就可以，现在需要适应多种处理方法，设 计方案该如何支持？
比如付费：现金、储蓄卡、信用卡、支付宝、微信
1.3 GRASP rule6: Polymorphism(多态)
Name: Polymorphism(多态)
Problem:
如何处理依据类型不同而有不同行为的一类需求？
How to handle alternative behaviors based on type?
How to create pluggable software components?
Solution:
使用多态操作为依据类型变化的行为 进行职责分配
When related alternatives or behaviors vary by type (class), assign responsibility for the
behavior using polymorphic operations to the types for which the behavior varies
Corollary(推论):
不要去测试对象的类型或者条件逻辑，并以此选择相应的行为
Do not test for the type of an object and use conditional logic to perform varying alternatives
based on type
即，不要使用条件逻辑，而是为不同的类定义相同名字的方法
That is, don’t use conditional logic, but assign the same name to services (methods) in
different classes
不同的类实现了相同的接口、或者有一个共同的父类（继承）The different classes usually
implement a common interface or are related in an implementation hierarchy with a
common superclass