4、操作契约 Operation Contracts
“用例描述”的补充
强调: 用例中重要的动作，其开始与结束是需要一些约束
4.5 操作契约的后置条件
定义Definition
后置条件描述了领域对象状态的变化 describe changes in the state of objects in the domain model
状态变化包括
是否创建、删除了对象？instances created / deleted
对象间的关系是否发生变化？associations formed or broken
有对象的属性变化了吗？and attributes changed
为什么要后置条件？ Why post-condition
也不是始终都需要的 they aren't always necessary
如果开发人员很容易明白该如何去做，就不用写操作契约了
If developers can comfortably understand what to do without them, then avoid writing contracts 操作契约比较细节地、精确地定义了一个操作应该承担的职责
契约，是面向对象分析过程中非常有用的工具
把问题说清楚、无歧义
先不考虑设计，集中于分析 what must happen 而不是how it is to be accomplished
4.6 创建后置条件
建议 Apply the following advice to create contracts
Identify system operations from the SSDs .
For system operations that are complex and perhaps subtle in their results,
or which are not clear in the use case, build a contract
To describe the postconditions, use the following categories:
instance creation and deletion
attribute modification
associations formed and broken
常见错误
描述契约
(better) A SalesLineItem was created
(worse) A SalesLineItem is created; or Create a SalesLineItem
忘记对象之间的关系发生了变化
The SalesLineItem was associated with the Sale (association formed)
4.7 后置条件 & 领域模型
后置条件体现在领域模型的对象 postconditions are expressed in the context of the Domain Model objects What instances can be created?
those from the Domain Model
What associations can be formed?
those in the Domain Model
and so on
在定义契约的过程中，经常触发对领域模型的修改
记录新的概念类、属性 ？
领域模型是否有新的关系 ？
4.8 对领域模型的修改