Use case diagram mainly used to describe the relationship between "the user needs, the system function unit", the requirement analysis phase, often by the use case diagram, function of the system from the user's perspective is described, a graphical visualization of the way as the development team and the customer exchange, while also helping form a unified language.
A use case described in FIG.
Use case diagram (Use Case Diagrame): describes how one wants to use a system, related user, the user needs the service and system provided by the system requires service user to provide a more clearly displayed so as to make the system easier for the user to understand these elements use, but also for developers to implement the final element. The reason why the use case diagram is crucial, because the user does not care about the implementation and the internal structure of the system, only care about the external characteristics of dynamic presentation of the product. The use case diagram is exactly the description of the characteristics of the external view of the software product that from a user perspective rather than to describe the demand from the developer's perspective, analysis of the product's features and dynamic behavior.
Second, the basic elements
1, participants (the Actor) , in a system external roles or external systems that interact directly with the system. By communication with customers to determine the stakeholders, and to determine the participants.
- Role: usually assume the role of a specific person, which is the most common participants.
- External system: such as OA CRM system to operating system, to facilitate sending notifications, calls for the OA system, CRM system as a participant in this external system.
- Time: As there is a timing task operation or similar operation, the time as the participant
2, example
Through the description of the customer demand (mainly functional requirements), the development team through use cases to reflect the functions and services, through the interaction of participants and use cases to achieve the same objectives with the customer development team.
3, relationship
1) generalization relationship between participants and participants
For example Tencent user, including two part micro-channel users and QQ users, but the use of service Tencent, Tencent user need only to this case, generalization can be employed, as a white arrow pointing triangular.
2) the relationship between participants and use cases
参与者与用例间是简单的关联关系，一个参与者可以有着多个用例
3）用例与用例间的泛化关系
用例之间可以存在泛化关系，比如常见的支付，可以选择微信支付、支付宝支付等等，但是这个操作就是支付。泛化关系采用三角空心箭头。
4）用例与用例间的包含关系
包含关系用来把一个较复杂用例所表示的功能分解成较小的步骤。通常可以这么理解，由基础用例向复杂用例转换的过程。但是最终被参与者直接操作的还是基础用例。包含关系的图形为虚线箭头加<<include>>，箭头指向复杂用例。
5）用例与用例间的扩展关系
扩展关系是指用例功能的延伸，相当于为基础用例提供一个附加功能。当特定条件出现时，该扩展用例的行为才会被执行。扩展关系的图形为虚线箭头加上<<<exclude>>>，箭头指向基础用例。
三、简单案例
依照常见的购买操作，设计简单的用例操作，当然，实际中的用例远不止这么简单，本次只是将以上几种关系融入进来，设计简单的案例。
至此，针对UML用例图的相关内容做了大概的总结，需求分析阶段，利用用例图，来方便与客户形成统一语言，也方便活动图的设计。
2020-01-22,望技术有成后能回来看见自己的脚步