[读书笔记-- Learning UML 2.0, OReilly, April 2006] Use case diagram

 

读书笔记： Learning UML 2.0, OReilly, April 2006

 

(一) Use case diagram

 

Rule of thumb to specify a Use Case:

A use case is something that provides some measurable result to the user or an external system.

 

Approaches:

1.       Capture an initial set of actors;

2.       Find cases where the system is used to complete a specific job;

 

Use case descriptions:

Table 2-1. Some types of information that you can include in your use case descriptions

Use case description detail	What the detail means and why it is useful
Related Requirements	Some indication as to which requirements this use case partially or completely fulfills.
Goal In Context	The use case's place within the system and why this use case is important.
Preconditions	What needs to happen before the use case can be executed.
Successful End Condition	What the system's condition should be if the use case executes successfully.
Failed End Condition	What the system's condition should be if the use case fails to execute successfully.
Primary Actors	The main actors that participate in the use case. Often includes the actors that trigger or directly receive information from a use case's execution.
Secondary Actors	Actors that participate but are not the main players in a use case's execution.
Trigger	The event triggered by an actor that causes the use case to execute.
Main Flow	The place to describe each of the important steps in a use case's normal execution.
Extensions	A description of any alternative steps from the ones described in the Main Flow.

Example:

Table 2-2. A complete use case description for the "Create a new Blog Account" use case

Use case name	Create a new Blog Account
Related Requirements	Requirement A.1.
Goal In Context	A new or existing author requests a new blog account from the Administrator.
Preconditions	The system is limited to recognized authors and so the author needs to have appropriate proof of identity.
Successful End Condition	A new blog account is created for the author.
Failed End Condition	The application for a new blog account is rejected.
Primary Actors	Administrator.
	Secondary Actors	Author Credentials Database.
	Trigger	The Administrator asks the CMS to create a new blog account.
Main Flow	Step	Action
	1	The Administrator asks the system to create a new blog account.
	2	The Administrator selects an account type.
	3	The Administrator enters the author's details.
	4	The author's details are verified using the Author Credentials Database.
	5	The new blog account is created.
	6	A summary of the new blog account's details are emailed to the author.
Extensions	Step	Branching Action
	4.1	The Author Credentials Database does not verify the author's details.
	4.2	The author's new blog account application is rejected.

 

2.2.       Use Case Relationships

2.2.1.     The <<include>> Relationship

2.2.2.     Special Cases

   2.2.3.    The <<extend>> Relationship

 

 

2.3  Use Case Overview Diagrams

           Use Case Overview diagrams give you an opportunity to paint a broad picture of your system's context or domain. Unfortunately, Use Case Overviews are badly named as they don't usually contain any use cases…. they are more a placeholder and starting point to for the rest of your model's detail.