UML study Note 2

Use case diagram)

Basic elements:
Actors: actors are translated into participants and roles, which seem to be more suitable and easier to understand. Participants in Microsoft Chinese VISIO.
Relationship)
Process)
Package)
System Boundary)

Links are used to indicate the elements related to actors. Actors may be related to other actors or use cases. Actors are related to use cases to indicate that they receive information from the use case or lose the input or information to the use case.
The use case diagram includes three types of relationships:
Communicates)
Uses (usage relationship)
Extends)

The communicates relationship indicates that the actors receive information from the use case or provide input or information to the use case. More likely, it indicates the interaction between actors and use cases, that is, the tasks contained in the execution cases of actors. Communicates are often called associations ).

The uses relationship indicates that the use case uses the functions provided by another use case.

The extends relationship indicates that one use case can use the function of another use case.

Difference between uses and extends:
Extends indicates an optional decision.
Uses indicates a required decision.

Sequence digraphs)
A sequence chart is a graphical method for recording scenes (scenarios. Each time you want to record a series of actions, you can build a sequence chart.
Sequence Chart elements:
Object (Objects)
Activations)
Message)
Notes)

Objects are persistent elements in the solution. They may be tables, physical forms, or classes.
Activation is the execution of the process, including the time it waits for the execution of the nested process.
A message is an action executed between objects. A message indicates the communication between two objects and represents an action.
Message: connects to the connection points of two different object lifeline. This type of message indicates an action, which is related to the method or operation of the receiving object. The object receiving the message executes the action on the request. A message can contain parameters that are passed to the method to be executed.
Message (CALL): It seems to be the same as the message.
Message (return): the message that returns information to the object.
Message (asynchronous): The sending object sends the message to the receiving object and does not wait for the response of the receiving object.
(This paragraph is hard to understand and not as good as this one:
Simple Message indicates a Simple control flow. It is used to describe how to pass between objects without considering the communication details.
Synchronous Message indicates the nested control flow.
An operation call is a typical one-step message.
After a message is sent, the caller must wait for the message to return. The caller can continue to perform his/her own operations only after the message processing operation is completed.
Asynchronous Message indicates the Asynchronous control flow.
After the caller sends a message, the caller can continue to perform his/her operations without waiting for the message to be returned.
Asynchronous messages are mainly used to describe concurrent behaviors in real-time systems.
Simple messages and synchronous messages are always unclear.
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