UML sequence diagram [easy to understand]

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Concept

Sequence diagram （Sequence Diagram） Describes the time sequence in which messages are delivered between objects , Used to express the order of behavior in use cases , It's an interaction diagram that emphasizes the chronological order of messages . in other words , Sequence diagrams describe classes and messages exchanged between classes to complete the desired behavior .

Content

The sequence diagram includes 4 Elements , They are objects （Object）、 Lifeline （Lifeline）、 Activate （Activation） And news （Message）.

1. object （Object） The object represents the role of the object in the sequence diagram in the interaction . The symbols of objects in the sequence diagram are the same as those used in the object diagram , Both use rectangles to contain object names , And the object name is underlined , As shown in the figure below ：

Placing the object at the top of the sequence diagram means that the object already exists at the beginning of the interaction , If the position of the object is not at the top , Then the representation object is created in the process of interaction （ Objects can be created at the beginning of an interaction , It can also be created during interaction ）.

1. Lifeline （Lifeline） The lifeline is a vertical dotted line , Indicates the existence of objects in the sequence diagram for a period of time . The bottom center of each object has a lifeline . The lifeline is a timeline , From the top of the sequence diagram to the bottom , The time taken depends on the duration of the interaction . The combination of object and lifeline is called the lifeline of object ; The lifeline of an object contains a rectangular object icon and the lifeline below the icon , As shown in the figure below ：

1. Activate （Activation） Sequence diagrams can describe the activation of objects （Activation） And deactivation （Deactivation）. Activation represents the period during which the object in the sequence diagram performs an operation . Activation means that the object is occupied to complete a task , Deactivation means that the object is idle , Waiting for news . stay UML in , To indicate that the object is active , You can widen the lifeline of an object into a rectangle , As shown in the figure below ：

1. news （Message） Messages are classes that define the exchange of information in interaction and collaboration , It is used to model the communication content between entities . Messages are used to pass information between entities , Allow entities to request other services , Class roles communicate by sending and receiving messages . It can trigger an operation 、 Evoke a signal or cause the creation or revocation of the target object . Message sequences can be represented by two graphs ： Sequence diagrams and collaboration diagrams （ Sequence diagrams emphasize the chronological order of messages , The collaboration diagram emphasizes the relationship between the objects exchanging messages ）. stay UML in , Messages are represented by arrows , The type of arrow indicates the type of message , The message arrow refers to the receiving party , As shown in the figure below ：

Time series diagram modeling strategy

（1） Set the context of the interaction , These contexts can be systems 、 Subsystem 、 operation 、 class 、 A script for use cases and collaboration ; （2） By identifying the role of the object in the interaction , According to the importance of the object , Place it in the sequence diagram from left to right ; （3） Set the lifeline of each object . In general , Objects exist in the whole process of interaction , But it can also be created and undone during interaction ; （4） Start with the information that triggers an interaction , Draw the following messages from top to bottom between lifelines ; （5） Set the activation period of the object , This can visualize the point in time when the actual calculation occurs 、 Visualize the nesting of messages ; （6） If you need to set time or space constraints , You can attach appropriate time and space constraints to each message ; （7） Attach pre or post conditions to each message of a control flow , This can explain this control flow in more detail .

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