Sequence diagram definition  : Describes the relationship between objects The time order in which messages are delivered , Used to represent... In a use case The order of behavior , It's about emphasizing Message chronology Interaction graph of ;

The things that a sequence diagram describes : The sequence diagram describes in the system The interaction between classes , Model these interactions as The message exchange , The sequence diagram describes Classes and exchanges between classes With The message of the expected behavior completed , Each message in the sequence diagram represents An operation of class   perhaps The trigger event that causes the state machine to change ;

The sequence diagram shows  : The objects involved in the interaction are in the sequence diagram The top is horizontal , At the bottom of each object is drawn a The vertical dotted line , object A Image object B Send a message , Use one A solid line with an arrow Express , The solid line starts from the object A The dotted line at the bottom , Terminate in object B The dotted line at the bottom ; The solid arrows are placed horizontally ,  The closer to the top, the sooner it's sent .

Sequence diagram trajectory  : The sequence diagram provides Over time Clear   visualization Track of ;

1 The sequence diagram consists of

The sequence diagram consists of  : The sequence diagram consists of four elements   object (Object) Lifeline (Lifeline) Activate (Activation) news (Message);

1.1 object (Object)

object  : The object in the sequence diagram plays the role of object in the interaction ;

Symbols of objects  : Objects in the sequence diagram are represented in the same way as objects in the diagram , Use rectangles to include object names , And the object name is underlined ;

Object creation time  : Objects can be created at the beginning of an interaction , It can also be created during interaction ;

--  At the top  : If the position of the object is at the top of the sequence diagram , Indicates that the object already exists at the beginning of the interaction ;

--  Not at the top  :  If the position of the object is not at the top , So the object is created in the process of interaction ;

1.2 Lifeline (Lifeline)

Lifeline  : The lifeline is a vertical dotted line , This dotted line Represents the existence of an object , In the sequence diagram , There is a lifeline at the bottom of each object ;

Lifeline function  : The lifeline is a timeline , From the sequence diagram Top to bottom All exist , Its length Depending on the time of interaction ;

The lifeline of the object  : The combination of the object and the lifeline is the lifeline of the object , The concept Contains object icons as well as The lifeline icon below the object   Pictured above ;

1.3 Activate (Activation)

Activate (Activation) : Represents the object in the sequence diagram The period during which an operation is performed , Activation period can be understood as in semantics {} The content in , It means that we should Object is occupied to complete a task ;

Deactivation (Deactivation) : Refers to the object At rest , Waiting for a message to activate this object ;

The active representation  : When the object is active , Lifelines can be widened into rectangles , This rectangular bar becomes the active bar ;

The time to activate and to de activate  :

--  Activate  : Object activation is at the top of the activation bar ;

--  Deactivation  : Activate the bottom of the bar to activate , It usually happens when a message leaves the object lifeline ;

1.4 news (Message)

Message concept  : Definition   Interaction   and   Collaboration   in   Exchange information   Class , Yes Between objects   Communication content modeling ;

Message action  :

--  Action type  : The message allows Passing information between entities ( Pass parameters ), allow The entity requests other services , Between objects send out and receive news communicate ;

--  Produce results  :  News can Trigger operation Evoke signals , Or make the goal objects creating or The destruction ;

Asynchronous and synchronous communication of messages  :

--  asynchronous communication  :  Messages are signals When , After sending the signal , Wait for the other party to trigger the corresponding method , This is clear Named Asynchronous communication between objects ;

--  Synchronous communication  : direct Method of calling object , Execute the method and return the result , This kind of operation with return control mechanism is synchronous communication ;

The difference between messages in a sequence diagram and a collaboration diagram  : The message in the sequence diagram emphasizes The order , The messages in the collaboration diagram emphasize the importance of exchanging messages The relationship between objects ;

Message type  :

--  : Draw messages between two objects ;

--  : Procedure calls between two objects ;

--  : Asynchronous messages between two objects ;

--  : The message returned in the procedure call ;

--  : Draw reflexive messages ;

2. Example of sequence diagram

Sample introduction  :

--  Use scenarios  : Car rental company ;

--  Objects used  : Customer ( Customer ), Worker ( Personnel ), Order ( request ), Record ( Record ), Car ( automobile );

--  Workflow  : ① Customer towards Worker Ask for a taxi , ② Worker Check request , ③ Customer payment , ④ Worker Fill in Record , ⑤ Worker Pick up the car ;

Sequence diagram  :

3. Object creation and revocation

object anchors  :

--  Top  : The default position of an object in a sequence diagram is at the top of the sequence diagram , This indicates that the object already exists between the beginning of the interaction ;

--  middle  : If the object is in the middle of the sequence diagram , Created during interaction when describing objects ;

How to create objects during interaction  :

--  The message points to the object  : The message is the construction method , The arrow points to the rectangle represented by the object ;

--  The message points to the activation bar  : The message is the construction method , The arrow points to the activation bar below the object ;

Undo object  : If you want to undo an object in the sequence diagram , Because if there are several objects at the same time , Without special operations, the life cycle will be the same ;

--  design sketch  : In the picture below Customer and Order Object undone , Because they basically don't participate in the later interaction ;

4. Sequence diagram modeling technology

Timing of sequence diagram use  : Modeling dynamic behavior , When it comes to emphasizing the order in which time unfolds information , Use sequence diagrams ;

Sequence diagram management strategy  :

--  Single control flow  : A single sequence diagram can only show one control flow ;

--  Multiple sequence diagrams  : The complete flow of control is complex , You can draw multiple sequence diagrams , A trunk sequence diagram , Multiple branch sequence diagrams , Use packages to manage these sequence diagrams ;

Time series diagram modeling strategy  :

--  Set the interaction context  : The environment of the interaction , Which objects are included , What kind of system does it belong to , Subsystem , Related operations class Use cases, etc ;

--  The order of objects  : According to the importance of the object , From left to right in the sequence diagram ;

--  Object lifeline  : Objects usually exist throughout the interaction , It can also be created and undone during interaction ;

--  Message permutation  : After triggering the interactive information , Messages are drawn in chronological order from top to bottom , The first message stay After the message sent above ;

--  Activation period settings  : You can take the actual operation of Point in time news Show it clearly ;

--  Time and space constraints  : Each message can be attached with the right time and Space constraints ;

--  Pre and post conditions  : Each message can be attached with In front of or After Conditions ;

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