UML sequence diagram (sequence diagram)

Sequence diagram (Sequence Diagram)： Is an interaction diagram that emphasizes the order of message passing between objects , Also known as sequence diagram or sequence diagram . Describes how objects interact with each other through messages during the execution of a use case or operation , Explains how messages are sent and received between objects and the order in which they are sent .

form ：

1、 Lifeline ： Indicates when an object exists

2、 news ： Messages are generally divided into synchronous messages （Synchronous Message）, Asynchronous messaging （Asynchronous Message） And return messages （Return Message）. As shown in the figure below ：

a. Sync message

The sender of the message passes process control to the receiver of the message , Then pause the activity , Wait for the response message from the message recipient .

b. Asynchronous messaging （Asynchronous Message）

      After the sender of the message sends the message to the receiver of the message , Don't wait for a response , You can start another activity .

c. Return message （Return Message）

       The return message indicates that... Is returned from the procedure call

d. Self related messages

       Represents the call of a method itself and the call of another method by a method in an object

Add ：

1、 Interactive clips (Interaction Fragment)： A complex sequence diagram can be divided into several small blocks , Each small piece is called an interaction fragment , Surrounded by a large box .

2、 Common operators include ：

1) alt: Multiple paths , When the condition is true .

2) opt: Optional , Only if the condition is true .

3) par: parallel , Each fragment is executed concurrently .

4) loop: loop , Fragments can be executed many times .