Interactions

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• Objects communicate with each other by sending messages. Sending a message is another name for a member function call. – Some C++ examples of member function calls ...

• objectName.messageName( );

• objectPointer->messageName( );

• (*objectPointer).messageName( );

• An interaction is a set of messages exchanged among a set of objects in order to accomplish a specific goal.

Introduction

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• Interactions model the dynamic aspects of a system by showing the message traffic between a group of objects.

• Showing the time-ordering of the message traffic is a central ingredient of interactions.

• Graphically, a message is represented as a directed line that is labeled.

Introduction

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Introduction

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• Context– Interactions appear whenever an object is linked to

another object.

• Links– A link is an instance of an association.

– Links must exist for interactions to occur.

– The multiplicity on both ends of a link is 1, since it is an instance of an association.

Terms and Concepts

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Links and Associations

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• Messages– A message is the vehicle by which communication

between objects is achieved.

– A call is the most common type of message.

– The return of data as a result of a function call is also considered a message.

– A message may result in a change of state for the receiver of the message.

– The receipt of a message is considered an instance of an event.

Terms and Concepts

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Sequence Diagram

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• Sequencing– A sequence of messages is rooted in some process,

thread or hardware event.

– A sequence will continue as long as the process that started it lives.

– Messages in sequence are ordered by time.

Terms and Concepts

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• Creation, Modification, and Destruction– There are three predefined stereotypes that can adorn

an interaction for purposes of showing the life span of a link or, more commonly, the life span of an object participating in the link.

• <<new>> -- interaction results in the creation of a new object.

• <<destroyed>> -- interaction results in the destruction of an existing object.

• <<transient>> -- link will be destroyed before the completion of the interaction.

Terms and Concepts

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• Representation– Interactions are be modeled using interaction diagrams.

– There are two types of interaction diagrams.• Sequence diagrams emphasize the time ordering of the

interactions.

• Collaboration diagrams emphasize the structural organization of the objects participating in interactions.

– Both types convey the same information, but with a different emphasis.

– The type of diagram used is largely a matter of personnel preference.

Terms and Concepts

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• It is impossible to model all possible interactions within a system.

• Only model those interactions that are interesting or shed light on important aspects of the system.

• A system of even modest complexity may require several interaction diagrams.

• It does not matter whether you use interaction or collaboration diagrams to model interactions.

Common Modeling Techniques

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Sequence / Collaboration Diagram Comparison

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Sequence / Collaboration Diagram Comparison

Interaction Diagrams

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• Interaction diagrams ...– Are used to model the dynamic aspects of a system.

– Aid the developer visualize the system as it is running.

– Are storyboards of selected sequences of message traffic between objects.

• After class diagrams, interaction diagrams are the most widely used diagrams in UML.

• Interaction diagrams commonly contain objects, links and messages.

Introduction

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• There are two types of interaction diagrams …– Sequence diagrams emphasize the time ordering of

message traffic between objects.

– Collaboration diagrams emphasize the structural relationships between objects that send and receive messages.

• Both types of diagrams are semantically equivalent and can easily be converted from one format to the other.– See figure 18-1, page 245.

Introduction

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• Sequence diagrams are build around an X-Y axis.– Objects are aligned (in most cases) at the top of the

diagram, parallel to the X axis.

– Messages travel (in most cases) parallel to the X axis.

– Time passes from top to bottom along the Y axis.

– Where a message arrow appears relative to the Y axis and other message arrows, determines the relative time the message is sent.

– Sequence diagrams most commonly show relative timings, not absolute timings.

– Links between objects are implied by the existence of a message.

Sequence Diagrams

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Sequence Diagrams

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• A collaboration diagram is a graph where the objects (vertices) are connected by links (arcs).

• Each link is adorned with the relevant message traffic that can travel across the link.

• Each message is assigned a sequence number to show the time order of the message.– The sequence number may be prefixed with an

expression to show iteration.

Collaboration Diagrams

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Collaboration Diagrams

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• Determine what scenarios need to be modeled.• Identify the objects that play a role in the scenario.• Lay the objects out in a sequence diagram left to

right, with the most important objects to the left.– Most important in this context means objects that are

the principle initiators of events.

• Draw in the message arrows, top to bottom.– Adorn the message as needed with detailed timing

information.

Modeling Control Flow by Time

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Modeling Control Flow by Time

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• Determine what scenarios need to be modeled, then identify the objects that play a role in the scenario.

• Place the most important objects in the center of the diagram.

• Place links between the appropriate objects.• Draw message arrows next to the appropriate

links.• Number the message arrows to show time

ordering of messages.

Modeling Control Flow by Organization

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Modeling Control Flow by Organization