systemcomposer.interaction.Message

R2026b

Message between lifelines in interaction

Since R2024a

Description

A Message object represents a communication between two lifelines of an interaction. On the sequence diagram representing the interaction, a message is shown as an arrow. Messages have labels that can describe the communication occurrence either formally or informally. A message corresponds to a connector connecting ports of two components in an architecture model.

A message label defines the conditions under which a message occurs and what the message validates. A message label has a trigger, an optional guard, and an optional constraint in the form of trigger[guard]{constraint}.

• Trigger — The identifying event for this message. The trigger format depends on the communication type:

  • Signal-based communication: The trigger follows the format direction(signal [+|-] value), which specifies a triggering edge with a direction and an expression. The direction can be:

    • rising — The edge expression is rising from strictly below zero to a value equal to or greater than zero.

    • falling — The edge expression is falling from strictly above zero.

    • crossing — The edge expression is either rising or falling past zero.

    For example, in the message label rising(sw-1){sw==1}, the trigger rising(sw-1) fires when the sw signal rises from below 1 to a value of 1 or above.

  • Message-based communication: The trigger is in the format port, which specifies the name of the input message port and represents the arrival of a message. For example, in the message label switchEvent[switchEvent==1], the trigger switchEvent fires when a message arrives through the switchEvent port.

• Guard (optional) — A MATLAB^® Boolean expression in square brackets that is an additional condition to determine whether the message occurs. Evaluation of the guard only occurs once the software detects a valid trigger. During execution, the sequence diagram waits to proceed to the next message until the guard expression evaluates to true. For example, in switchEvent[switchEvent==1], the guard [switchEvent==1] requires the value at switchEvent to be 1 for the message to occur.

• Constraint (optional) — A MATLAB Boolean expression in curly brackets that specifies an expected value of inputs to the destination lifeline. During execution, the evaluation of the constraint determines whether the sequence diagram shows a pass or fail for that message. If a constraint expression is not specified, the evaluation of the trigger determines whether the sequence diagram shows a pass or fail for that message. For example, in rising(sw-1){sw==1}, the constraint {sw==1} validates that sw equals 1 when the message fires.

Creation

Access Message objects via the RootFragment property of the corresponding systemcomposer.interaction.Interaction object. Iterate over the root fragment to inspect the systemcomposer.interaction.MessageEvent objects within the fragments. Access the Message property of the message event to inspect the corresponding Message object.

Properties

Type — Message type

"Signal" | "Message"

Message type, specified as one of these options:

• "Signal" for signal-based communication

• "Message" for Simulink^® message-based communication

Data Types: string

Condition — Message label

string

Message label, specified as a string.

Data Types: string

Start — Start of message

message event object

Start of message, specified as a systemcomposer.interaction.MessageEvent object.

End — End of message

message event object

End of message, specified as a systemcomposer.interaction.MessageEvent object.

RelatedElements — Connectors represented by message

array of connector objects | array of physical connector objects

Connectors represented by message, specified as an array of systemcomposer.arch.Connector or systemcomposer.arch.PhysicalConnector objects.

ExternalUID — Unique external identifier

character vector

Unique external identifier, specified as a character vector. The external ID is preserved over the lifespan of the element and through all operations that preserve the UUID.

Data Types: char

UUID — Universal unique identifier

character vector

Universal unique identifier, specified as a character vector.

Example: '91d5de2c-b14c-4c76-a5d6-5dd0037c52df'

Data Types: char

Object Functions

move	Move message within interaction
destroy	Remove model element

Examples

Inspect Sequence Diagrams with Programmatic Interfaces

You can use read-only API workflows to navigate sequence diagrams in System Composer™ and display information about each element.

Open Traffic Light Example

Open the traffic light example architecture model so that you can inspect the sequence diagrams visually and confirm the programmatic outputs.

model = systemcomposer.openModel("TLExample");

To view the sequence diagrams associated with the model, on the System Composer toolstrip, navigate to Modeling > Sequence Diagram.

Programmatically Navigate Sequence Diagram

Collect the sequence diagrams represented by interactions that contain specific interactions of elements in the model.

interactions = model.getInteractions;

For the first interaction, extract the name of the sequence diagram.

disp("The first sequence diagram is called " + interactions(1).Name + ".")

The first sequence diagram is called PressDetection.

For this sequence diagram, display each lifeline and the component the lifeline represents.

for i = 1:length(interactions(1).Lifelines)
    disp("The " + interactions(1).Lifelines(i).Name + ...
    " lifeline represents the " + ...
    interactions(1).Lifelines(i).RelatedElements.Name + ...
    " component.")
end

The source lifeline represents the source component.
The poller lifeline represents the poller component.
The switch lifeline represents the switch component.

Display the contents of one message in the root fragment.

disp("The sequence diagram message starting at the " + ...
    interactions(1).RootFragment.Operands.Fragments(1).Name + ...
    " message end is of type " + ...
    string(interactions(1).RootFragment.Operands.Fragments(1).Message.Type) + ...
    " and the message label is " + ...
    interactions(1).RootFragment.Operands.Fragments(1).Message.Condition + ".")

The sequence diagram message starting at the switchout message end is of type Signal and the message label is rising(sw-1).

Use Iterator Utility to Step Through Sequence Diagram

Step through the Inhibit sequence diagram using the Iterator utility.

interaction = model.getInteraction('Inhibit');
interaction.open

Display the annotation from the interaction.

disp(interaction.Annotations.Content)

When inhibit is true, it means pedestrian crossing is not controlled by a walk signal on this intersection.

Use an iterator to navigate through all elements of a sequence diagram before extracting their properties.

iterator = systemcomposer.interaction.Iterator(interaction.RootFragment);
next = iterator.next;
while ~isempty(next)
    disp(class(next))
    next = iterator.next;
end

systemcomposer.interaction.RootFragment
systemcomposer.interaction.Operand
systemcomposer.interaction.MessageEvent
systemcomposer.interaction.MessageEvent
systemcomposer.interaction.MessageEvent
systemcomposer.interaction.MessageEvent
systemcomposer.interaction.AltFragment
systemcomposer.interaction.Operand
systemcomposer.interaction.MessageEvent
systemcomposer.interaction.MessageEvent
systemcomposer.interaction.MessageEvent
systemcomposer.interaction.MessageEvent
systemcomposer.interaction.Operand
systemcomposer.interaction.MessageEvent
systemcomposer.interaction.MessageEvent

More About

Definitions

Term	Definition	Application	More Information
Interaction	An interaction specifies how each part of a system should interact as a sequence of message exchanges.	Use interactions to describe operational system behaviors.	Describe System Behavior Using Sequence Diagrams
Sequence diagram	A sequence diagram is a visual representation of an interaction.	Use sequence diagrams to visually specify how each part of a system should interact.	Describe System Behavior Using Sequence Diagrams
Lifeline	A lifeline represents an instance of a component as a participant of an interaction.	A lifeline corresponds to a component in an architecture.	Describe Interactions with Lifelines and Messages
Message	A message represents communication between two lifelines. Messages have labels to specify the expected condition for the message to occur.	A message label has a trigger, an optional guard, and an optional constraint where a trigger represents the identifying event for this message, a guard represents an additional condition to determine whether the message occurs, and a constraint is an expression that is expected to be true when this message occurs.	Describe Interactions with Lifelines and Messages
Gate	A gate represents the root of an architectural hierarchy.	A gate allows you to describe the exchange of messages between the architecture and its environment.	Describe Interactions with Lifelines and Messages
Annotation	An annotation describes the elements of a sequence diagram.	Use annotations to provide detailed explanations of elements or workflows captured by sequence diagrams.	Annotate Sequence Diagrams
Fragment	A fragment encloses a group of lifelines and messages within an interaction to allow for the specification of more complex patterns of interaction.	A fragment defines the type of ordering logic such as looping and alternatives. Fragments can have one or more operands.	Model Complex Interactions with Fragments and Operands
Operand	An operand is a region in a fragment, or group of messages. The condition of an operand specifies whether the messages inside the operand execute.	The condition of an operand can specify constraints on the input signal of a lifeline as a MATLAB Boolean expression.	Model Complex Interactions with Fragments and Operands
Duration constraint	A duration constraint defines a constraint on elapsed time between a start and an end occurrence.	Use duration constraints to explicitly express a constraint on the duration between a start occurrence and an end occurrence.	Specify Timing Constraints Between Message Events with Duration Constraints

Version History

Introduced in R2024a