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Weak sequencing fragment in interaction

Since R2024a


    A SeqFragment object is a container for operands in a sequence diagram that specifies the behavior of contained elements.

    A Seq Fragment represents that the messages in its operand are weakly sequenced. Weak sequencing is also the default interpretation of messages on an interaction, and so this fragment is often not explicitly required.

    Weak sequencing means that the order of message events on a lifeline must be followed, but there is no ordering constraint between message events on different lifelines, except that for a given message, the send event occurs before the receive event.

    A weak sequencing fragment is drawn as a box in the sequence diagram, with the label Seq in the box header. The operand is then drawn within this box as a separate section.


    Access Fragment objects via the RootFragment property of the corresponding systemcomposer.interaction.Interaction object. Access the Operand property of the root fragment and then, if there are fragments within the root operand, access the Fragment property of the root operand and iterate over it to view details about each of the fragments.


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    Operands, specified as an array of systemcomposer.interaction.Operand objects.

    Lifelines covered, specified as an array of systemcomposer.interaction.Lifeline objects.

    Parent operand, specified as an systemcomposer.interaction.Operand object.

    Universal unique identifier, specified as a character vector.

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

    Data Types: char


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    This example shows how to use read-only API workflows to navigate sequence diagrams in System Composer™ and display information about each of the elements.

    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");

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

    The press detection sequence diagram in the views gallery of the top model.

    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 SignalSequence.

    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.")
    The ped lamp lifeline represents the ped lamp component.
    The lampController lifeline represents the lampController component.
    The poller lifeline represents the poller component.
    The source lifeline represents the source component.

    Now, 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');

    Inhibit sequence diagram from the top model.

    Display the annotation from the interaction.

    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 =;
    while ~isempty(next)
        next =;

    More About

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    Version History

    Introduced in R2024a