Value Lifeline

Description

A Lifeline is the path an object takes across a measure of time, indicated by the x-axis. There are two sorts: Value Lifelines (defined here) and State Lifelines, both used in Timing diagrams.

A Value Lifeline shows the Lifeline's state across the diagram, with parallel lines indicating a steady state. A cross between the lines indicates a transition or change in state.

This is an example of a Value Lifeline:

See UML Superstructure Specification, v2.1.1, Figure 14.30, p.520.

Transition point properties

A Value Lifeline consists of a set of transition points. Each transition point can be defined with these properties:

Property	Description
At time	Specifies the starting time for a change of state.
Transition to	Indicates the state to which the Lifeline is to change.
Event	Describes the occurring event.
Timing constraints	Refers to the time taken for a state to change within a Lifeline, or the time taken to transmit a message.
Timing observations	Provides information on the time of a state change or sent message.
Duration constraints	Pertains to a Lifeline's period at a particular state. The constraint could be instigated by a change of state within a Lifeline, or that Lifeline's receipt of a message.
Duration observations	Indicates the interval of a Lifeline at a particular state, begun from a change in state or message receipt.

Example properties

In the example diagram, the 10ms transition point has these properties:

Property	Text
At Time	10ms
Transition to	Waitcard
Event	Switch
Timing constraints	–
Timing observations	–
Duration constraints	d..3*d
Duration observations	–

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Learn more

OMG UML Specification:

The OMG UML specification (UML Superstructure Specification, v2.1.1, p.518) states:

Shows the value of the connectable element as a function of time. Value is explicitly denoted as text. Crossing reflects the event where the value changed.