State Lifeline

Description

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

A State Lifeline follows discrete transitions between states, which are defined along the y-axis of the timeline. Any transition has optional attributes of timing constraints, duration constraints and observations. An example of a State Lifeline is shown here:

See UML Superstructure Specification, v2.1.1, figure 14.29, p. 519.

Transition point properties

A State 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 changes.

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 (e.g. t..t+3).

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 OK transition point has these properties:

Property

Value

At Time

68 ms

Transition to

Idle

Event

OK

Timing constraints

t..t+3

Timing observations

Duration constraints

Duration observations

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Learn more

OMG UML Specification:

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

This is the state of the classifier or attribute, or some testable condition, such as an discrete enumerable value.

It is also permissible to let the state-dimension be continuous as well as discrete. This is illustrative for scenarios where certain entities undergo continuous state changes, such as temperature or density.