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.
An example of a Value Lifeline is shown below:
See UML Superstructure Specification, v2.0, figure 351, p. 453.
A Value Lifeline consists of a set of transition points. Each transition point can be defined with the following properties:
Property |
Description |
At time |
Specifies the starting time for a change of state. |
Transition to |
Indicates the state to which the Lifeline will 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. |
In the example diagram above, the 10ms transition point has these properties:
Property |
Text |
At Time |
10ms |
Transition to |
Waitcard |
Event |
Switch |
Timing constraints |
?/span> |
Timing observations |
?/span> |
Duration constraints |
d..3*d |
Duration observations |
?/span> |
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OMG UML Specification
The OMG UML specification (UML Superstructure Specification, v2.0, p. 451) 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."