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State Lifeline
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 below:
See UML Superstructure Specification, v2.1.1, figure 14.29, p. 519.
A State 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 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.
|
In the example diagram above, the OK transition point has these properties:
Property |
Value |
|---|---|
At Time |
18 ms
|
Transition to |
Idle
|
Event |
OK
|
Timing constraints |
t..t+3
|
Timing observations |
-
|
Duration constraints |
-
|
Duration observations |
-
|
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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.
