A Transition may own a set of Triggers, each of which specifies an Event whose occurrence, when dispatched, may trigger traversal of the Transition. A Transition trigger is said to be enabled if the dispatched Event occurrence matches its Event type.

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Transitions
Source
Unified Modeling Language 2.5.1
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About Object Management Group copyright in text extracts quoted from OMG specifications for educational purposes
Snippet kind
INFO
UML keywords
State
StateMachine
Transition
Trigger
Transition::trigger
Event
Event occurrence
Trigger::event
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A Transition may own a set of Triggers, each of which specifies an Event whose occurrence, when dispatched, may trigger traversal of the Transition. A Transition trigger is said to be enabled if the dispatched Event occurrence matches its Event type.
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When multiple triggers are defined for a Transition, they are logically disjunctive, that is, if any of them are enabled, the Transition will be triggered.
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A Transition represents an arc between exactly one source Vertex and exactly one Target vertex (the source and targets may be the same Vertex). It may form part of a compound transition, which takes the StateMachine from one steady State configuration...
A Transition is a single directed arc originating from a single source Vertex and terminating on a single target Vertex (the source and target may be the same Vertex), which specifies a valid fragment of a StateMachine Behavior.
NOTE. The duration of a Transition traversal is undefined, allowing for different semantic interpretations, including both “zero” and non-“zero” time.
Transitions are executed as part of a more complex compound transition that takes a StateMachine execution from one stable state configuration to another.
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When multiple triggers are defined for a Transition, they are logically disjunctive, that is, if any of them are enabled, the Transition will be triggered.
The semantics of a Transition depend on its relationship to its source Vertex. Three different possibilities are defined, depending on the value of the Transition’s kind attribute ...
kind = external means that the Transition exits its source Vertex. If the Vertex is a State, then executing this Transition will result in the execution of any associated exit Behavior of that State.
kind = local is the opposite of external, meaning that the Transition does not exit its containing State (and, hence, the exit Behavior of the containing State will not be executed). ...
kind = local ... However, for local Transitions the target Vertex must be different from its source Vertex. A local Transition can only exist within a composite State.
kind = internal is a special case of a local Transition that is a self-transition (i.e., with the same source and target States), such that the State is never exited (and, thus, not re-entered), which means that no exit or entry Behaviors are executed ...
kind = internal ... This kind of Transition can only be defined if the source Vertex is a State.
Transitions of the kind internal are not shown explicitly in diagrams.
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