Activity diagrams are flowcharts that decompose
a system into activities that correspond to states. These diagrammatic
elements, called actions, are member function calls within
a given operation.
In contrast to normal states (as in
statecharts), actions in activity diagrams terminate on completion
of the activity, rather than as a reaction to an externally generated
event.
Each action can have an entry action, and must have
at least one outgoing control flow. The implicit event trigger on
the outgoing control flow is the completion of the entry action. If
the action has several outgoing transitions, each must have its own
guard condition.
Actions have the following constraints:
- Outgoing transitions from actions do not include
an event signature. They can include guard conditions and actions.
- Actions have non-empty entry actions.
- Actions do not have internal transitions or exit
actions, nor do activities.
- Outgoing transitions on actions have no triggering
events.