Unified Modeling Language Reference Manual, The, Second Edition

change trigger

A trigger on a state machine transition that becomes enabled if a specified change event occurs.

Semantics

A change trigger specifies a change event by a Boolean expression. There are no parameters to the event. The change event occurs when the condition becomes true (after having been false) because of a change to one or more variables on which the condition depends. When the event occurs, the change trigger fires and enables its transition, provided any attached guard condition is true.

A change trigger implies a continuous test for the condition. In practice, however, by analyzing the times at which the inputs to the condition can change, an implementation can often perform the test at well-defined, discrete times so that continuous ...