Section 14.6. Composite States

14.6. Composite States

A key difference between UML state diagrams and other non-UML state diagrams you may be familiar with is that UML allows concurrent states, or being in multiple states at the same time. Composite states are what makes this possible.

Suppose the troll in the Neutral state is doing two things at the same time: Searching and Pacing. You can model two simultaneous states by using a composite state, as shown in Figure 14-18.

Figure 14-18. Composite states contain one or more state diagrams; if they contain more than one state diagram, then the state diagrams execute in parallel

A composite state is a state that contains one or more state diagrams. Each diagram belongs to a region, and regions are divided by a dotted line. A state in a region is referred to as a substate of the composite state.

Composite states work as follows: when the composite state becomes active, the initial pseudostate of each region becomes active, and the contained state diagrams begin executing. The contained state diagrams are interrupted if a trigger on the composite state occurs. In Figure 14-18, the substates will be halted when a trigger on the composite statesee opponentoccurs.

If substates have behavior that can run to completion, then the composite state is complete when every region's state diagram is complete.