Decomposing an Activity

An activity can be broken down into subactivities. This works much the same as a superstate and substates do on a state diagram. You can just show the superstate on the parent diagram, or you can show the superstate and its internal behavior inside it, as I have in Figure 9-3. In this case, I've given the delivery activity both a start and an end. You can also draw transitions directly into, or out of, the subsidiary diagram. The advantages of the explicit start and end states are that the delivery activity can be used in other contexts, and the parent diagram is decoupled from the contents of the subsidiary diagram.