Section 10.6. Event Occurrences

10.6. Event Occurrences

Event occurrences are the smallest building blocks of interaction diagrams; they represent moments in time when something happens. Sending and receiving a message are the most common types of event occurrences, though technically they can be any action associated with an object. For example, if object1 sends a message to object2, there are two event occurrences, a message send and a message receive. UML carefully defines interaction fragments as a set of event occurrences where ordering is significant because they represent events over time.

Each type of interaction diagram notation (sequence, communication, etc.) has a way of expressing the time-sensitive nature of the event occurrences. In a sequence diagram the event occurrences are ordered along the lifelines and are read from top to bottom. Figure 10-16 shows a sequence diagram with three of the event occurrences labeled (as mentioned earlier, any action associated with an object is an event occurrence, but to keep the diagram from getting out of control only three are labeled in the figure).

Figure 10-16. A sequence diagram with three event occurrences labeled