15.1 Generic event model

Table 15.1 gives a review of the major roles in a generic event model. Figure 15.1 shows them in a schematic way.

Figure 15.1. Characteristics of the event source, object and consumer in a generic event model.

Table 15.1. Description of different roles in a generic event model

Role	Description
Event source	The object which potentially causes an event to happen an event can be any special occurrence. The event source provides a way for interested event consumers to register their interest with this event source. The event source typically keeps a list of registered event consumers, so that when the event occurs the registered consumers in the list are notified.
Event consumer	The object which is interested in listening to a particular event. An event consumer contains a special method called the event handler. This method takes in the event object as a parameter. Event sources and event consumers can have a many-to-many relationship one event consumer can register with many event sources, and one event source can have multiple event consumers registered for the same event.
Event object	When an event occurs in the event source, a new event object is created. This event object is then passed over to the event consumer's event handler method as a parameter. An event object encapsulates event-specific information (e.g. the label of the button clicked) which the event consumer can extract.

Let us examine the sequence of actions in the generic event model.

1. The event consumer registers its interest with an event source and provides the name of the event handler that should be invoked when the event occurs.

2. The event source keeps a reference to each registered event consumer in an internal list.

3. When the event occurs, the event source creates a new instance of an event object and passes into it event-specific information.

4. The event source goes down the list of registered event consumers and invokes each event handler, passing in the event object as a parameter.

5. The event handler of the event consumer runs. Event information can be obtained from the event object passed in.

Figure 15.2 depicts this sequence.

Figure 15.2. Sequence of action in a generic event model.