ActionScript 3.0 Design Patterns

Key OOP Concepts Used with the Observer Pattern | 285

the observer from the notification process. In fact,every singleSubjectmethod refer-

ences theObserver. Likewise, theObserverinterface, with its singleupdate( )method,

references the notification process from the subject. TheConcreteSubjectprovides a

property,stateWork, for holding the state about which the observers are notified. It is

also responsible for notifying the subscribed observers. Finally, through theupdate( )

method, theConcreteObserverkeeps a state value consistent with the subject, and,

through a state property, stores the current state.

Key OOP Concepts Used with the Observer Pattern

One object-oriented design principle put forth in the original design pattern book by

Gammaet alis tofavor object composition over class inheritance. That is, when possi-

ble, design your programs using object composition instead of beginning with a

superclass and then accomplishing reuse through inheritance. To understand the rea-

son for this dictum, we need to look atobject composition.

To appreciate the advantage of object composition over inheritance, we need to

understand another principle put forth by Gamma, Helm, Johnson and Vlissides:

Program to an interface, not an implementation.

Let’s first look at why programming to an interface is favored over programming to

an implementation.

Choose the Interface

In a nutshell, according to Gammaet al,programming to an interface is better than

programming to an implementation because it reduces implementation dependen-

cies between subsystems. This increases software flexibility. As a result, changes can

be made at runtime.

The idea of programming to an interface does not literally mean an ActionScript 3.0

constructinterfaceas opposed to aclass. That is, you can program to the interface

Figure 8-2. Observer pattern class diagram

Subject

‹‹interface››

Subscribe Observer

Unsubscribe Observer

Notify Observer

Observer

‹‹interface››

Update()

observers

ConcreteSubject

Subscribe Observer

Unsubscribe Observer

Notify Observer

ConcreteObserver

subjectsUpdate()

state work