ActionScript 3.0 Design Patterns

294 | Chapter 8: Observer Pattern

Note that this script programs to the interface to instantiate the different observer

instances, but must program to theConcreteSubjectdirectly to implement it. How-

ever, we could have typed the observers asConcreteObserver, and then used the

instances asObservertypes in the parameters for subscribing. In either case, we

would have been following the dictum of programming to the interface.

Open a new Flash document, and, in the Document class window, type inTestSub,

and then test the application. Your output should appear as the following:

*|*Concrete Subject*|*

=Concrete Observer=

=Concrete Observer=

=Concrete Observer=

Observer 0 Light is on

Observer 1 Light is on

Observer 2 Light is on

Observer 0 Light is off

Observer 1 Light is off

As you can see, a single instance used theConcreteSubjectconstructor and three

used theConcreteObserverconstructor, as indicated by the respectivetrace( )out-

puts. The observer state was first set to “on” and, as expected, the three observer

instances indicated the correct state. Then, a single observer was unsubscribed, and

the state was set to “off.” With two observers now, only two indicated the state.

Example: Adding States and Identifying Users

The minimal Observer design pattern example had only a single state. You might

think that adding additional states may be complicated, but you’ll see that it’s very

easy. One of the key features about design patterns is that they’re flexible, and add-

ing states illustrates this flexibility.

Another issue that can be important in some applications using the Observer pattern

is getting to know the observers. In the minimal example in the previous section, the

observers were identified by their array index, which changes when observers are

removed. However, using a property to identify observers is quite simple as well.

mySub.setLight("on");
mySub.unsubscribeObserver(subObserver);
mySub.setLight("off");
}
}
}

Example 8-9. TestSub.as (continued)