ActionScript 3.0 Design Patterns

Object and Class Adapters | 179

and their functionality to figure out if they’ll fit the bill. We can then develop an

adapter class that implements the target interface. There’s no point to adapting an

existing class if it doesn’t provide some benefit, such as savings in development time

compared to developing a whole new class.

Object and Class Adapters

An adapter can exploit an existing class to implement functionality in two ways. The

adapter can use eithercompositionorinheritanceto access an existing class. If the

adapter subclasses the existing class, it will inherit the methods and properties. If the

adapter uses composition to access the existing class, the adapter class will hold a

reference to an instance of the existing class, and use its methods and properties for

implementation. In OOP, composition is generally preferred over inheritance. How-

ever, as we will see in subsequent examples, there are good arguments for both

approaches depending on the context. Either way, the idea is to use the properties

and methods of the existing class to provide most or some of the functionality

required.

Object Adapters

When the adapter usescompositionto access the existing class, it’s known as an

object adapter. Object adapters store a reference to an instance of the existing class.

Therefore, an object adapteruses an existing class.

All adapter classes implement aninterface. In the class diagram shown in Figure 5-2,

the interface calledITargetdeclares a single method calledrequest( ). The class dia-

gram shows that a reference to an instance of theAdapteeclass is stored in the vari-

ableadaptee. When implementing therequest( )method, the adapter references this

instance.

Minimalist example of an object adapter

Example 5-1 through Example 5-4 show the implementation of an object adapter in

ActionScript 3.0. The different files are called:Adaptee.as(existing class),ITarget.as

(required interface),Adapter.as(adapter), andMain.as(client; also thedocument

class for the Flash document).

Figure 5-2. Class diagram of an object adapter

adpatee->specificRequest()

ITarget

request()

Adaptee

specificRequest()

Adapter

request()

adpatee