ActionScript 3.0 Design Patterns.pdf - VideoTutorials-bg.com

Example 5-4. Main.as (continued)
}
}
public function Main( )
{
var target:ITarget = new Adapter( );
target.request( );
}
The client class Main creates an instance of the adapter class and calls the method
request( ) defined in the ITarget interface. The client sees only the interface that’s
implemented by the adapter. Object adapters are like traffic cops, as they intercept
method calls and decide what to do with them. Some of the method calls are directed
to the appropriate methods in the Adaptee class. The Adaptor class keeps tight control
of access to the existing class by implementing only the ITarget interface. The client
doesn’t even know that the adapter’s using an existing class. The Adaptee class could
have many public methods and properties that aren’t accessible through the adapter.
Because the Adaptor class creates an instance of the Adaptee class, there’s tight
coupling between them. This doesn’t allow the adapter to use a subclass of Adaptee
or use a totally different existing class without modifying existing Adapter code.
Using a parameterized adapter class
A variation on the previous example of the adapter is to require the client to pass an
instance of the existing class when creating an adapter. You can do this by using a
parameterized constructor in the adapter class. The following changes are needed in
the Adapter class constructor.
public function Adapter(a:Adaptee)
{
this.adaptee = a;
}
This is desirable in many cases, as it reduces the coupling between the Adapter and
Adaptee classes, providing more flexibility. However, this puts the burden on the client
to create an instance of Adaptee class and pass it to the adapter.
var adaptee:Adaptee = new Adaptee( );
var target:ITarget = new Adapter(adaptee);
The disadvantage of doing it this way is that the existing class is no longer hidden
from the client. The client creates the instance of Adaptee that is passed to the
Adaptor. This could potentially cause disruption if the client inadvertently manipulates
the Adaptee instance.
Class Adapters
When the adapter uses inheritance to access an existing class, it’s known as a class
adapter. The adapter class extends the existing class and has an is-a relationship with
Object and Class Adapters | 181