Object and Class Adapters | 181The client classMaincreates an instance of the adapter class and calls the method
request( )defined in theITargetinterface. 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 theAdapteeclass. TheAdaptorclass keeps tight control
of access to the existing class by implementing only theITargetinterface. The client
doesn’t even know that the adapter’s using an existing class. TheAdapteeclass could
have many public methods and properties that aren’t accessible through the adapter.
Because the Adaptorclass creates an instance of theAdaptee class, there’stight
couplingbetween them. This doesn’t allow the adapter to use a subclass ofAdaptee
or use a totally different existing class without modifying existingAdapter 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
theAdapter class constructor.
public function Adapter(a:Adaptee)
{
this.adaptee = a;
}This is desirable in many cases, as it reduces the coupling between theAdapterand
Adapteeclasses, providing more flexibility. However, this puts the burden on the cli-
ent to create an instance ofAdaptee 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 ofAdapteethat is passed to the
Adaptor. This could potentially cause disruption if the client inadvertently manipu-
lates theAdaptee instance.
Class Adapters
When the adapter usesinheritanceto access an existing class, it’s known as aclass
adapter. The adapter class extends the existing class and has anis-arelationship with
public function Main( )
{
var target:ITarget = new Adapter( );
target.request( );
}
}
}
Example 5-4. Main.as (continued)