ActionScript 3.0 Design Patterns

Program to Interfaces over Implementations | 45

TheMyObjectclass’s sole method isworksForRequest( ). The method is encapsulated,

so it should have only a single way to launch its operations. Fortunately, all we need

to do is create an instance of the class, and add the method to the instance to make it

work, as Example 1-38 shows.

The output tells the whole story:

This was requested by a client

The client in this case is simply an instance of the object whose method it requests.

The client/object relationship is through the request. Often the client adds specific

details through a parameter, but the concept of a request is usually nothing more

than invoking the method with an instance of the object that owns the method.

Program to Interfaces over Implementations

To understand why the authors of design patterns encourage programming to inter-

faces over implementations, you need to first understand the general goal of flexibil-

ity and reusability. Second, you need to appreciate the problem of managing

dependency in large programs.

As we have noted in this chapter, the overall goal of design patterns is to create reus-

able code. In order to meet this overall goal, the code must be flexible. This doesnot

mean that your application runs better or compiles faster. All it does is help you cre-

ate code that you can reuse in other projects. The more time and effort you spend,

the larger the team engaged in working with the code, the more important this over-

all goal.

Managing Dependency

The need for software flexibility leads to the need to manage dependency. When

your code depends on a specific implementation, and the implementation changes,

Example 1-38. MyClient.as

package
{
import flash.display.Sprite;

public class MyClient extends Sprite
{
var myClient:MyObject;
public function MyClient( ):void
{
myClient=new MyObject( );
myClient.worksForRequest( );
}
}
}