ActionScript 3.0 Design Patterns

Program to Interfaces over Implementations | 47

Using Complex Interfaces

As you saw in Example 1-36, the program typed the instances to the implementation

and not the interfaces as was done in Example 1-20. This was caused by the key

methods being part of classes that implemented an interface and extended a class.

Because of the way in which the different display objects need to be employed, this

dilemma will be a common one in using Flash and ActionScript 3.0. Fortunately, a

solution is at hand. (The solution may be considered aworkaroundinstead of the

correct usage of the different structures in ActionScript 3.0. However, with it, you

can create methods that require someDisplayObjectstructure and program to the

interface instead of the implementation.)

If the interface includes a method to include aDisplayObjecttype, it can be an inte-

gral part of the interface. BecauseSpriteis a subclass of theDisplayObject, its inclu-

sion in the interface lets you type to the interface when the class you instantiate is a

subclass ofSprite (or some other subclass of theDisplayObject, such asMovieClip.)

To see how this works, the application made up of Example 1-39 and Example 1-40

creates a simple video player. TheVidPlayerclass builds the structural details of the

video playing operations. To do so requires that it subclass theSpriteclass. By plac-

ing a getter method with aDisplayObjecttype in theIVidinterface, the application

sets up a way that the client can program to the interface.

The implementation of the IVid interface includes a key element. The

displayObject( )function is implemented to theDisplayObjectclass in building the

VidPlayer class.

Example 1-39. IVid.as

package
{
import flash.display.DisplayObject;

public interface IVid
{
function playVid(flv:String):void;
function get displayObject( ):DisplayObject;
}
}

Example 1-40. VidPlayer.as

package
{
import flash.net.NetConnection;
import flash.net.NetStream;
import flash.media.Video;
import flash.display.Sprite;
import flash.display.DisplayObject;