ActionScript 3.0 Design Patterns

Minimalist Example of a Composite Pattern | 207

the components. We can think oftouch( )as a common operation that applies to all

components in the mobile. Generally, the touch operation makes the mobile compo-

nent rotate. If you touch a composite node, it will rotate not only itself, but all its

children as well. Touching a leaf node such as a fish will rotate only that compo-

nent, as leaf nodes have no children. The fact that the client manipulating the com-

posite structure does not have to worry about whether the operation’s being carried

out on a leaf or a composite node is one of the key features of the composite pattern.

Key Features of the Composite Pattern

The composite pattern streamlines the building and manipulation of comple xstruc-

tures that are composed of several related pieces.

• Complex structures are built as hierarchical trees.


• The components of the structure can be individual components (primitives or
  indivisible objects) or composite components that hold a collection of other
  components.


• They allow clients to treat both individual components (leaf nodes) and compos-
  ite components (composite nodes) the same way, simplifying the interface.

Minimalist Example of a Composite Pattern

This example implements the composite shown in the class diagram in Figure 6-2.

TheComponent.as file (Example 6-1) contains the Component abstract class that

defines the interface for both leaf and composite nodes. The Leaf.as file

(Example 6-2) contains theLeafclass, and theComposite.asfile (Example 6-3) con-

tains theCompositeclass. BothLeafandCompositeclasses extend theComponentclass

and provide necessary implementations. TheMain.asfile (Example 6-4) contains the

client classMain (also known as thedocument class for the Flash document).

Example 6-1. Component.as

package
{
import flash.errors.IllegalOperationError;

// ABSTRACT Class (should be subclassed and not instantiated)
public class Component
{
public function add(c:Component):void
{
throw new IllegalOperationError
("add operation not supported");
}

public function remove(c:Component):void
{