ActionScript 3.0 Design Patterns

Example: Music Playlists | 217

Key OOP Concepts in the Composite Pattern

The key OOP concept embedded in the composite pattern ispolymorphism. Poly-

morphism can be broadly defined as the ability of objects instantiated from different

classes to respond to the same method calls in specific ways. This is possible because

the method signature’s the same, even though the objects are instantiated from dif-

ferent classes. In short, the methods in the different classes show a common inter-

face to clients.

Since the composite pattern allows clients to treat both leaf and composite nodes the

same way through a common interface, it’s a good example of polymorphism. This is

due to both leaf and composite classes implementing the same component interface

by subclassing the abstract component class.

Implementations of operations truly exemplify polymorphism. In the minimalist

examples, theoperation( )method was implemented one way in the leaf nodes and

another way in the composite nodes. The composite nodes have to forward the

operation( )call to all its child nodes. Theoperation( )responds differently in both

leaf and composite nodes, but the client doesn’t need to know or care about these

differences. The client doesn’t see the different implementations; it sees only the

interface.

Example: Music Playlists

Have you created music playlists in your favorite digital music jukebox? If so, have

you taken the next step and embedded playlists inside other playlists to create ever

larger playlists? This is a perfect application to implement a composite pattern. A

library of digital music contains songs and playlists. A song is a primitive object,

Figure 6-5. Deleting components from the tree structure

root

Composite 1

Composite 2

leaf 6

leaf 2

leaf 3 leaf 4 leaf 5

leaf 2

root

Composite 1

Composite 2

leaf 6

leaf 2

leaf 4 leaf 5

leaf 2

root

Composite 1 leaf 6

leaf 2 leaf 2

Display tree First child of second

child of ‘root’ removed

Second child of

‘root’ removed

Second child

of root

First child of second

child of root

Second child

of root