Composite

aka        recursive composition                                                                                 Object Structural      p. 163

Intent

Compose objects into tree structures.    Towards uniform treatment of objects and  compositions

Motivation

Having to distinguish between objects and container objects makes an application complex

Key Idea

An abstract class that represents both primitives and their containers

Applicability

1)       part whole hierarchies

2)       uniform treatment of compositions of objects and  individual objects

Participants

Component, Leaf, Composite, Client

Collaborations

Clients use the Component interface to interact with objects in the structure

   If aLeaf, request handled directly

   If aComposite, forward request to child components (possibly performing additional operations)

Consequences

Defines class hierarchies of primitives and composites, which are interchangeable in all(?) method calls

Client can treat primitives and composites uniformly

Easier to add new kinds of components

Prb:  resulting  design can be overly general

Implementation

1)       explicit parent references

2)       sharing components ???

3)       maximize the component interface  (define many common operations) – a class could end up  defining operations NOT meaningful to its subclasses

4)       where should child-management operations be defined (declared in Component, impl in Leaf; or decl and impl only in Composite?)  tradeoff between safety and transparency  (transparency better here)

5)       Component implement a list of Components?  (child pointer in the base class = space penalty)

6)       Child ordering

7)       Cache to improve performance (but need cache invalid interface)

8)       Who should delete components?   (relevant only in languages w/o garbage collection, e.g.,…..)

9)       Which data structure to store components?

Known Uses

View Class of ST MVC, UI toolkit/frameworks, RTL ST compiler, portfolio/assets, Command pattern

Related Patterns

Chain of responsibility, decorator, flyweight, Iterator, interpreter, visitor

redesign cause

Additional refs

http://  

questions

What is the distinction between component and composite