Composite Pattern

1. Composite Pattern Jim Fawcett CSE791 – Design Patterns Summer 2000

2. Intent • Compose objects into tree structures to represent part-whole hierarchies. • Composite lets clients treat individual objects and compositions of objects in a uniform way.

3. Motivation – Dynamic Structure

4. Motivation – Static Structure

5. Applicability • Use the Composite pattern when: • You need to represent part-whole hierarchies of objects • You want clients to ignore the differences between parts and wholes (individual objects and compositions of objects) • The compositions are created dynamically – at run time – so use composite: • when you need to build a complex system from primitive components and previously defined subsystems. • This is especially important when the construction process will reuse subsystems defined earlier.

6. Structure

7. Participants – Plan A • Component (Graphic) • declares interface for objects in the composition • implements default behavior for the interface common to all classes, as appropriate • declares an interface for accessing and managing its child components • (optional) defines an interface for accessing a component’s parent in the recursive structure, and implements it if that’s appropriate • Client • manipulates objects in the composition through the Component interface

8. Participants – Plan A • Leaf • Represents leaf objects in the composition. A leaf has no children. • Defines behavior for primitive objects in the composition. • Composite (picture) • Defines behavior for components having children. • Stores child components. • Implements child-related operations in the Component interface.

9. Structure

10. Participants – Plan B • Component (Graphic) • declares interface for objects in the composition • Client • manipulates objects in the composition through the Composite interface

11. Participants – Plan B • Leaf • Represents leaf objects in the composition. A leaf has no children. • Defines behavior for primitive objects in the composition. • Composite (picture) • Defines behavior for components having children. • declares an interface for accessing and managing its child components • Stores child components. • Implements child-related operations in the Component interface.

12. Collaborators • Plan A:Clients use the Component class interface to interact with objects in the composite structure. • Plan B:Clients use the Composite class interface to interact with objects in the composite structure. • If the recipient is a Leaf then the request is handled directly. • If the recipient is a composite, then it usually forwards request to its child components.

13. Tree Structure

14. Consequences • The Composite pattern: • Defines class hierarchies consisting of primitives and composites • When a client expects a primitive it can also take a composite. • Makes it easier to add new components. • Newly defined primitives or composites will work automatically with existing structures and client code. • Can make your design too general. • Since its easy to add new components it is hard to restrict the components of a composite. • Composite doesn’t support using the type system to support these constraints for you, since all components have the same base type.

15. Know Uses • Authors cite many windowing toolkits. • The FORTH language defines its statements using an extensible dictionary which implements the Composite pattern.

16. Related Patterns • Decorator – very similar structure and intent • Iterator can be used to traverse Composites.