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Composite Design Pattern

Composite Design Pattern. Motivation – Dynamic Structure. Motivation – Static Structure. Applicability. Use the Composite pattern when: You need to represent part-whole hierarchies of objects

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Composite Design Pattern

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  1. Composite Design Pattern

  2. Motivation – Dynamic Structure

  3. Motivation – Static Structure

  4. 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.

  5. Structure

  6. 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

  7. 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.

  8. Structure

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

  10. 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.

  11. 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.

  12. Tree Structure

  13. 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.

  14. Directory / File Example – Object Structure / • Directory = Composite • File = Leaf bin/ user/ tmp/ file1 subdir/ file3 file2 file5 file4 Composite Composite Composite Composite Leaf Leaf Composite Leaf Leaf Leaf

  15. Directory / File Example – Classes Leaf Class: File Composite Class: Directory • One class for Files (Leaf nodes) • One class for Directories (Composite nodes) • Collection of Directories and Files • How do we make sure that Leaf nodes and Composite nodes can be handled uniformly? • Derive them from the same abstract base class

  16. Directory / File Example – Structure Abstract Base Class: Node Leaf Class: File Composite Class: Directory

  17. Directory / File Example – Operation Abstract Base Class: Node size() in bytes of entire directory and sub-directories Composite Class: Directory size () Sum of file sizes in this directory and its sub-directories Leaf Class: File size () of file long Directory::size () { long total = 0; Node* child; for (int i = 0; child = getChild(); ++i; { total += child->size(); } return total; }

  18. Inventory Example – Problems to Consider Abstract Base Class: Inventory Item Backpack Cloth Bag Jeweled Dagger Ruby Ring Wine Bottle Leaf Class: Object Composite Class: Container Do you really want all leaf Operations to be defined in Component class? Is Wine Bottle a Composite or does it have an operation for Fill(Liquid)? If Ruby is pried from Dagger, how will this be handled? Would you allow user to put other items in the wine bottle? How do you handle the relative size of items to be placed in a backpack?

  19. Consequences • Solves problem of how to code recursive hierarchical part-whole relationships. • Client code is simplified. • Client code can treat primitive objects and composite objects uniformly. • Existing client code does not need changes if a new leaf or composite class is added (because client code deals with the abstract base class). • Can make design overly general. • Can’t rely on type system to restrict the components of a composite. Need to use run-time checks.

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