Composite Design Pattern

Motivation – Dynamic Structure

Motivation – Static Structure

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.

Structure

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

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.

Structure

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

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.

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.

Tree Structure

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 / bin/user/tmp/ file1 file2 subdir/file3 file4 file5 Composite Leaf Directory = Composite File = Leaf

15 Directory / File Example – Classes 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 Leaf Class: FileComposite Class: Directory

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

17 Directory / File Example – Operation Abstract Base Class: Node size() in bytes of entire directory and sub- directories Leaf Class: File size () of file Composite Class: Directory size () Sum of file sizes in this directory and its sub-directories 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 Leaf Class: Object Composite Class: Container Backpack Cloth BagJeweled Dagger RubyRingWine Bottle 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.