composite - Harsh4999/Design-Patterns GitHub Wiki

Composite

Use

• When we have a part-whole relationaship or hierarchy of objects and we want to be abe to treat all objects in this hierarchy uniformly
• Allows us to treat in same way when they are in a tree structure
• Its not simple composition

UML

• Component = abstract base class which defines behaviour common to all classes it adds method which allows the access to children in base class
• Leaf = Class which represnts the leaf node implementation for operations and functionality which is required by the client
• Composite = Stores child components

Implementation

• Component must declare all methods that are applicable to both leaf and composite
• We have to choose who defines the children management operations component or composite.
• We implement the composite an operation invoked on composite is propogated to all its children
• In leaf nodes we have to handle the non-applicable operations like add/remove a child if they are defined in component
• Decision needs to be made about where child management operations are deinfined. Defining them on component provides transparency but leaf nodes are forced to implement those methods
• Eg: UIViewRoot