BTS530: Major Project Planning and Design The Composite Pattern All References and Material From: Design Patterns,by (GOF!) E.Gamma, R.Helm, R.Johnson, J.Vlissides, Addison-Wesley, 2001

Composite The Problem: How do you treat a group or composition structure of objects the same way (polymorphically) as a non- composite object?

Composite A Suggestion/Solution: Define classes for composite and atomic object so that they implement the same interface.

Composite Classic example: Graphics applications users can group components into larger components which can be grouped to form larger components…and so on. problem: the code must treat primitive and container objects differently even though the user might treat them the same Composite pattern uses recursive composition so clients don’t have to make a distinction Design Patterns, p.163

Graphic draw() add(Graphic g) remove(Graphic) getChild(int) Line draw() Picture draw() add(Graphic g) remove(Graphic) getChild(int) Rectangle draw() Text draw() 1…n forall g in graphics g.draw() add g to list of graphics graphics Design Patterns, p.163

aPicture aLineaRectangle aTextaLineaRectangle Design Patterns, p.164

Component operation() add(Component) remove(Component) getChild(int) Composite operation() add(Component) remove(Component) getChild(int) Leaf operation() 1…n forall c in children c.operation() Client children Design Patterns, p.164

aComposite aLeaf Design Patterns, p.165

Composite Component (Graphic): declares the interface for objects in the composition implements common default behaviour declares an interface for managing child components Leaf (Rectangle, Line, Text) has no children defines behaviour for primitives Composite (Picture) defines behaviour for components having children stores child components implements child-related operations through the component interface Design Patterns, p.165

Composite The Client: uses the Component class interface to interact with objects in the composite structure If recipient is Leaf request handled directly If recipient is Composite request is usually forwarded to child components, possibly performing additional operations before and/or after forwarding Design Patterns, p.165

Composite Exercise draw a sequence diagram to illustrate: draw line l draw circle c group l and c into graphicA draw box b1 draw box b2 group b1 and b2 into graphicB group graphicB into graphicA select c in graphicA (assume lines, circles, boxes exist but not graphics)