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Decorator Design Pattern
- 1. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Decorator Pattern Adeel Riaz Muhammad Shehyyar Muhammad Owais Sara Rehmatullah Ayesha Mehfooz Advisor: Muhammad Qasim Pasta PAF KIET Fall 11PAF-KIET CoCIS Dept.
- 2. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Concept of Decorating in Real World Before Decorating After Decorating PAF-KIET CoCIS Dept.
- 3. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Problem?? PAF-KIET CoCIS Dept.
- 4. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Decorator Pattern Approach PAF-KIET CoCIS Dept.
- 5. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Problem?? PAF-KIET CoCIS Dept. Whenever a team member becomes a team lead, we have to create a new object of team lead and the previous object that points to that employee (team member) may be destroyed. Another case is when an employee can perform responsibilities of a team member as well as those of a team lead or a manager can perform team leads responsibilities. In that case you need to create two objects for the same employee which is totally wrong.
- 6. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Decorator Pattern Approach PAF-KIET CoCIS Dept. Now, if we want to change responsibilities of an employee to manager we just need a new Manager (Decorator) and assigning that employee to it will solve our problem. Same is the case when a team lead’s responsibilities are revoked, and some other member becomes team lead, we just need to swap employee objects within TeamMember and TeamLead decorators.
- 7. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Intent The Decorator Pattern attaches additional responsibilities to an object dynamically. Decorators provide a flexible alternative to Sub-classing for extending functionality. PAF-KIET CoCIS Dept.
- 8. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Motivation • Sometimes we want to add responsibilities to individual objects not to an entire class. • Inheriting responsibilities from another class attaches them to every subclass instance statically. This is inflexible. PAF-KIET CoCIS Dept.
- 9. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Graphical user interface toolkit PAF-KIET CoCIS Dept.
- 10. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Structure PAF-KIET CoCIS Dept.
- 11. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Example PAF-KIET CoCIS Dept.
- 12. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Applicability PAF-KIET CoCIS Dept. •To add responsibilities to individual objects dynamically and transparently, that is, without affecting other objects. •For responsibilities that can be withdrawn. •When extension by sub-classing is impractical. Sometimes a large number of independent extensions are possible and would produce an explosion of subclasses to support every combination. Or a class definition may be hidden or otherwise unavailable for sub-classing.
- 13. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Participants • Component (VisualComponent) defines the interface for objects that can have responsibilities added to them dynamically. • ConcreteComponent (TextView) defines an object to which additional responsibilities can be attached. • Decorator maintains a reference to a Component object and defines an interface that conforms to Component‘s interface. • ConcreteDecorator (BorderDecorator, ScrollDecorator) adds responsibilities to the component. PAF-KIET CoCIS Dept.
- 14. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Participants PAF-KIET CoCIS Dept.
- 15. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Collaborations PAF-KIET CoCIS Dept. •Decorator forwards requests to its Component object. It may optionally perform additional operations before and after forwarding the request.
- 16. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Consequences • More flexible than static inheritance. • Avoids feature laden classes high up in hierarchy. • Lots of little objects that look alike. So it is hard to learn and debug. • A decorator and its components are not identical. So checking object identification can cause problems. PAF-KIET CoCIS Dept.
- 17. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Implementation Several issues should be considered when applying the Decorator pattern: 1. Interface conformance: A decorator object’s interface must conform to the interface of the component it decorates. 2. Omitting the abstract Decorator class: If only one responsibility is needed, don’t define abstract Decorator. Merge Decorator’s responsibility into the ConcreteDecorator. 3. Keeping Component classes light weight: Component class should be dedicated to defining an interface, no other functions. Keep it light and simple. A complex Component class might make Decorator too costly to use in quantity. 4. Changing the skin of an object versus its guts: Decorator classes should act as a layer of skin over an object. If there’s a need to change the object’s guts, use Strategy pattern. PAF-KIET CoCIS Dept.
- 18. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Builder VS Decorator PAF-KIET CoCIS Dept.
- 19. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level PAF-KIET CoCIS Dept. Builder VS Decorator
- 20. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Related Patterns PAF-KIET CoCIS Dept. •Adapter: A decorator is different from an adapter in that a decorator only changes an object‘s responsibilities, not its interface; an adapter will give an object a completely new interface. •Strategy : A decorator lets you change the skin of an object; a strategy lets you change the guts. These are two alternative ways of changing an object.