Design Patterns

1. Design Patterns Source: “Design Patterns”, Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides And http://java.sun.com/blueprints/patterns Created By Eshcar Hilel

2. Agenda • Design Patterns - Why? • Model-View-Controller Pattern • What Is the Problem? • Solution, Structure, Responsibilities • Class Diagram • Observer Pattern • (Composite Pattern) • Strategy Pattern • MVC variants examples Design Patterns

3. Design Patterns - Why? • Designing OO software is hard • Designing reusable OO software – harder • Experienced OO designers make good design • New designers tend to fall back on non-OO techniques used before • Experienced designers know something – what is it? Design Patterns

4. Design Patterns - Why? • Expert designers know not to solve every problem from first principles • They reuse solutions • These patterns make OO designs more flexible, elegant, and ultimately reusable Analogy II – Telenovela!! Analogy I - Novelists and playwrights: “Tragically Flawed Hero” (Macbeth, Hamlet...) “The Romantic Novel” Design Patterns

5. Two Major Principles of Object-Oriented Design: Dynamic binding, polymorphism... • Program to an interface, not an implementation. • Favor object compositions over class inheritance. White-box reuse vs. Black-box reuse See more of this in… OOD Design Patterns

6. Model-View-Controller Pattern • MVC consists of three kinds of objects: • Model – the application object • View – UI (screen presentation) • Controller – defines the way the UI reacts to user inputs Design Patterns

7. MVC – What Is the Problem? • The same enterprise data needs to be accessed when presented in different views: e.g. HTML, JFC/swing, XML • The same enterprise data needs to be updated through different interactions • Supporting multiple types of views and interactions should not impact the components that provide the core functionality of the enterprise application Design Patterns

8. MVC – Solution • Separate core business model functionality from the presentation and control logic that uses this functionality • Allows multiple views to share the same enterprise data model • Makes supporting multiple clients easier to implement, test, and maintain Design Patterns

9. MVC Structure Design Patterns

10. MVC – Responsibilities • Model - the model represents enterprise data and the business rules that govern access to and updates of this data • View -the view renders the contents of a model. It accesses enterprise data through the model and specifies how that data should be presented • Controller - the controller translates interactions with the view into actions to be performed by the model Design Patterns

11. MVC – Class Diagram Design Patterns

12. MVC – Class Diagram Strategy Pattern Observer Pattern Design Patterns

13. MVC – Class Diagram(including interfaces) Design Patterns

14. MVC – Class Diagram(including interfaces) controller view model Design Patterns

15. Observer PatternClass Diagram Design Patterns

16. Observer Pattern • Define a one-to-many dependency, all the dependents are notified and updated automatically • The interaction is known as publish-subscribe or subscribe-notify • Avoiding observer-specific update protocol: pull model vs. push model • Other consequences and open issues Design Patterns

17. Observer Pattern Sequence Diagram Design Patterns

18. Composite Pattern • View can be nested • Described by the Composite design pattern seen in the lecture Design Patterns

19. Strategy Pattern – Class Diagram Design Patterns

20. Strategy Pattern • Define family of algorithms, encapsulates them and make them interchangeable • Eliminate conditional statements • Cons: communication overhead, increase number of objects Design Patterns

21. MVC Variants

22. Three Tier Model Design Patterns

23. MVC in Eliza ElizaUI <<ElizaModelInterface>> implements ElizaModel Design Patterns

24. The End