1. Design Patterns Alan Shalloway, James Trott, Design Patterns Explained, Addison-Wesley, 2002. Gamma, Helm, Johnson, Vlissides, Design Patterns, Elements of Reusable Object-Oriented Software, Addison-Wesley, 1995.

2. Perspectives in software development: Conceptual: represent concepts. Ignore how software implements it. (SRS) Specification: look at the software interfaces, not at the implementation Implementation: write code

3. Design Pattern Describes –problem that occurs repeatedly –the core of the solution to that problem Solution can be reused –not necessary to implement it the same way

4. Where do you see Design Patterns? Seminars and conferences. Technical journals. Text books and trade journals. You don’t really know OO until you understand these. They may help you understand OO better.

5. Where did they come from? Patterns start with a premise and answer two questions: –Premise: the goodness of a design is objective: not entirely in the eyes of the beholder. –Q1: What is present in good designs that is not present in poor ones? –Q2: What is present in poor designs that is not present in good ones?

6. Pattern: A solution to a problem in a context Name –Becomes part of our vocabulary –Allows abstract discussion of problem and approaches –Allows discussion of issues instead of implementation Purpose –Description of the desirable thing –Problems that prevent the desirable solution explicitly described

7. Vocabulary Example: find number in sorted list Find middle value and compare value we are looking for to that one. If found, we’re done. Otherwise, if we are looking for something bigger, set the new bottom to the current middle. Otherwise, set the new top to the current middle. Repeat the above step until the list can’t be divided further. If it’s not in the last two positions, it’s not in the list.

8. Example 2: find number in sorted list Use binary search.

9. Example 2: find number in sorted list Use binary search. This assumes that you know what binary search is. But if you know what binary search is, the discussion is greatly simplified. Algorithms are not patterns. This is an analogy.

10. Description of patterns: Name Intent Problem Solutionhow does the pattern solve the problem Participants Consequencesinvestigate forces at play ImplementationNote: this is not the pattern itself. References (GoF?)

11. Elements Pattern name Problem –Explains the problem and its context –may include a list of conditions needed before application of solution makes sense Solution Consequences

12. Elements Pattern name Problem Solution –Describes elements of design, relationships, responsibilities, and collaborations –Does not describe a particular implementation –Abstract description and how a general arrangements of objects solves problem Consequences

13. Elements Pattern name Problem Solution Consequences –Results and trade-offs of applying pattern –Needed to evaluate tradeoffs between solutions –Frequently time/space trade-offs

14. Why patterns? Reuse existing, high-quality solutions to common problems –incorporates a great deal of experience –may describe things often overlooked or subtle that prevent good solutions

15. Why patterns? Reuse existing, high-quality solutions to common problems –incorporates a great deal of experience –may describe things often overlooked or subtle that prevent good solutions Focus on issues not implementation –Shift thinking to higher level –Decide if design is right, not just working –Learn how to design –Improve code: smaller hierarchies, more modifiable code.

16. Strategies suggested by patterns 1.Design to interfaces

17. Strategies suggested by patterns 1.Design to interfaces 2.Favor composition over inheritance

18. Strategies suggested by patterns 1.Design to interfaces 2.Favor composition over inheritance 3.Find what varies and encapsulate it. Consider what should vary in your design. Consider what you want to be able to change without redesign (as opposed to what might force you to redesign) Encapsulate this

19. Catalog of Patterns GOF = “Gang of four” (1995) –Erich Gamma –Richard Helm –Ralph Johnson –John Vlissides 23 patterns Others since then

20. GOF: 23 Patterns Abstract Factory Adapter Bridge Builder Chain of Responsibility Command Composite Decorator Facade Factory Method Flyweight Interpreter Iterator Mediator Memento Observer Prototype Proxy Singleton State Strategy Template Method Visitor

21. Design Pattern Classification We classify design patterns by two criteria: –Purpose: Reflects what the Pattern does. –Scope: Specifies whether the pattern applies primarily to classes or to objects.

22. Design Pattern Purposes Patterns can have either creational, structural, or behavioral purpose. –Creational patterns: concern the process of object creation. –Structural patterns: deal with the composition of classes or objects. –Behavioral patterns: characterize the ways in which classes or objects interact and distribute responsibility.

23. Design Patterns Scopes Class patterns deal with relationships between classes and their subclasses. –These relationships are established through inheritance, so they are static—fixed at compile-time. Object patterns deal with object relationships, which can be changed at run- time and are more dynamic.

24. In General Almost all patterns use inheritance to some extent. The only patterns labeled "class patterns" are those that focus on class relationships. Note that most patterns are in the Object scope.

25. Solving Problems Patterns help you identify non-obvious abstractions –e. g. Strategy describes pattern for interchangeable families of algorithms Patterns can guide decisions about what is an object

26. How to Select a Design Pattern Consider how design patterns solve design problems Scan Intent sections Study how patterns interrelate and patterns of like purpose Consider what should be variable in your design

27. How to use a pattern Read the pattern paying attention to Applicability and Consequences Study the Structure, Participants, and Collaborations Look at Sample Code Choose good names for participants Define the classes, interfaces, and operations Implement the operations to carry out the responsibilities and collaborations

28. GOF Design Patterns App Android for $.99 Take a look at it… might be helpful for exam and/or project

29. Design Patterns: Model-View Separation

30. Motivation: Suppose you have a system that needs to run on a variety of devices –Maybe a cell phone and a PDA –Maybe Windows and with a command line –...

31. Motivation: Suppose we support both the command line and the GUI interfaces. What changes? What stays the same?

32. Motivation: Suppose we support both the command line and the GUI interfaces. What changes? What stays the same? How do you design to handle this?

33. Model-View Separation Pattern Model: The domain layer of objects. (objects that contain data and operations). View: The presentation layer of objects (windows, applets, reports).

34. Model-View Context Context/Problem –It is desirable to: de-couple domain (model) objects from windows (views) support increased reuse of domain objects minimize the impact of changes in the interface upon domain objects

35. Model-View Context Context/Problem It is desirable to de-couple domain (model) objects from windows (views), to support increased reuse of domain objects, and minimize the impact of changes in the interface upon domain objects. Solution Define the domain (model) classes so that they do not have direct coupling or visibility into the window (view) classes, and so that application data and functionality is maintained in domain classes, not window classes.

36. Model-View Window Configuration display( ) addEntry( ) query( ) Model View Goal: Classes in Model should not have direct visibility to classes in View.

37. Model-View Separation Motivation Motivation Focus more on the domain processes rather than on computer interfaces. Allow separate development of the model and user interface layers. Minimize the impact of changes in the interface upon the domain layer. Allow new views to be easily connected to an existing domain layer.

38. Problem Window Configuration display( ) addEntry( ) query( ) Model View Model classes talk to View classes. Worse displayMessage ( )

39. Model-View Separation Pattern Window Configuration display( ) addEntry( ) query( ) Model View query( ) View classes talk to Model classes. Better

40. Model-View Separation Pattern Configuration Old Window display( ) addEntry( ) query( ) Model View query( ) New Window display( ) The View Layer can be modified without affecting the Model layer.

41. Model-View Separation Pattern Configuration addEntry( ) query( ) Model View query( ) New Window display( ) When the Window needs to display data it queries the Model objects “Polling Model”

42. Model-View Separation Pattern Problem: Domain objects need to communicate with windows to cause a real- time ongoing display update as the state of information in the domain object changes. - Monitoring applications - Simulation applications Solution: Indirect Visibility

43. Model-View Separation Pattern with Indirect Visibility Named Publish-Subscribe Pattern Context/Problem: A change in state (an event) occurs within a Publisher of the event and other objects are dependant on or interested in this event (Subscribers to the event). The Publisher should not have direct knowledge of its subscribers. Solution: Define an event notification system so that the Publisher an indirectly notify Subscribers. Event Manager or Model View Controller (MVC).

44. Model-View Separation Pattern with Indirect Visibility http://gmoeck.github.com/2011/03/10/sproutcore-mvc-vs-rails-mvc.html

45. Model-View Separation Pattern with Indirect Visibility

46. Design Patterns: Façade

47. Façade Intent: want to simplify the use of an existing system. Need to define an interface that is a subset of the existing one. Problem: –You only need a subset of a complex system or need to interact in a particular way. –The API for the subset is simpler than the API for the entire subsystem. –You may want to hide the original system, or you may want to use a subset of the system and add functionality.

48. Façade Solution: The façade presents a new interface Participants: presents a specialized interface to the client to make it easier to use Consequences: –Simplifies the use of the subsystem. –Some functionality may not be available to the client. Implementation: –Define a new class (or classes) that has the required interface. –Have this class use the existing system.

49. Façade Example Huge database interface, but we only need a few of the functions. Rather than everyone learning the database interface, create the interface you need, and build a façade.

50. Without Façade ClientADatabaseClientBElementModel

51. With Façade ClientADatabaseClientBElementModelDBFacade

52. Other Patterns of Interest State Design Pattern Strategy Design Pattern Singleton Design Pattern Adapter Design Pattern