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Eran Toch technion.ac.il/~erant

Winter 2007. Abstracting Solutions with Design Patterns. Eran Toch http://www.technion.ac.il/~erant. Agenda. What are Design Patterns? Composite Pattern Façade Pattern 3-Tier Pattern Go Back to a Safe Place Pattern Summary. Patterns in Architecture.

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Eran Toch technion.ac.il/~erant

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  1. Winter 2007 Abstracting Solutions with Design Patterns Eran Toch http://www.technion.ac.il/~erant

  2. Agenda • What are Design Patterns? • Composite Pattern • Façade Pattern • 3-Tier Pattern • Go Back to a Safe Place Pattern • Summary

  3. Patterns in Architecture • Does this room makes you feel happy? • Why? • Light (direction) • Proportions • Symmetry • Furniture • And more… Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  4. What is a Design Pattern? A description of a recurrent problem and of the core of possible solutions. In Short, a solution for a typical problem Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  5. Why do we need Patterns? • Reusing design knowledge • Problems are not always unique. Reusing existing experience might be useful. • Patterns give us hints to “where to look for problems”. • Establish common terminology • Easier to say, "We need a Façade here“. • Provide a higher level prospective • Frees us from dealing with the details too early • In short, it’s a “reference” Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  6. History of Design Patterns Architecture Christopher Alexander The Timeless Way of BuildingA Pattern Language: Towns, Buildings, Construction 1970’ Object OrientedSoftware Design Gang of Four (GoF)Design Patterns: Elements of Reusable Object-Oriented Software 1995’ Other Areas: HCI, Organizational Behavior, Education, Concurent Programming… Many Authors 2007’ Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  7. Structure of a design pattern* • Pattern Name and Classification • Intent • a Short statement about what the pattern does • Motivation • A scenario that illustrates where the pattern would be useful • Applicability • Situations where the pattern can be used *According to GoF Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  8. Structure – cont’d • Structure • A graphical representation of the pattern • Participants • The classes and objects participating in the pattern • Collaborations • How to do the participants interact to carry out their responsibilities? • Consequences • What are the pros and cons of using the pattern? • Implementation • Hints and techniques for implementing the pattern Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  9. Classification of GoF Patterns Taken from Vince Huston’s site about Design Patterns http://home.earthlink.net/~huston2/dp/patterns.html Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  10. Agenda • What are Design Patterns? • Composite Pattern • Façade Pattern • 3-Tier Pattern • Go Back to a Safe Place Pattern • Summary

  11. An Example Client Files Folders Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  12. A Solution? • Folder: • For each action (delete, display, copy etc), there is special treatment for files and folders. • Explorer: • Each type of object is manipulated separately • Scalability: • What if there are more types of elements (disks, CD, USB…) Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  13. The Composite Solution Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  14. Composite: Structure • An abstract base class (Component) specifies the uniform behavior. • Primitive and Composite classes are subclassed. • Composite Manages components uniformly, using add and remove. Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  15. Composite: Consequences • Good • It makes it easy to add new kinds of components. • It makes clients simpler. • Bad • It makes it harder to restrict the type of components of a composite. Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  16. Composite: When to Use? • Tree structures that represent part-whole hierarchies. • Uniform access. Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  17. Composite: Known Uses Graphical User Interface Arithmetic Expressions A * (B + (C * D)) * A () + B * C D Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  18. Agenda • What are Design Patterns? • Composite Pattern • Façade Pattern • 3-Tier Pattern • Go Back to a Safe Place Pattern • Summary

  19. Façade – an Example • A robot has four classes: • Camera (that can identify objects) • Arm (that can move) • Pillars (that can grab) Subsystem: operator Subsystem: mechanics Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  20. Problems • How much does the operator needs to know regarding the mechanics? • For instance, we want to identify an object and move it to some predefined location. • Problem: No encapsulation • Operator needs to know a lot: structure + behavior • Preconditions • Agility • oldLocation = Camera.identify(object); • Arm.move(oldLocation); • Pliers.close(); • Arm.move(newLocation); • Pliers.open(); Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  21. Façade Pattern Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  22. Façade - Definition • Intent • Provide a unified interface to a set of interfaces in a subsystem. • Façade defines a higher-level interface that makes the subsystem easier to use. • Applicability • Use the Façade pattern • to provide a simple, default view of a complex subsystem. • to layer your subsystems. Use a façade as the entry point to each layer. Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  23. Façade Structure Operator interacts only with the Façade class The Façade Class knows and controls the subsystem objects Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  24. Façade - Consequences • Good: • Shields clients from subsystem components • It promotes weak coupling between the subsystem and its clients. • Façades help layer a system • Bad: • Duplicated methods • Lower visibility of subsystem functionality Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  25. Façade – the System Level • General patterns for constructing subsystems. • Each subsystem is represented by a façade interface • Inner details are encapsulated Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  26. More Structural Patterns • Adapter - Match interfaces of different classes • Bridge - Separates an object’s interface from its implementation • Decorator - Add responsibilities to objects dynamically • Flyweight - A fine-grained instance used for efficient sharing • Proxy - An object representing another object Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  27. Agenda • What are Design Patterns? • Composite Pattern • Façade Pattern • 3-Tier Pattern • Go Back to a Safe Place Pattern • Summary

  28. Bank Services: Multiple Channels Web SMS * Taken from a bank Leumi commercial * Taken from a bank Leumi commercial Terminal Branch Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  29. Managing Systems in Banks • Doing things twice • Same data? • Same processes? • Views similar? • Agility • Legislation changes processes. • Business needs drive new services, new views • Manageability • Different teams working on views, processes, data. Web Terminal Date Date Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  30. 3-Tier Pattern • Intent • Breaking the application to three parts: • Boundary (view) • Control (process) • Entity (data) • Centralizing the business logic of the application from the scattered methods to the control classes • Origin • Not a GoF pattern • By Ivar Jacobson • Based on the MVC (Model-View-Controller) pattern See: Rumbaugh et al, Object Oriented Modeling and Design, Prentice Hall, 1991. Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  31. 3-Tier Structure Represents the system’s interface to the outside world Represents application processes Represents consistent data Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  32. Crossing from Use-Cases into 3-Tier • 3-tier allow us to translate use-case requirements into an analysis model in a more intuitive way • Let’s look at the following use-case: Accounting system Order handling Sales person Inventory system Inventory reporting Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  33. Step 1 – Boundary Tier In the simplest form, each actor gets an interface: However, it can be more complicated. Several interfaces can serve one actor, generalization can be used, and several actors can share interfaces Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  34. Boundary Tier Member Distribution • Boundary tier objects are created when the session starts and are destroyed when the session ends. • Contains: • Methods operated by the user. • Attributes of the interface. Interface configuration properties Pull messages Push messages Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  35. Step 2 – Control Tier We add a control tier. Basically, every use-case gets a control class that handles it’s process Controls can be decomposed into sub-processes. Again, inheritance can be used Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  36. Control Tier Member Distribution • Control objects are created when the action (use-case) starts and are destroyed when it ends • Contain mainly methods, some of them double the ones in the boundary tier objects • May contain attributes that are used for the process’s flow management Process state variables Flow management of other control objects Calling interface objects Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  37. Step 3 – Entity Tier We add the Entity classes. These classes compose most of the classes in the regular class-diagram mode Entity classes have mainly associations, rather than dependencies, because they are consistent through the processes Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  38. Entity Tier Member Distribution • Entity tier objects represent the consistent data in the system. They are created and destroyed according to the processes that control them • Contain mainly attributes and some data-handling methods Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  39. 3-Tier Consequences • Good: • Re-use of Model components • Flexible processes • Easier support for new types of views • Bad: • Increased design complexity. Web Terminal Stocks data Money transfer Date Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  40. Behavioral Patterns • Chain of Responsibilities - A way of passing a request between a chain of objects • Command - Encapsulate a command request as an object • Interpreter - A way to include language elements in a program • Iterator - Sequentially access the elements of a collection • Mediator - Defines simplified communication between classes • Memento - Capture and restore an object's internal state Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  41. Behavioral Patterns – cont’d • Observer - A way of notifying change to a number of classes • State - Alter an object's behavior when its state changes • Strategy - Encapsulates an algorithm inside a class • Template Method - Defer the exact steps of an algorithm to a subclass • Visitor - Defines a new operation to a class without change Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  42. Agenda • What are Design Patterns? • Composite Pattern • Façade Pattern • 3-Tier Pattern • Go Back to a Safe Place Pattern • Summary

  43. Human / Computer Interaction Patterns • Human / Computer Interaction is about how users interact with computers • Some interactions are recurrent and can be expressed by patterns: • Form • Control Panel • WYSIWYG Editor • Hierarchical Set • Map of Navigable Spaces • Go Back to a Safe Place • More… Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  44. Computers can be SCARY Sometimes an innocent user gets into a state she don’t want to be in… And then, the terror!!! Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  45. Solution • Provide a way to go back to a checkpoint of the user's choice. The "Home" button and the “Back” The “undo" feature Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  46. Agenda • What are Design Patterns? • Composite Pattern • Façade Pattern • 3-Tier Pattern • Go Back to a Safe Place Pattern • Summary

  47. Relevancy to the Course • Most of the design patterns are relevant to the design and coding phases • However, some of them are very relevant as a tool for the analysis phase Chain of resp. Visitor Flyweight 3-Tier Façade Composite Singleton Prototype Analysis Design Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  48. Summary • Patterns learned: • Composite, Façade, 3-Tier, Go Back to a Safe Place. • Advantages of Design Patterns: • They capture expertise and make it accessible to non-experts. • They form a vocabulary that helps developers communicate better. • They help people understand a systems more quickly when it is documented with the patterns it uses. Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  49. References • Design Patterns: Elements of Reusable Object-Oriented Software, Gamma E. et el., 1995, Addison-Wesley. • The original patterns for architecture by C. Alexanderhttp://www.enumerable.com/dev/leveltwo/patternsframe.htm?/dev/leveltwo/../apl/twopanel-apl-towns.htm • A course from Bob Tarr from UMBC Universityhttp://www.research.umbc.edu/~tarr/dp/fall00/cs491.html • The Design Patterns Java Companion, James W. Cooper (an online version of the book)http://www.patterndepot.com/put/8/JavaPatterns.htm • A Site dedicated to Design Patterns by Vince Hustonhttp://home.earthlink.net/~huston2/dp/patterns.html Intro | Composite | Façade | 3-Tier| Safe Place | Summary

  50. References – con’d • Seven Habits of Successful Pattern Writers, John Vlissideshttp://hillside.net/patterns/papers/7habits.html • COMMON GROUND: A Pattern Language for Human-Computer Interface Design, Jenifer Tidwell, http://www.mit.edu/~jtidwell/common_ground_onefile.html • Design patterns for .NET: http://www.dofactory.com/Patterns/Patterns.aspx • Patterns for Concurrent, Parallel, and Distributed Systemshttp://www.cs.wustl.edu/~schmidt/patterns-ace.html • J2EE Patterns Cataloghttp://java.sun.com/blueprints/patterns/catalog.html Intro | Composite | Façade | 3-Tier| Safe Place | Summary

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