1 / 35

Design Patterns

Don’t reinvent the wheel. Design Patterns. The Design Patterns Book. Design Patterns. “simple and elegant solutions to specific problems in object-oriented software design”

chin
Download Presentation

Design Patterns

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Don’t reinvent the wheel Design Patterns

  2. The Design Patterns Book

  3. Design Patterns • “simple and elegant solutions to specific problems in object-oriented software design” • If there are “tried and true” approaches that have worked on similar problems in the past, you can benefit by adopting them. • Design patterns can enhance the vocabulary of design • The people you are talking to have to know the names and meanings of the patterns

  4. Patterns are Reusable Solutions • Each pattern has • A pattern name • Naming the pattern provides powerful short-hand for design solutions • A problem description • The problem in general terms • A solution • Describes the design • A set of consequences • Trade-offs involved in applying the pattern

  5. Inheritance versus Composition • Object Inheritance (is a) • Whitebox reuse • Internals of parent class often visible • Breaks encapsulation • Can’t change at runtime • Non-abstract base classes define part of physical representation • Directly supported by language (polymorphism) • Easy to modify an existing implementation • Object Composition (has a) • Blackbox reuse • Internals of delegate class not visible • Can change at runtime (limited by base class data type) • Fewer implementation dependencies • Smaller hierarchies

  6. Prefer Composition to Inheritance • Delegation • A object contains a delegate object • Requests to the object are explicitly referred to the delegate, similarly to how a subclass implicitly refers functions that have not been overridden to its parent class. • Disadvantage • Dynamic, parameterized software is harder to understand

  7. Singleton • Intent: • Ensure a class only has one instance, and provide a global point of access to it. • Motivation • It’s important for some classes to have exactly one instance. • We often need global access to just objects • Applicability • Exactly one instance of a class must be accessible to clients from a well-known access point • The sole instance should be extensible by subclassing

  8. Implementation in C++ • Static Singleton* Instance(); • Instance does one of the following • Creates an instance of the class and returns it, storing the value as a static private data member • Returns a previously created instance • Protected Singleton constructor • External code cannot create an instance of the singleton class

  9. Singleton Example • SpriteLand • Design Patterns uses Instance() • We use getSpriteLand() • Look at SpriteLand implementation

  10. Multithreading Considerations for Singleton from “Head First Design Patterns” • Need some synchronization to prevent the creation of multiple instances • Use “synchronized” keyword for Java constructor • private static synchronized Singleton getInstance() • Create the single instance before any calls to the singleton class • private static Singleton instance_ = new Singleton(); • Use “double-checked locking” • If (instance_ == null) Synchronized (Singleton.class) { if (instance_ == null) { uniqueInstance = new Singleton();

  11. Consider needs for mutual exclusion in other singleton methods. • Remember, access to data member in the singleton are shared and might require synchronization.

  12. Creational Patterns • Factory • Abstract Factory • Builder • Prototype • Singleton

  13. Factory Method (Virtual Constructor) • Define an interface for creating an object, but let subclasses decide which class to instantiate. • class TowerDefenseGame • { • // Factory methods • virtual Path* makePath(); • virtual Tower* makeMonkeyTower(); • } • // The TowerDefenseGame can be subclassed to //EasyTowerDefenseGame and DifficultTowerDefenseGame • makePath() and makeMonkeyTower() are overridden so that EasyTowerDefenseGame::makePath() returns an instance of class EasyPath, and DifficultTowerDefenseGame::makePath() return an instance of class DifficultPath.

  14. Abstract Factory (or Kit)(Factory that uses delegation rather than inheritance) • Provide an interface for creating families of related or dependent objects without specifying their concrete classes. • class ComponentFactory • { • Path* makePath() = 0; • Tower* makeMonkeyTower() = 0; • }; • class TowerDefense • { • public: • Path* makePath() { componentFactory->makePath(); } • private: • ComponentFactory* componentFactory_; • };

  15. Prototype • Specify the kinds of objects to create using a prototypical instance, and create new objects by copying the prototype. • class TowerDefenseGame • { • public: • TowerDefenseGame(Path* prototypePath, • Tower* prototypeMonkeyTower) : prototypePath_(prototypePath), prototypeTower_(prototypeTower) {} • Path* makePath() { return prototypePath_.clone(); } • Tower* makeMonkeyTower() { return prototypeTower_.clone(); } • private: • Path* prototypePath_; • Tower* prototypeTower_; • }

  16. Structural Design Patterns • How classes and objects are composed to form larger structures • Structural class patterns use inheritance • Structural object patterns use composition

  17. Adapter (aka Wrapper) • Convert the interface of a class into another interface clients expect. • Can use multiple inheritance • Structural class pattern • Can use composition • Structural object pattern

  18. Bridge (aka Handle/Body aka PIMPL) • Decouple an abstraction from its implementation so that the two can vary independently. • C++ • Pointer to implementation • class myClass • { • public: • // Define public interface private: myClassImpl* implementation; • };

  19. Uses for Bridge • Completely divorce public interface from implementation. (C++ headers) • Allow different implementations to be assigned at runtime. • You want to share an implementation between multiple objects

  20. Some Sample Patterns (from the Design Patterns Book) • Singleton • Ensure a class only has one instance, and provide a global point of access to it. • Factory Method • Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses. • Bridge • Decouple an abstraction from its implementation so that the two can vary independently • Façade • 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.

  21. Composite • Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly. • Problem – We want to build composite objects from primitive objects, and then use the composites as though they are primitives. • Abstract base class represents both primitives and their containers. • Windows Forms use this pattern. • Windows have components that can also be windows • Messages sent to windows are forwarded to their components.

  22. Decorator • Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality. • Decorators are implemented as classes that contain an instance (component) of the thing that they are decorating • Example • Window contained in a ScrollingDecorator which in turn is contained in a BorderDecorator. Window requests are routed first to the outer-most decorator and forward to inner decorators until finally reaching the window.

  23. Decorator Implementation • Decorator must conform to interface of decorated component. • Decorators should be light-weight (focus on interface, not data storage). • Change the skin of an object rather than change the guts.

  24. Facade • Provide a unified interface to a set of interfaces in a subsystem • Provide a single-point of contact between subsystems • Provide a task-oriented interface to a set of components (compile, rather than scan, parse, generate code, optimize, etc). • Decouple dependencies between components and component clients.

  25. Flyweight • Use sharing to support large numbers of fine-grained objects efficiently • Flyweight is a shared object that can be used in multiple contexts simultaneously • Intrinsic state • What is stored inside the flyweight • Independent of flyweight’s context • Extrinsic state • Client objects track the extrinsic state and pass it to member functions that need it. • Example • Flyweight for each letter in a text document • Position and typographic style are extrinsic state

  26. Flyweight Applicability • Application uses a large number of objects • Most object state can be extrinsic • Many instances can make use of a shared object.

  27. Proxy • Provide a surrogate or placeholder for another object to control access to it. • Uses • An expensive class can be replaced by a proxy until it must be created • Control access based on a set of access rights • Smart pointers!

  28. Behavioral Patterns • Behavioral class patterns • Use inheritance • Behavioral object patterns • Use composition

  29. Chain of Responsibility • Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it. • Each object, starting with the first in in a chain of objects, get a chance to handle the request, or pass it along the chain. • Example • Windows

  30. Command • Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue, or log requests, and support undoable operations. • Use when you want to build an object that invokes a command (like a UI button), and you don’t want to tie a button press to a hard-coded function. • Delegates in C# come to mind • Execute() can store state to provide an Unexecute() function.

  31. Iterator (aka Curso) • Provide a way to access the elements of an aggregate (collection) object sequentially without exposing its underlying representation. • Support variation in traversal method • Preorder, postorder, inorder tree traversal • Simplify the interface • Use of multiple iterators allows us to keep track of multiple places in the aggregate.

  32. Mediator • Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently. • Each object knows only of the Mediator • Reduce coupling between components • Mediator must know objects and interactions • Centralizes control • Simplifies object protocols

  33. Memento (aka Token) • Without violating encapsulation, capture and externalize an object’s internal state. • Often used to restore state via undo operations • Originator (the class whose state will be saved as a memento) supports the methods • CreateMemento() // Create a memento with state • SetMemento(Memento m) // Restore state • Mementos are opaque objects that are used only for setMemento operations

  34. Observer (aka publish-subscribe) • Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. • Example: • Spreadsheet data used to generate a grid-view of the data and several different types of data • When the underlying data changes, all presentations must be updated. • The “Subject” knows its observers • Subject notifies each observer of change • Upon notification, observers request data from subject • In Model/View/Controller pattern, model is subject, views are observers.

  35. State • Allow an object to alter its behavior when its internal state changes. The object will appear to change its class. • Abstract “State” class contains a pointer to concrete state classes that to whom all requests are delegated. • Example: • Person class might be implemented as an abstract state where • Sleeping • Working • Playing provide concrete implementations of “answerPhone()”

More Related