Some Cool Design Patterns, Practices, and Principles

Some Cool Design Patterns, Practices, and Principles

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  Presented by ArsalanAhmed
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  What Are DesignPatterns?  Well-understood code patterns  Solve common problems  Create common pattern vocabulary  Speed up design and development  Encourage creating design standards
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  Useful Design Principles Prefer Composition over Inheritance  Program to abstractions instead of concrete implementations  Design loosely coupled components  Encapsulate what varies  Hide the likely-to-change code behind an interface
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  Time-Tested Design Principles S – Single Responsibility Principle  One reason to change per class  O – Open-Closed Principle  Classes open for extension, closed for modification  L – Liskov Substitution Principle  Classes can be substituted if designed to be referenced via their public contracts  I – Interface Segregation Principle  Interfaces should not be bloated  D – Dependency Inversion Principle  Classes should not be responsible for creating their dependencies
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  Anti-Patterns  Commonly used Often seem harmless  Degrade overall design  Avoid like a plague!
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  Example Anti Patterns The Arrowhead  Constructor Over-Injection  Gold Plating  Interface Bloat  God Object  Magic Literals  Reinventing The Wheel
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  The Gang OfFour  Authors of the canonical book Design Patterns: Elements of Reusable Object- Oriented Software  Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides  Main theme 1: Program to an interface instead of an implementation  Main theme 2: Favor composition over inheritance
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  GoF Design Patterns Creational: Abstract Factory, Builder, Factory Method, Prototype, Singleton  Structural: Adapter, Bridge, Composite, Decorator, Façade, Flyweight, Proxy  Behavioral: Chain of Responsibility, Command, Interpreter, Iterator, Mediator, Memento, Observer, State, Strategy, Template, Visitor
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  Enterprise Design Patterns Patterns found in enterprise applications  Major proponent: Martin Fowler of Thoughtworks  Enterprise patterns go beyond GoF patterns  Often specific to technology stacks
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  Pattern: The Command Source:dofactory.com
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  What Makes ItUseful?  Repeatable pattern to execute tasks as objects  Allows parameterizing command invokers with different commands  Allows undo operations  Meta-Command Pattern allows creating composite commands  Decouples worker objects from client objects
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  Practical Uses  Backgroundprocessing agents  Allow third-party vendors to create and distribute custom commands  Supports queuing requests  Facilitates logging requests  By serializing/deserializing objects to/from log files  Log contains history of all command objects  Allows implementing transaction semantics
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  Command Pattern Demo
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  Pattern: The Decorator Source:dofactory.com
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  What Makes ItUseful?  Facilitates implementing the open- closed principle  Allows adding new functionality without modifying existing classes  Promotes composition over inheritance  Promotes iterative development
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  Practical Uses  Dynamicallycreating specialized objects  Provide flexible API with object decorators  Example uses in the .Net framework  BufferedStream decorates MemoryStream
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  Decorator Pattern Demo
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  Before Using TheDecorator
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  After Using TheDecorator
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  Pattern: Lazy Load Avoid performing time-consuming operations unless unavoidable  Load data from database or an external service as late as possible  Cache fetched data for as long as feasible  Reduce database and/or network load
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  Code Sample Private SharedReadOnly Property DTSearchIndexLocation() As String Get If String.IsNullOrEmpty(_dtSearchIndexLocation) Then ' fetch from database and assign to _dtSearchIndexLocation End If Return _dtSearchIndexLocation End Get End Property
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  Design Patterns In.Net  Observer  Iterator  Decorator  Adapter  Factory  Strategy  Composite  Template Method  Intercepting Filter  Page Controller
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  Further Study  Writingsof The Godfather of Enterprise Design Patterns a.k.a. Martin Fowler  Martinfowler.com  Head-First Design Patterns by O’Reilly  Writings of Uncle Bob a.k.a. Robert C. Martin  ButUncleBob.com, blog.objectmentor.com  Presentations by Jeremy Miller  codebetter.com/blogs/jeremy.miller