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Some Cool Design Patterns, Practices, and Principles

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itsarsalan

Design patterns are reusable code solutions to common programming problems. They create a common vocabulary that allows designers and developers to communicate effectively. Some key design patterns include the Command pattern, which encapsulates executing operations as objects, and the Decorator pattern, which allows adding new functionality to objects without modifying them. Using patterns like Lazy Load avoids unnecessary processing until needed.

Related topics:
Design Principles • Software Design Patterns
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Presented by Arsalan Ahmed
What Are Design Patterns?  Well-understood code patterns  Solve common problems  Create common pattern vocabulary  Speed up design and development  Encourage creating design standards
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
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
Anti-Patterns  Commonly used  Often seem harmless  Degrade overall design  Avoid like a plague!
Example Anti Patterns  The Arrowhead  Constructor Over-Injection  Gold Plating  Interface Bloat  God Object  Magic Literals  Reinventing The Wheel
The Gang Of Four  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
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
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
Pattern: The Command Source: dofactory.com
What Makes It Useful?  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
Practical Uses  Background processing 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
Command Pattern Demo
Pattern: The Decorator Source: dofactory.com
What Makes It Useful?  Facilitates implementing the open- closed principle  Allows adding new functionality without modifying existing classes  Promotes composition over inheritance  Promotes iterative development
Practical Uses  Dynamically creating specialized objects  Provide flexible API with object decorators  Example uses in the .Net framework  BufferedStream decorates MemoryStream
Decorator Pattern Demo
Before Using The Decorator
After Using The Decorator
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
Code Sample Private Shared ReadOnly 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
Design Patterns In .Net  Observer  Iterator  Decorator  Adapter  Factory  Strategy  Composite  Template Method  Intercepting Filter  Page Controller
Further Study  Writings of 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

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Some Cool Design Patterns, Practices, and Principles

  • 2. What Are Design Patterns?  Well-understood code patterns  Solve common problems  Create common pattern vocabulary  Speed up design and development  Encourage creating design standards
  • 3. 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
  • 4. 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
  • 5. Anti-Patterns  Commonly used  Often seem harmless  Degrade overall design  Avoid like a plague!
  • 6. Example Anti Patterns  The Arrowhead  Constructor Over-Injection  Gold Plating  Interface Bloat  God Object  Magic Literals  Reinventing The Wheel
  • 7. The Gang Of Four  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
  • 8. 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
  • 9. 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
  • 11. What Makes It Useful?  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
  • 12. Practical Uses  Background processing 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
  • 15. What Makes It Useful?  Facilitates implementing the open- closed principle  Allows adding new functionality without modifying existing classes  Promotes composition over inheritance  Promotes iterative development
  • 16. Practical Uses  Dynamically creating specialized objects  Provide flexible API with object decorators  Example uses in the .Net framework  BufferedStream decorates MemoryStream
  • 18. Before Using The Decorator
  • 19. After Using The Decorator
  • 20. 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
  • 21. Code Sample Private Shared ReadOnly 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
  • 22. Design Patterns In .Net  Observer  Iterator  Decorator  Adapter  Factory  Strategy  Composite  Template Method  Intercepting Filter  Page Controller
  • 23. Further Study  Writings of 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

Editor's Notes

  • #3: Say more with less, shared vocabulary speeds up development and reduces chances of confusion and misunderstanding.
  • #7: Constructor Over Injection: If the class can function without a particular dependent object, it shouldn’t demand it in the constructor.