DESIGN PATTERN NOTES

Introduction

Pattern

The outline of reuasble solution to a general problem encounter in a particular context.

Types of design patterns

• Behavioral patterns: Deal with responsibles and communication between objects (Ex: Strategy)
• Creational patterns: Deal with instantialation of objects (Ex: Singleton)
• Structural patterns: Deal with composition and relations (Ex: Adapter)

Stratergy pattern

Notes:

• Define a family of algorithms, encapsulate each one and make them interchangable
• Let the algorithm vary independently from clients that use it.
• The stratergy pattern useful for situations where it is necessary to dynamically swap the algorithms used in application.
• Use composition instead of inheritance

Model:

Singleton pattern

Notes

• Singleton pattern is a convention for ensuring one and only one object is instantiated for a given class.
• The Singleton pattern ensures a class has only be instance and provides a point to access to it.
• Use private constructor to guarantee that no morethan one instance will be created.
• The single instance must also be accessible to all classes that require it.
• 2 types of initializations:
  • Eager initialization: initiating the class at the time of class loading. Drawbacks: always initialized whether it is being used or not.
  • Lazy initatialization: initiated only when it is required, save you form instanting the class when you don't need it.
• Thread safe solutions:
  • Synchroized GetInstance(): expensive
  • Eager initialization: always initialized whether it is being used or not.
  • Double checked locking: best practice

Model

Observer pattern

Notes

• The Observer design pattern defines a one-to-many dependency between objects, so that when one object changes states, all of its dependents are notified and updated automatically.
• A newspaper subscription service with its publisher and subscribers is a good way to visualize observer pattern.
• The Observer pattern provides an object design where subjects and observers are loosely coupled.
• You should use this pattern in your application where multiple objects are dependent on the state of one object.
• Applications: Event Listener, Email subscription, Youtube Notification.

Model

Adapter pattern

Notes

• A wrapper allows 2 incompatible interfaces to work together.
• The Adapter pattern onverts the interface of a class into another interface the client expect.
• How clients use adapter
  • The client makes a request to the adapter by calling a method on it using the target interface.
  • The adapter translates the request into one or more calls on the adaptees ising adaptee interface.
  • The client recieves the results of the call and never knows there is an adpter doing the translation.
• There are two types of adapters
  • Object adapter: Use composition, Adapter has an Adaptee.
  • Class adapter: Use inheritance, Adapter is subclass of both Target and Adaptee.
• Remember that adapter pattern doesn't create new responsibilities.

Illustration

(source: https://www.amazon.com/Head-First-Design-Patterns-Brain-Friendly/dp/0596007124)

Model

Class Adapter

(source: https://vngeeks.com/adapter-design-pattern/)

Object Adapter

(source: https://www.dotnettricks.com/learn/designpatterns/adapter-design-pattern-dotnet)

Decorator pattern

Notes

• The Decorator pattern provides a flexible alternative to subclassing for extending functionality
• Decorators attached additional responsibilities to an object dynamically
• Decorator pattern involves a set of decorator classes that are used to wrap around concrete components
• Decorator classes mirror the type of the components (Have the same type, through inheritance or interface implementation)
• Decorators change the behavior of their components by adding new functionality before / after method class to the component.
• Drawback: Decorators can results in many small objects in design and overuse can be complex.
• Application: File reader

Model

(source: https://www.oreilly.com/library/view/learning-javascript-design/9781449334840/ch09s14.html)