Design Patterns :

• These are typical solution to common problems in software design.
• Each design pattern is like a blueprint to solve a particular design problem.
• The pattern is not a specific piece of code, but a general concept for solving a particular problem.
• Patterns are often confused with algorithms, because both concepts describe typical solutions to some known problems
• While an algorithm always defines a clear set of actions that can achieve some goal, a pattern is a more high-level description of a solution.
• NOTE : The code of the same pattern applied to two different programs may be different.
• In General there are 23 Design Patterns, ever software engineer should know.
• Design patterns can speed up the development process by providing tested, proven development paradigms.
• Design patterns are common architectural approaches
• Popularized by the Gang of Four book (1994) ==> Element of Reusable Object-Oriented Software
• Translated to many lang - C#, C++, Java, JS
• Universally relevant
  • Internalized in some programming langages.
  • Libraries

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Why should I learn patterns ?

• Design patterns are a toolkit of tried and tested solutions to common problems in software design.
• knowing patterns is still useful because it teaches you how to solve all sorts of problems using principles of object-oriented design.
• Design patterns define a common language that you and your teammates can use to communicate more efficiently.

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Classification of Design patterns :

• The most basic and low-level patterns are often called idioms.
• The most universal and high-level patterns are architectural patterns.
• Design patterns are typically split into 3 categories this is called Gamma Categorization after Erich Gamma one of GoF Author.
• 3 patterns :
  • Creational patterns
  • Structural patterns
  • Behavioral patterns
• Creational patterns :
  • provide object creation mechanisms that increase flexibility and reuse of existing code.
  • Deals with the creation (construction of objects)
  • Explicit (constructor) v/s implicit (D.I, reflection, etc)
  • Wholesale (single statement) v/s piecewise (step-by-step)
• Structural patterns :
  • explain how to assemble objects and classes into larger structures, while keeping the structures flexible and efficient.
  • Concerned with the structure (ex- class members)
  • Manay patterns are wrappers that mimc the underlying class interface
  • Stress the importance of good API design
• Behavioral patterns :
  • take care of effective communication and the assignment of responsibilities between objects.
  • They are all different; no central theme.
  • They are unique in there ways.

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Creational patterns

• provide object creation mechanisms that increase flexibility and reuse of existing code.
• Different Creational patterns are :
  • Factory Method
  • Abstract Method
  • Builder
  • Prototype
  • Singleton

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Structural patterns

• explain how to assemble objects and classes into larger structures, while keeping the structures flexible and efficient.
• Different Structural patterns are :
  • Adapter
  • Bridge
  • Composite
  • Decorator
  • Facade
  • Flyweight
  • Proxy

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Behavioral patterns

• take care of effective communication and the assignment of responsibilities between objects.
• Behavioral Structural patterns are :
  • Chain of Responsibility
  • Command
  • Interpreter
  • Iterator
  • Mediator
  • Memento
  • Observer
  • State
  • Strategy
  • Template Method
  • Visitor

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Anti-pattern:

• Pattern which is supposed to be bad
• example: God Object (A God object is an object that knows too much or does too much)
• Single Responsibility Principle --> Antipattern is God Object. [Thus, SRP recommends to if a class is having more than one responsibilities then just create a new class which can manage those responsibilities]
• To Tackle this God Object Anti-Pattern : A common programming technique is to separate a large problem into several smaller problems and create solutions for each of them.

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NOTE: In js we cannot have private methods.

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Summary

Creational Patterns

• Builder
  • Separate component for when object construction gets too complicated.
  • Can create mutually cooperating sub-builders
  • Often has fluent interface
• Factories
  • Factory method more expressive than constructor
  • A separate class with factory method is a Facotry
  • Class hierarchies can have corresponding hirerarchies of factories (Abstract Factory)
• Prototytype
  • Creation of object from an existing object
  • Requires either explicit deep copy or copy through serialization
  • Additional work required to preserve type
• Singleton
  • When you need to ensure just a single instance exists
  • Can return same object from constructor on every call
  • Direct dependence on a Singleton is dangerous

Structural Patterns

• Adapater
  • Converts the interface you get to the interface you need.
• Bridge
  • Decopule abstraction from implementation
• Composite
  • Allows client to treat individual objects and componsitions of objects uniformly
• Decorator
  • Attach additional repsonsibilites to object without modifiying those objects or inheriting from them
  • Decorators are composable with each other
• Facade
  • Provide a single unified interface over a set of systems/interfaces
• Flyweight
  • Memory saving technique
  • Efficently support very lalrge numbers of similar objects
• Proxy
  • Provide a surrogate object that forwards calls to the real object while performing additional functions
  • Ex: access acontrol, communication, logging etc

Behavioral Patterns

• Chain of Responsibility
  • Allow components to process information/events in a chain
  • Each element in the chain refers to next element
  • Make a list and go through it
• Command
  • Encapsulate a request into a separate object
  • Good for audit, replay, undo/redo
  • Part of CQS/CQRS
• Interpreter
  • Transform textual input into object-oriented structures
  • Used by interpreters, compilers, static analysis tools, etc
  • Compiler Theory is a separate branch of computer science
• Iterator
  • Provide an interface for accessing elements of an aggregate object
  • Objects can be made iterable (for loop)
• Mediator
  • Provides mediation services between two objects
  • Ex: message passing, chat room
• Memento
  • Yields tokens representing system states
  • Tokens do not allow direct manipulation, but can be used in appropriate APIs
• Observer
  • Allows notifications of changes/happenings in a component
• State
  • We model systems by having one of a possible state and transitions between these states
  • Such a system is called a state machine
  • Special framworks exists to orchestrate state machines
• Strategy & Template Method
  • Both define a skeleton algo with details filled in by implementor
  • Strategy uses oridinary composition, template method uses inheritance
• Vistior
  • Allows non-intrusive addition of functionality to hirearchies

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