Examples of sofware design patterns
A running list of examples of design patterns listed in Design Patterns: Elements of Reusable Object-Oriented Software.
Create objects, rather than having to instantiate objects directly. This gives the program more flexibility in deciding which objects need to be created for a given case.
Abstract factory groups object factories that have a common theme.
Builder constructs complex objects by separating construction and representation.
Factory method creates objects without specifying the exact class to create.
Prototype creates objects by cloning an existing object.
Singleton restricts object creation for a class to only one instance.
Concern class and object composition. They use inheritance to compose interfaces and define ways to compose objects to obtain new functionality.
Adapter allows classes with incompatible interfaces to work together by wrapping its own interface around that of an already existing class.
- Database adapters (eg. mysql2 gem)
Bridge decouples an abstraction from its implementation so that the two can vary independently.
- ActiveRecord logging does this where
- Also caching
Composite composes zero-or-more similar objects so that they can be manipulated as one object.
- As I understand it this is effectively any structure that can
has_and_belongs_to_manyof itself so for example the categorization in Supplybunny
Decorator dynamically adds/overrides behaviour in an existing method of an object.
Facade provides a simplified interface to a large body of code.
- Integrating with Lazada API where to get the full details of the product have to call get info and get details
Flyweight reduces the cost of creating and manipulating a large number of similar objects.
Proxy provides a placeholder for another object to control access, reduce cost, and reduce complexity.
def price @price ||= get_expensive_price end
Specifically concerned with communication between objects.
Chain of responsibility delegates commands to a chain of processing objects.
Command creates objects which encapsulate actions and parameters.
- Sidekiq and DelayedJob (when you just pass a worker in)
Interpreter implements a specialized language.
- All the domain specific languages, for example
Iterator accesses the elements of an object sequentially without exposing its underlying representation.
Mediator allows loose coupling between classes by being the only class that has detailed knowledge of their methods.
- Vue actions
Memento provides the ability to restore an object to its previous state (undo).
Observer is a publish/subscribe pattern which allows a number of observer objects to see an event.
- Rails observers
State allows an object to alter its behavior when its internal state changes.
- Acts as a state machine
Strategy allows one of a family of algorithms to be selected on-the-fly at runtime.
- Devise authentication strategies (eg. LDAP, OAuth, Basic)
Template method defines the skeleton of an algorithm as an abstract class, allowing its subclasses to provide concrete behavior.
Visitor separates an algorithm from an object structure by moving the hierarchy of methods into one object.
- Arel uses it here
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