Want to increase your coding efficiency and write code that’s not like spaghetti? Here are 23 software design patterns that are sure to help.
The Gang of Four Design Patterns
The following software design patterns are often referred to as the Gang of Four design patterns. They were named the Gang of Four after the four authors that documented them in the book Design Patterns: Elements of Reusable Object-Oriented Software.
In this post, I will list the name and intent of each pattern from the Gang of Four book. In future posts I will write a detailed explanation of each pattern and how it can be used used with the Unity game engine and C#.
Note: As I write about each pattern, I will link to it from the list below. If you would like to be notified when I have published each pattern you can subscribe to the newsletter or bookmark this page.
Each of the patterns in this article fall under one of three categories: Creational, Structural or Behavioral.
Creational Patterns
Creational patterns deal with the best ways to instantiate new and related objects. The 5 creational patterns from the Gang of Four are:
- Abstract Factory: Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
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Builder: Separate the construction of a complex object from its representation so that the same construction process can create different representations.
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Factory Method: Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.
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Prototype: Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.
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Singleton: Ensure a class only has one instance, and provide a global point of access to it.
Structural Patterns
Structural patterns make it easier for you to design your application by identifying simple ways to create relationships between entities. The 7 structural patterns from the Gang of Four are:
- Adapter: Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn’t otherwise because of incompatible interfaces.
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Bridge: Decouple an abstraction from its implementation so that the two can vary independently.
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Composite: Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
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Decorator: Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
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Facade: Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higherlevel interface that makes the subsystem easier to use.
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Flyweight: Use sharing to support large numbers of fine-grained objects efficiently.
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Proxy: Provide a surrogate or placeholder for another object to control access to it.
Behavioral Patterns
Behavioral patterns deal with how objects communicate with each other. The 11 behavioral design patterns from the Gang of Four are:
- Chain of Responsibility: Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.
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Command: Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
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Interpreter: Given a language, define a represention for its grammar along with an interpreter that uses the representation to interpret sentences in the language.
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Iterator: Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
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Mediator: Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.
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Memento: Without violating encapsulation, capture and externalize an object’s internal state so that the object can be restored to this state later.
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Observer: Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
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State: Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
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Strategy: Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.
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Template Method: Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm’s structure.
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Visitor: Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.
Further Reading
More detailed discussion on the Gang of Four design patterns can be found in the following publications:
- Gamma, E. et al. 1994. Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley.
- Freeman, E. et al. 2004. Head First Design Patterns. O’Reilly Media.
- Bevis, T. 2012. C# Design Pattern Essentials. AbilityFirst.