knowledge-kitchen / course-notes

class: center, middle # Design Patterns Common problems and their solutions. --- # Agenda 1. [Overview](#overview) 2. [History](#history) 3. [Criticisms](#criticisms) 4. [Creational](#creational) 5. [Structural](#structural) 6. [Behavioral](#behavioral) 7. [Conclusions](#conclusions) --- name: overview # Overview -- ## Concept Design Patterns are good quality re-usable solutions to common problems. In other words, Design Patterns represent a set of best practices for particular contexts. --- template: overview ## What they offer Design Patterns describe a particular design problem. They do not prescribe a solution. Rather, they indicate a set of values to guide the designer toward a solution to the problem that is best for their particular situation. --- template: overview ## Nature in Flux There is no complete set of design patterns. - Some are well-established for particular contexts - Others are made up all the time and disappear as quietly as they came - Some patterns can be made redundant in languages that have built-in support for solving the problem. --- name: history # History -- ## Philosophy Seneca The Younger spoke of the value of abstract patterns versus their concrete implementations circa 65 AD in his letter to his friend, Lucilius, now referred to as "[On Sharing Knowledge](https://en.m.wikisource.org/wiki/Moral_letters_to_Lucilius/Letter_6)": > The way is long if one follows precepts, but short and helpful, if one follows patterns. --- template: history ## Architecture [A Pattern Language](https://en.wikipedia.org/wiki/A_Pattern_Language), by Christopher Alexander, Sara Ishikawa, Murray Silverstein - A seminal book published in 1977 - Layed out the concept of a pattern language, as applied to the architecture of progressively larger structures - nooks, rooms, homes, neighborhoods, and cities. - Inspired software engineers, including Kent Beck (inventor of Unit Testing) and Ward Cunningham (creator of first-ever wiki) to apply its concepts to the realm of code. --- template: history ## Architecture (continued)  --- template: history ## Architecture (continued)  --- template: history ## Software [Design Patterns: Elements of Reusable Object-Oriented Software](http://www.uml.org.cn/c%2B%2B/pdf/DesignPatterns.pdf), by Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides - An equally-seminal book published in 1994 - Colloquially known as the "Gang of Four" book after its four authors - First popular application of the concept of pattern languages to object-oriented software in Java - Still the standard reference today, and commonly discussed in job interviews! - outlined 23 object-oriented design patterns -- Creational - patterns related to the creation of objects -- Structural - how classes are designed using inheritance, composition, and aggregation -- Behavioral - communication between objects as a program is running --- name: criticisms # Criticisms -- ## Language Flaws A common criticism of Design Patterns is that their necessity indicates flaws in the programming language they are designed for. - [Read more about this view](http://wiki.c2.com/?AreDesignPatternsMissingLanguageFeatures) --- template: criticisms ## Overly-complicated Another criticism holds that Design Patterns can lead to overly complex and verbose code, when a simpler unpatterned solution might be simpler and easier to maintain. - [Read more about this view](https://web.archive.org/web/20120224000044/http://parand.com/say/index.php/2005/07/18/i-hate-patterns/) --- name: creational # Creational -- ## Prototype A fully initialized instance to be copied or cloned. Useful when the setup of an object is time/resource consuming, and you do not want to repeat it more than once, if possible. - [View more notes](https://web.archive.org/web/20210417112551/https://knowledge.kitchen/Design_patterns#Prototype) --- template: creational ## Builder Separates object construction from its representation. The builder pattern provides a simple interface to create an object that actually has a complex structure. Useful when there's a complex object structure that is complicated to build. - [View more notes](https://web.archive.org/web/20210417112551/https://knowledge.kitchen/Design_patterns#Builder) --- template: creational ## Singleton A class of which only a single instance is allowed to exist. Useful when you need only one object of that class to exist. - [View more notes](https://web.archive.org/web/20210417112551/https://knowledge.kitchen/Design_patterns#Singleton) --- name: structural # Structural -- ## Proxy An object representing another object. It's an imposter! Hides the complexity of the object it represents. Example: credit card is a proxy for a bank account. - [View more notes](https://web.archive.org/web/20210417112551/https://knowledge.kitchen/Design_patterns#Proxy) --- template: structural ## Facade A single class that represents the entire subsystem. Useful for web applications. Reducing network calls. Reduces coupling (bewteen web layer and back-end). Helps rollback all steps if one fails. - [View more notes](https://web.archive.org/web/20210417112551/https://knowledge.kitchen/Design_patterns#Facade) --- template: structural ## Decorator Adds responsibility to objects dynamically. Makes it easy to add behavior at runtime. - [View more notes](https://web.archive.org/web/20210417112551/https://knowledge.kitchen/Design_patterns#Decorator) --- name: behavioral # Behavioral -- ## Chain of responsibility A way of passing a request between a chain of objects. A message passes through a series of objects until one of them is able to handle it, at which point is doesn't get passed any further. - [View more notes](https://web.archive.org/web/20210417112551/https://knowledge.kitchen/Design_patterns#Chain_of_responsibility) --- template: behavioral ## Iterator Sequentially access the elements in a collection. Useful when the internal representation of a collection of objects isn't important, and you simply want to be able iterate through the objects in the collection. - [View more notes](https://web.archive.org/web/20210417112551/https://knowledge.kitchen/Design_patterns#Iterator) --- template: behavioral ## Observer A way of notifying a change to an object to a number of other objects. Objects can be registered to receive notifications. Situation: When one of the objects changes, it needs to let several of the other objects know. - [View more notes](https://web.archive.org/web/20210417112551/https://knowledge.kitchen/Design_patterns#Observer) --- name: conclusions # Conclusions -- This has been a brief, but inspirational introduction to design patterns. -- - Thank you. Bye.