Intro to Object Oriented Programming* *in Ruby Burlington Ruby - September 2012 - Peter Brown

Introduction OOP Overview Designing Objects Testing Refactoring OOP Tips class Presentation def language 'Ruby' end def topic 'OOP' end def totally_awesome? true end end

Procedural Programming Model a sequence of events Data and functions (procedures) are separate Perfect for procedural languages class DataProcessor def process data = get_data_from_file new_data = convert_to_format(data) write_data_to_file(new_data) end def get_data_from_file end def convert_to_format(data) end def write_data_to_file(new_data) end end processor = DataProcessor.new processor.process

What can be improved? Procedural programming alone can be: difficult to relate to real world objects tough to read and modify hard to test

Objects to the rescue! OOP is a shift in thinking Dividing code into small classes with a single responsibility: React to change Reuse code Self documenting

What is Object Oriented Programming? Design Refactoring Testing

Object Oriented Design Modeling Domain Objects Messages and Interfaces

What are Objects? Instance of a class Properties / State Instance variables (@var) Methods: Command vs Query

Everything in Ruby is an Object String Integer Float Symbol Method Array Hash File Regexp Exception Boolean

Object Example pete = Person.new pete.age = 30 pete.age # => 30 class Person def age=(value) @age = value end def age @age end end Class vs Object

Interface (API) Public methods exposed to consumers Consumers = developers or code Hide implementation (Encapsulation)

Public Methods [:!, :!=, :! ~, :<=>, :==, :===, :=~, :__id__, :__send__, :class, :clone, :define_singleton_method, :display, :dup, :enum_for, :eql?, :equal?, :extend, :freeze, :frozen?, :hash, :initialize_clone, :initialize_dup, :inspect, :instance_eval, :instance_exec, :instance_of?, :instance_variable_defined?, :instance_variable_get, :instance_variable_set, :instance_variables, :is_a?, :kind_of?, :method, :methods, :nil?, :object_id, :private_methods, :protected_methods, :public_method, :public_methods, :public_send, :respond_to?, :respond_to_missing?, :send, :singleton_class, :singleton_methods, :taint, :tainted?, :tap, :to_enum, :to_s, :trust, :untaint, :untrust, :untrusted?] class Person; end Person.new.methods # =>

Interfaces: Good vs Bad Predictable Familiar Consistent Dependable Hide implementation Unpredictable Unfamiliar Inconsistent Subject to change Expose implementation

Interfaces: A Comparison

Interfaces: A Comparison jack = Person.new('Jack', 35, 'brown') jill = Person.new('Jill', 31, 'red')

Interfaces: A Comparison jack = Person.new('Jack', 35, 'brown') jill = Person.new('Jill', 31, 'red') jack = Person.new(name: 'Jack', age: 35, hair: 'brown') jill = Person.new(age: 31, name: 'Jill', hair: 'red') bill = Person.new(age: 40, name: 'Bill')

Predictable Interfaces jack.name # => ‘Jack’ jack.hair # => ‘brown’ jack.male? # => true jack.siblings.size # => 2 jack.reset! # => Dangerous!

Designing Objects Classical Inheritance Sharing via Modules Composition

Classical Inheritance Object A is an Object B All objects have a superclass Methods automatically delegated to superclass class Beverage end class Beer < Beverage end

Classical Inheritance class Alarm def check notify if active? end def notify puts "#{self.class} is active!" end def active? raise NotImplementedError end end

Classical Inheritance class FireAlarm < Alarm def active? fire_detected? end private def fire_detected? true end end alarm = FireAlarm.new alarm.check # => FireAlarm is active!

Classical Inheritance class Person < Object end

Sharing via Modules Object A behaves like Object B Can be reused by other classes Classes can include multiple modules module Drinkable def drink puts "mmmm" end end class Beer include Drinkable end

Sharing via Modules class Alarm include Notifiable def check notify if active? end def active? raise NotImplementedError end end module Notifiable def notify puts "#{self.class} is active!" end end

Composition Object A has an Object B Create objects that are composed of other objects class Roof end class House def roof Roof.new end end

Composition 20.meters.to_feet # => 65.6168

Composition 20.meters.to_feet # => 65.6168 class Meter def initialize(value) @value = value end def to_feet @value * 3.28084 end end

Composition 20.meters.to_feet # => 65.6168 class Meter def initialize(value) @value = value end def to_feet @value * 3.28084 end end class Numeric def meters Meter.new(self) end end

Composition class Numeric def feet Foot.new(self) end def meters Meter.new(self) end end 20.feet.to_feet # => 20 20.meters.to_feet # => 65.6168

Composition house = Building.new(height: 6.meters) house.height.to_feet # => 19.68504 skyscraper = Building.new(height: 500.feet) skyscraper.height.to_feet # => 500

Composition class Building attr_reader :height def initialize(options={}) @height = options[:height] end end house = Building.new(height: 6.meters) house.height.to_feet # => 19.68504 skyscraper = Building.new(height: 500.feet) skyscraper.height.to_feet # => 500

Unit Testing Interfaces & BDD go hand in hand Makes refactoring less painful Testing Public vs Private methods

Unit Testing: RSpec describe Person do subject { Person.new(name: 'Jack') } context '#siblings' do it { should have(:no).siblings } specify 'adding siblings behaves like an array' do subject.siblings << Person.new(name: 'Jill') subject.siblings << Person.new(name: 'Bill') subject.should have(2).siblings end end end

Refactoring Improving internal design of existing code without changing external behavior

Code Smells Abusing Inheritance Defensive coding Class names that end in ‘er’ and `or` Complex tests Arrays representing objects Class uses methods from another class excessively Too many parameters Large classes Long methods

OOP Tips Avoid unexpected state change Think in terms of what an object does, not what it is (BDD) Don’t have to start with objects, add them as needed Code to an interface Be consistent Favor composition over inheritance Avoid tight coupling

ClassyEnum Gem Combines classes, enums and ActiveRecord Composition and Polymorphism https://github.com/beerlington/classy_enum

Learning More http://rubyrogues.com/ object-oriented- programming-in-rails-with- jim-weirich/ http://www.confreaks.com/ videos/763- rubymidwest2011-confident- code

Thanks! @beerlington pete@lette.us