Object Inheritance, Composition and Mixins in Ruby

Prerequisites

Objectives

By the end of this, developers should be able to:

Give two reasons why inheritance and Mixins are desirable.
Write a class which inherits from another class.
Write a mixin
Describe the difference between inheritance, composition, and mixins

Preparation

Fork and clone this repository.
Install dependencies with bundle install.

Ruby Inheritance

Let's discuss the code below:

class Car
  attr_reader :engine
  def initialize
    @engine = 1200
  end
end

class Ford < Car
end

focus = Ford.new
puts focus.engine

Whenever I create a new instance of a Ford, Ruby looks for a method called initialize. In this case, it doesn't find that method on the Ford class, so it finds the method on it's parent Car. focus is an instance of the Car class, so it inherits all of methods defined in the Car class.

Ruby Composition

class Album
  attr_accessor :tracks

  def initialize
    @tracks = []
  end
end

class Song
  def initialize(title)
    @title = title
  end
end

lemonade = Album.new()
lemonade.tracks << Song.new('Formation')

Sometimes, we want build more complex object by using specific instances of other objects. We can use composition to acheive this. In this case, we will add instances of the Song class to the tracks of our album.

Ruby Mixins

We want to make chunks of code that are resuable across of multiple classes. These "chunks" are called modules. Take a look at the code below:

module Sleepable
  def go_to_sleep
    # code here
  end
end

class Person
  include Sleepable
end

class Computer
  include Sleepable
end

In the code above we defined a module called Sleepable. We also define a Person class and a Computer class. By using the keyword include followed by the name of the module (in this case Sleepable) we have access to the methods we defined in our module. This is great because it allows us to keep our code D-R-Y, not to mention it allows us to be lazy developers (the good kind of lazy).

You will sometimes hear the word 'composition' when referring to mixins.

What should I use?

is-a: A Ford is a car. Inheritance creates a subclass - a class that has access to all of the methods of it's parent class. You should use it if your class is a type of it's parent class, like Ford is a type of car. A Ford is a more specialized, less abstract version of the Car class.

has-a: An Album has a song on it. We use composition when class instances contain instances of other classes.

behaves-as: Mixins are used when a behavior is shared between various classes. People and computers both share the sleep behavior in the example above. People and computers are very different - it wouldn't make sense for them to inherit from the Sleepable class.

Class or mixin?

When deciding whether to write a class or mixin, ask yourself if what you're writing will need an initialize function. If you do, you should be writing a class. Classes have state and behavior. Mixins are only for behavior - they don't hold state.

Lab: Write a Mixin

Open the file lib/equine.rb

Create a class Equine with a method eat_grass
Create three classes Horse, Zebra, and Mule that inherit from Equine
Create a mixin Ridable with a method ride
Include that method in the Horse and Mule class

Use bin/rake test to test your code.

Additional Resources

License

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