Modules

Learning Goals

Use modules to share functionality across classes

Introduction

In the previous lesson, we discussed the concept of inheritance. We learned that through subclasses (a certain class under a super class), the subclass has access to all of the methods of its parent class. Inheriting one class from another makes sense. The subclass can be understood as a child or subordinate of the super class. For example, a car is a type of vehicle, so it makes sense for the Car class to inherit from the Vehicle class.

Let's think about a slightly different type of example, one that is less hierarchical. We could easily envision writing an app that models the environment of a dance performance. Such an app might have a Dancer class. Dancers, we know, perform dances. Similarly, we could imagine a little kid going to see the Nutcracker ballet one Christmas, coming home and wanting to practice all of the ballet moves from the show. So, we might write a Kid class in which an instance of that class, our little kid who has gone to see the ballet, should have access to all those ballet moves (their performance skill notwithstanding).

This situation is not hierarchical, like our Car and Vehicle example. Instead, Kid and Dancer simply need to share some functionality, without being related in any other meaningful way.

This is where modules come in. Modules allow us to collect and bundle a group of methods and make those methods available to any number of classes. In this exercise, we'll be defining a Dance module and making it available to both the Dancer and Kid class.

Code Along 1: Including Module Methods as Instance Methods

Fork and clone this repo, and follow along with the walk-through below to get your code working. Get the tests to pass.

Step 1: Defining Our Module

We'll code our Dance module inside the lib/dance_module.rb file. Open up that file and define your module with the following code:

module Dance
end

Let's give our Dance module some fabulous moves:

module Dance
  def twirl
    "I'm twirling!"
  end

  def jump
    "Look how high I'm jumping!"
  end

  def pirouette
    "I'm doing a pirouette"
  end

  def take_a_bow
    "Thank you, thank you. It was a pleasure to dance for you all."
  end
end

Okay, now we'll define our Kid class and tell it to include the capabilities of the Dance module.

Step 2: Defining the Classes

Open up lib/kid.rb and define your Kid class:

class Kid
end

Let's do the same for the Dancer class in lib/dancer.rb:

class Dancer
end

Now we're ready to include our module in our classes:

Step 3: Including the Module

To lend our two classes all of the methods of the Dance module, we use the include keyword:

class Kid
  include Dance

  attr_accessor :name

  def initialize(name)
    @name = name
  end
end

class Dancer
  include Dance

  attr_accessor :name

  def initialize(name)
    @name = name
  end
end

If we use the include keyword, we allow our classes to use all of the methods of the included module as instance methods. We'll talk about how to lend a module's methods as class methods in a minute.

Now that we've included the module, open up bin/dance_party and get familiar with following code:

#!/usr/bin/env ruby

require_relative "../lib/kid.rb"
require_relative "../lib/dancer.rb"

angelina = Kid.new("Angelina")
mikhail_baryshnikov = Dancer.new("Mikhail")

puts "#{angelina.name} says: #{angelina.twirl}"
puts "#{mikhail_baryshnikov.name} says: #{mikhail_baryshnikov.take_a_bow}"

Now, run the file by typing ruby bin/dance_party and you should see the following output in your terminal:

Angelina says: I'm twirling!
Mikhail says: Thank you, thank you. It was a pleasure to dance for you all.

As you can see, we were able to include the #twirl instance method with our Kid instance, and the #take_a_bow method with our Dancer instance, even though those methods weren't defined directly in the classes! This is the benefit of using modules: we can share behavior between multiple classes without relying on inheritance.

Code Along 2: Extending Module Methods as Class Methods

In order to lend a module's methods to a class as class methods, we use the extend keyword. Let's write yet another module that we can extend into our classes as class methods. For the purposes of this example, let's create a shareable class method, metadata, which will report on some pertinent (shared) information regarding both classes.

Open up the lib/meta_dancing_module.rb and define the following module and methods:

module MetaDancing

  def metadata
    "This class produces objects that love to dance."
  end
end

Let's extend this module into both the Kid and Dancer classes:

class Dancer
  extend MetaDancing
end

class Kid
  extend MetaDancing
end

Now, open up the bin/extending file and familiarize yourself with the following code:

#!/usr/bin/env ruby

require_relative "../lib/kid"
require_relative "../lib/dancer"

puts Kid.metadata
puts Dancer.metadata

Run the file with ruby bin/extending and you should see the following output in your terminal:

This class produces objects that love to dance.
This class produces objects that love to dance.

Run the tests to make sure some of your tests are passing.

Note: Run the learn test command at this point, and you will pass almost all of the tests!

Code Along 3: Namespacing Modules

In the first code along, we built a module called Dance, which contained methods that we intended to be used as instance methods in the Dancer class.

In the second code along, we built the module MetaDancing, whose methods were intended to be used as class methods in the Kid and Dancer classes.

There are two drawbacks to this approach. First, if another developer looks at your modules, there is absolutely no way to determine how those methods are intended to be used. Are they class methods? Are they instance methods? Nobody knows!

Secondly, we had to build two separate modules that contained methods that were all related to the same functionality (dancing). But because there was no way to designate class methods versus instance methods, we were forced to define two separate modules, which violates the single responsibility principle. Wouldn't it be great if there was a way to define one module and specify which methods were intended as class methods and which methods as instance methods.

Guess what? There is! We're going to refactor the two modules into one, and use module namespacing to clarify how our code should be used.

module FancyDance
  module InstanceMethods

    def twirl
      "I'm twirling!"
    end

    def jump
      "Look how high I'm jumping!"
    end

    def pirouette
      "I'm doing a pirouette"
    end

    def take_a_bow
      "Thank you, thank you. It was a pleasure to dance for you all."
    end
  end

  module ClassMethods

    def metadata
      "This class produces objects that love to dance."
    end
  end
end

First, we define our FancyDance module. Then, inside the FancyDance module, we define a second module, InstanceMethods. Inside the InstanceMethods module, we place all our methods that we intend to be used as instance methods (twirl, jump, pirouette, take_a_bow). Next, we define a second nested module (nested inside of FancyDance) called ClassMethods. Inside, we place the metadata method, which we intend to be used as a class method.

So how do we use these nested modules?

class Dancer
  extend FancyDance::ClassMethods
  include FancyDance::InstanceMethods
end

class Kid
  extend FancyDance::ClassMethods
  include FancyDance::InstanceMethods
end

Note: remember to require the fancy_dance.rb file inside the dancer.rb and kid.rb, just like we did with our other file requirements.

We refer to the namespaced modules or classes with ::. This syntax references the parent and child relationship of the nested modules.

Remember, include is used to add functionality to our classes via instance methods. The InstanceMethods module inside the FancyDance module contains the methods twirl, jump, pirouette, and take_a_bow, which any instance of the Dancer or Kid class can do.

We can call:

angelina = Dancer.new
angelina.twirl
# => "I'm twirling!"
angelina.jump
# => "Look how high I'm jumping!"

buster = Kid.new
buster.jump
# => "Look how high I'm jumping!"
buster.take_a_bow
# => "Thank you, thank you. It was a pleasure to dance for you all."

Because we included the FancyDance::InstanceMethods nested module, we can call those instance methods on instances of our classes.

And extend is used to add functionality to our classes via class methods. We can now call the metadata class method on the Dancer and Kid classes:

Dancer.metadata
# => "This class produces objects that love to dance."
Kid.metadata
# => "This class produces objects that love to dance."

Namespacing Class Definitions

The same module namespacing syntax we used above can also be used to namespace classes. You'll encounter syntax like this when working with other Ruby libraries:

module ActiveRecord
  class Base
    # migration class definition
  end

  class Migration
    # migration class definition
  end
end

The reason for this namespacing is to avoid variable collision: what if our application also happened to have a class definition for Base or Migration? Thanks to namespacing, we can tell that these class definitions are specific to the ActiveRecord module.

To use those class definitions, we refer to them using the :: syntax:

ActiveRecord::Base

Namespacing vs Inheritance

Inheritance using the < syntax implies that a class is a type of another class. A BMW class should inherit from a Car class because a BMW is a type of car: class BMW < Car.

But what about the :: that we're using for our modules? The :: syntax just denotes a namespace. Doing BMW::Car just gives the BMW class access to all constants, instance methods, etc, without stating that a BMW is a type of car. The :: syntax carries all public items over to the inheriting class or module.

That's it! Now that we are familiar with several methods of sharing code between classes, you're ready to move on to the next few labs.

Conclusion

If you have a module whose methods you would like to be used in another class as instance methods, then you must include the module.

If you want a module's methods to be used in another class as class methods, you must extend the module.

principalkelvo/phase-3-ruby-oo-inheritance-modules