hadichloun / ruby-oo-inheritance-modules

Step I: Defining Our Module

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

Note: Run the learn test command to pass all the local tests for this lab. We will be altering the methods where these tests will no longer be valid.

Code Along III: Nested Modules

In the first code along, we built a module called Dance, which contained methods that we intended to be used as instances 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 nested module namespacing to clarify our code.

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 name-spaced 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
// returns "I'm twirling!"
angelina.jump
// returns "Look how high I'm jumping!"

buster = Kid.new
buster.jump
// returns "Look how high I'm jumping!"
buster.take_a_bow
// returns "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
// returns "This class produces objects that love to dance."
Kid.metadata
// returns "This class produces objects that love to dance."

:: versus <

Inheritance using the < syntax, implies that a class is a type of something. 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 name-space. 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.