The Magic of Class-level Instance Variables

In a previous Ruby Magic, we figured out how to reliably inject modules into classes by overwriting its .new method, allowing us to wrap methods with additional behavior.

This time, we’re taking it one step further by extracting that behaviour into a module of its own so we can reuse it. We’ll build a Wrappable module that handles the class extension for us, and we’ll learn all about class-level instance variables along the way. Let’s dive right in!

Introducing the Wrappable Module

In order to wrap objects with modules when they are initialized, we have to let the class know what wrapping models to use. Let’s start by creating a simple Wrappable module that provides a wrap method which pushes the given module into an array defined as a class attribute. Additionally, we redefine the new method as discussed in the previous post.

module Wrappable 
  @@wrappers = []

  def wrap(mod)
    @@wrappers << mod 
  end 

  def new(*arguments, &block)
    instance = allocate
    @@wrappers.each { |mod| instance.singleton_class.include(mod) } 
    instance.send(:initialize, *arguments, &block)
    instance
  end 
end

To add the new behavior to a class, we use extend. The extend method adds the given module to the class. The methods then become class methods. To add a module to wrap instances of this class with, we can now call the wrap method.

odule Logging 
  def make_noise 
    puts "Started making noise"
    super 
    puts "Finished making noise"
  end
end

class Bird
  extend Wrappable 

  wrap Logging

  def make_noise
    puts "Chirp, chirp!"
  end
end

Let’s give this a try by creating a new instance of Bird and calling the make_noise method.

bird = Bird.new 
bird.make_noise
# Started making noise
# Chirp, chirp!
# Finished making noise

Great! It works as expected. However, things start to behave a bit strange once we extend a second class with the Wrappable module.

module Powered
  def make_noise 
    puts "Powering up"
    super 
    puts "Shutting down" 
  end 
end 

class Machine 
  extend Wrappable 

  wrap Powered

  def make_noise 
    puts "Buzzzzzz" 
  end 
end

machine = Machine.new 
machine.make_noise
# Powering up
# Started making noise
# Buzzzzzz
# Finished making noise
# Shutting down

bird = Bird.new 
bird.make_noise
# Powering up
# Started making noise
# Chirp, chirp!
# Finished making noise
# Shutting down

Even though Machine hasn’t been wrapped with the Logging module, it still outputs logging information. What’s worse - even the bird is now powering up and down. That can’t be right, can it?

The root of this problem lies in the way we are storing the modules. The class variable @@wrappables is defined on the Wrappable module and used whenever we add a new module, regardless of the class that wrap is used in.

This get’s more obvious when looking at the class variables defined on the Wrappable module and the Bird and Machine classes. While Wrappable has a class method defined, the two classes don’t.

Wrappable.class_variables # => [:@@wrappers]
Bird.class_variables # => []
Machine.class_variables # => []

To fix this, we have to modify the implementation so that it uses instance variables. However, these aren’t variables on the instances of Bird or Machine, but instance variables on the classes themselves.

In Ruby, classes are just objects

This is definitely a bit mind boggling at first, but still a very important concept to understand. Classes are instances of Class and writing class Bird; end is equivalent to writing Bird = Class.new. To make things even more confusing Classinherits from Module which inherits from Object. As a result, classes and modules have the same methods as any other object. Most of the methods we use on classes (like the attr_accessor macro) are actually instance methods of Module.

Using Instance Variables on Classes

Let’s change the Wrappable implementation to use instance variables. To keep things a bit cleaner, we introduce a wrappers method that either sets up the array or returns the existing one when the instance variable already exists. We also modify the wrap and new methods so that they utilize that new method.

module Wrappable
  def wrap(mod)
    wrappers << mod
  end

  def wrappers
    @wrappers ||= []
  end

  def new(*arguments, &block)
    instance = allocate
    wrappers.each { |mod| instance.singleton_class.include(mod) }
    instance.send(:initialize, *arguments, &block)
    instance
  end
end

When we check the instance variables on the module and on the two classes, we can see that both Bird and Machine now maintain their own collection of wrapping modules.

Wrappable.instance_variables #=> []
Bird.instance_variables #=> [:@wrappers]
Machine.instance_variables #=> [:@wrappers]

Not surprisingly, this also fixes the problem we observed earlier — now, both classes are wrapped with their own individual modules.

bird = Bird.new 
bird.make_noise
# Started making noise
# Chirp, chirp!
# Finished making noise

machine = Machine.new 
machine.make_noise
# Powering up
# Buzzzzzz
# Shutting down

Supporting Inheritance

This all works great until inheritance is introduced. We would expect that classes would inherit the wrapping modules from the superclass. Let’s check if that’s the case.

module Flying
  def make_noise
    super
    puts "Is flying away"
  end
end

class Pigeon < Bird
  wrap Flying

  def make_noise
    puts "Coo!"
  end
end

pigeon = Pigeon.new
pigeon.make_noise
# Coo!
# Is flying away

As you can see, it doesn’t work as expected, because Pigeon is also maintaining its own collection of wrapping modules. While it makes sense that wrapping modules defined for Pigeon aren’t defined on Bird, it’s not exactly what we want. Let’s figure out a way to get all wrappers from the entire inheritance chain.

Lucky for us, Ruby provides the Module#ancestors method to list all the classes and modules a class (or module) inherits from.

Pigeon.ancestors # => [Pigeon, Bird, Object, Kernel, BasicObject]

By adding a grep call, we can pick the ones that are actually extended with Wrappable. As we want to wrap the instances with wrappers from higher up the chain first, we call .reverse to flip the order.

Pigeon.ancestors.grep(Wrappable).reverse # => [Bird, Pigeon]

Ruby’s #=== method

Some of Ruby’s magic comes down to the #=== (or case equality) method. By default, it behaves just like the #== (or equality) method. However, several classes override the #=== method to provide different behavior in case statements. This is how you can use regular expressions (#=== is equivalent to #match?), or classes (#=== is equivalent to #kind_of?) in those statements. Methods like Enumerable#grep, Enumerable#all?, or Enumerable#any? also rely on the case equality method.

Now we can call flat_map(&:wrappers) to get a list of all wrappers defined in the inheritance chain as a single array.

Pigeon.ancestors.grep(Wrappable).reverse.flat_map(&:wrappers) # => [Logging]

All that’s left is packing that into an inherited_wrappers module and slightly modifying the new method so that it uses that instead of the wrappers method.

module Wrappable 
  def inherited_wrappers
    ancestors
      .grep(Wrappable)
      .reverse
      .flat_map(&:wrappers)
  end

  def new(*arguments, &block)
    instance = allocate
    inherited_wrappers.each { |mod|instance.singleton_class.include(mod) }
    instance.send(:initialize, *arguments, &block)
    instance
  end
end

A final test run confirms that everything is now working as expected. The wrapping modules are only applied to the class (and its subclasses) they are applied on.

bird = Bird.new
bird.make_noise
# Started making noise
# Chirp, chirp!
# Finished making noise

machine = Machine.new
machine.make_noise
# Powering up
# Buzzzzz
# Shutting down

pigeon = Pigeon.new
pigeon.make_noise
# Started making noise
# Coo!
# Finished making noise
# Is flying away

That’s a wrap!

Admittedly, these noisy birds are a bit of a theoretic example (tweet, tweet). But inheritable class instance variables are not just cool to understand how classes work. They are a great example that classes are just objects in Ruby.

And we’ll admit that inheritable class instance variables might even be quite useful in real life. For example, think about defining attributes and relationships on a model with the ability to introspect them later. For us the magic is to play around with this and get a better understanding of how things work. And open your mind for a next level of solutions. 🧙🏼‍♀️

As always, we’re looking forward to hearing what you build using this or similar patterns. Just chirp to @AppSignal on Twitter.

Benedikt Deicke is a software engineer and CTO of Userlist.io. On the side, he’s writing a book about building SaaS applications in Ruby on Rails. You can reach out to Benedikt via Twitter.

Originally published at blog.appsignal.com on October 2, 2018.