Module: Module::Concerning

Included in:
Module
Defined in:
lib/concerning/module_concerning.rb

Overview

We often find ourselves with a medium-sized chunk of behavior that we’d like to extract, but only mix in to a single class.

We typically choose to leave the implementation directly in the class, perhaps with a comment, because the mental and visual overhead of defining a module, making it a Concern, and including it is just too great.

Using comments as lightweight modularity:

class Todo
  # Other todo implementation
  # ...

  ## Event tracking
  has_many :events

  before_create :track_creation
  after_destroy :track_deletion

  def self.next_by_event
    # ...
  end

  private
    def track_creation
      # ...
    end
end

Trying on a noisy embedded module:

class Todo
  # Other todo implementation
  # ...

  module EventTracking
    extend ActiveSupport::Concern

    included do
      has_many :events
      before_create :track_creation
      after_destroy :track_deletion
    end

    module ClassMethods
      def next_by_event
        # ...
      end
    end

    private
      def track_creation
        # ...
      end
  end
  include EventTracking
end

Once our chunk of behavior starts pushing the scroll-to-understand it boundary, we give in and move it to a separate file. At this size, the overhead feels in good proportion to the size of our extraction, despite diluting our at-a-glance sense of how things really work.

Mix-in noise exiled to its own file:

class Todo
  # Other todo implementation
  # ...

  include TodoEventTracking
end

Introducing Module#concerning.

By quieting the mix-in noise, we arrive at a natural, low-ceremony way to do bite-sized modularity:

class Todo
  # Other todo implementation
  # ...

  concerning :EventTracking do
    included do
      has_many :events
      before_create :track_creation
      after_destroy :track_deletion
    end

    class_methods do
      def next_by_event
        # ...
      end
    end

    private
      def track_creation
        # ...
      end
  end
end

Todo.ancestors
# => Todo, Todo::EventTracking, Object

This small step forward has some wonderful ripple effects. We can:

  • grok the behaviors that compose our class in one glance

  • clean up junk drawer classes by encapsulating their concerns

  • stop leaning on protected/private for “internal stuff” modularity

Instance Method Summary collapse

Instance Method Details

#concern(topic, &module_definition) ⇒ Object

A low-cruft shortcut to define a concern.

concern :EventTracking do
  ...
end

module EventTracking
  extend ActiveSupport::Concern

  ...
end



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# File 'lib/concerning/module_concerning.rb', line 134

def concern(topic, &module_definition)
  const_set topic, Module.new {
    extend ActiveSupport::Concern
    module_eval(&module_definition)
  }
end

#concerning(topic, &block) ⇒ Object

Define a new concern and mix it in.



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# File 'lib/concerning/module_concerning.rb', line 119

def concerning(topic, &block)
  include concern(topic, &block)
end