require "active_support/concern" class Module # = Bite-sized separation of concerns # # 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. # # Extracting a plain old Ruby object to encapsulate it and collaborate or # delegate to the original object is often a good choice, but when there's # no additional state to encapsulate or we're making DSL-style declarations # about the parent class, introducing new collaborators can obfuscate rather # than simplify. # # The typical route is to just dump everything in a monolithic class, perhaps # with a comment, as a least-bad alternative. Using modules in separate files # means tedious sifting to get a big-picture view. # # = Dissatisfying ways to separate small concerns # # == Using comments: # # class Todo # # Other todo implementation # # ... # # ## Event tracking # has_many :events # # before_create :track_creation # after_destroy :track_deletion # # private # def track_creation # # ... # end # end # # == With an inline module: # # Noisy syntax. # # class Todo # # Other todo implementation # # ... # # module EventTracking # extend ActiveSupport::Concern # # included do # has_many :events # before_create :track_creation # after_destroy :track_deletion # end # # private # def track_creation # # ... # end # end # include EventTracking # end # # == Mix-in noise exiled to its own file: # # 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 # increased overhead can be a reasonable tradeoff even if it reduces our # at-a-glance perception of how things work. # # 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 # separate bite-sized concerns. # # class Todo # # Other todo implementation # # ... # # concerning :EventTracking do # included do # has_many :events # before_create :track_creation # after_destroy :track_deletion # end # # private # def track_creation # # ... # end # end # end # # Todo.ancestors # # => [Todo, Todo::EventTracking, Object] # # This small step has some wonderful ripple effects. We can # * grok the behavior of our class in one glance, # * clean up monolithic junk-drawer classes by separating their concerns, and # * stop leaning on protected/private for crude "this is internal stuff" modularity. module Concerning # Define a new concern and mix it in. def concerning(topic, &block) include concern(topic, &block) end # A low-cruft shortcut to define a concern. # # concern :EventTracking do # ... # end # # is equivalent to # # module EventTracking # extend ActiveSupport::Concern # # ... # end def concern(topic, &module_definition) const_set topic, Module.new { extend ::ActiveSupport::Concern module_eval(&module_definition) } end end include Concerning end