ruby_dependency_injection.rb

module Injectable
  class Placeholder
    def initialize(target)
      @target = target
    end

    def injected_value
      @target.generate
    end
  end
  def self.included(klass)
    klass.extend Generate
  end
  module Generate
    def injectable
      Placeholder.new(self)
    end
  end
end

class Email
  include Injectable
  attr_reader :address
  def initialize(address)
    @address = address
  end
  def self.generate
    new "email#{rand}@example.com"
  end
end

require 'logger'
class Logger
  include Injectable
  def self.generate
    new STDOUT
  end
end

class Binding
  def inject
    local_variables.each do |name|
      current = local_variable_get(name)
      if current.class <= Injectable::Placeholder
        local_variable_set(name, current.injected_value)
      end
    end
  end

  # Patched in https://bugs.ruby-lang.org/issues/8773
  def local_variables
    eval("local_variables")
  end
end

def run
  regular = "not injected"
  logger = Logger.injectable
  email = Email.injectable

  puts "The variables are not instantiated - they're merely placeholders:"
  p regular
  p logger
  p email

  binding.inject

  puts ''
  puts "Now they've been realized:"
  p regular
  p logger
  p email

  puts ''
  logger.info(email.address)
end

run