rails--rails/activesupport/lib/active_support/lazy_load_hooks.rb

module ActiveSupport
  # lazy_load_hooks allows rails to lazily load a lot of components and thus
  # making the app boot faster. Because of this feature now there is no need to
  # require <tt>ActiveRecord::Base</tt> at boot time purely to apply
  # configuration. Instead a hook is registered that applies configuration once
  # <tt>ActiveRecord::Base</tt> is loaded. Here <tt>ActiveRecord::Base</tt> is
  # used as example but this feature can be applied elsewhere too.
  #
  # Here is an example where +on_load+ method is called to register a hook.
  #
  #   initializer 'active_record.initialize_timezone' do
  #     ActiveSupport.on_load(:active_record) do
  #       self.time_zone_aware_attributes = true
  #       self.default_timezone = :utc
  #     end
  #   end
  #
  # When the entirety of +activerecord/lib/active_record/base.rb+ has been
  # evaluated then +run_load_hooks+ is invoked. The very last line of
  # +activerecord/lib/active_record/base.rb+ is:
  #
  #   ActiveSupport.run_load_hooks(:active_record, ActiveRecord::Base)
  @load_hooks = Hash.new { |h,k| h[k] = [] }
  @loaded = Hash.new { |h,k| h[k] = [] }

  def self.on_load(name, options = {}, &block)
    @loaded[name].each do |base|
      execute_hook(base, options, block)
    end

    @load_hooks[name] << [block, options]
  end

  def self.execute_hook(base, options, block)
    if options[:yield]
      block.call(base)
    else
      base.instance_eval(&block)
    end
  end

  def self.run_load_hooks(name, base = Object)
    @loaded[name] << base
    @load_hooks[name].each do |hook, options|
      execute_hook(base, options, hook)
    end
  end
end