rails--rails/actionpack/lib/action_controller/form_builder.rb

module ActionController
  # Override the default form builder for all views rendered by this
  # controller and any of its descendants. Accepts a subclass of
  # +ActionView::Helpers::FormBuilder+.
  #
  # For example, given a form builder:
  #
  #   class AdminFormBuilder < ActionView::Helpers::FormBuilder
  #     def special_field(name)
  #     end
  #   end
  #
  # The controller specifies a form builder as its default:
  #
  #   class AdminAreaController < ApplicationController
  #     default_form_builder AdminFormBuilder
  #   end
  #
  # Then in the view any form using +form_for+ will be an instance of the
  # specified form builder:
  #
  #   <%= form_for(@instance) do |builder| %>
  #     <%= builder.special_field(:name) %>
  #   <% end %>
  module FormBuilder
    extend ActiveSupport::Concern

    included do
      class_attribute :_default_form_builder, instance_accessor: false
    end

    module ClassMethods
      # Set the form builder to be used as the default for all forms
      # in the views rendered by this controller and its subclasses.
      #
      # ==== Parameters
      # * <tt>builder</tt> - Default form builder, an instance of +ActionView::Helpers::FormBuilder+
      def default_form_builder(builder)
        self._default_form_builder = builder
      end
    end

    # Default form builder for the controller
    def default_form_builder
      self.class._default_form_builder
    end
  end
end