What if we used dependency injection for Rails controller code?

controller_source.rb

class ControllerSource
  Response = Struct.new(:controller, :injector) do
    def redirect_to(path, *args)
      controller.redirect_to(controller.send("#{path}_path", *args))
    end

    def render(*args)
      ivars = {}
      if args.last.is_a?(Hash) && args.last.has_key?(:ivars)
        ivars = args.last.delete(:ivars)
      end

      ivars.each do |name, val|
        controller.instance_variable_set("@#{name}", val)
      end

      controller.render *args
    end

    def respond_with(klass, *args)
      obj = klass.new(*args)
      format = controller.request.format.symbol
      if obj.respond_to?(format)
        injector.dispatch obj.method(format)
      end
    end
  end

  def initialize(controller)
    @controller = controller
  end

  def params;    @controller.params; end
  def session;   @controller.session; end
  def flash;     @controller.flash; end
  def now_flash; @controller.flash.now; end

  def response(injector)
    Response.new(@controller, injector)
  end
end

injector.rb

require 'ostruct'

class Injector
  attr_reader :sources

  def initialize(sources)
    @sources = sources + [self]
  end

  def dispatch(method, overrides = {})
    args = method.parameters.map {|_, name|
      source = (
        [OpenStruct.new(overrides)] +
        sources
      ).detect {|source| source.respond_to?(name) }
      if source
        dispatch(source.method(name), overrides)
      else
        UnknownInjectable.new(name)
      end
    }
    method.call(*args)
  end

  def injector
    self
  end

  class UnknownInjectable < BasicObject
    def initialize(name)
      @name = name
    end

    def method_missing(*args)
      ::Kernel.raise "Tried to call method on an uninjected param: #{@name}"
    end
  end
end

registration.rb

module Controller
  class Registration
    def update(response, now_flash, update_form)
      form = update_form

      if form.save
        response.respond_with SuccessfulUpdateResponse, form
      else
        now_flash[:message] = "Could not save registration."
        response.render action: 'edit', ivars: {registration: form}
      end
    end

    SuccessfulUpdateResponse = Struct.new(:form) do
      def html(response, flash, current_event)
        flash[:message] = "Updated details for %s" % form.name
        response.redirect_to :registrations, current_event
      end

      def js(response)
        response.render json: form
      end
    end
  end
end

registration_source.rb

module Controller
  class RegistrationSource
    def current_event(params)
      Event.find(params[:event_id])
    end

    def current_registration(params, current_event)
      current_event.registrations.find(params[:id])
    end

    def current_organiser(session)
      Organiser.find_by_id(session[:organiser_id])
    end

    def authorized_organiser(current_event, current_organiser)
      unless current_organiser && current_organiser.can_edit?(current_event)
        raise UnauthorizedException
      end
    end

    def update_form(params, current_registration)
      Form::UpdateRegistration.build(
        current_registration,
        params[:registration]
      )
    end
  end
end

registration_spec.rb

require 'unit_helper'

require 'injector'
require 'controller/registration'

describe Controller::Registration do
  success_response = Controller::Registration::SuccessfulUpdateResponse

  let(:form)      { fire_double("Form::UpdateRegistration") }
  let(:response)  { fire_double("ControllerSource::Response") }
  let(:event)     { fire_double("Event") }
  let(:flash)     { {} }
  let(:now_flash) { {} }
  let(:injector)  { Injector.new([OpenStruct.new(
    response:      response.as_null_object,
    current_event: event.as_null_object,
    update_form:   form.as_null_object,
    flash:         flash,
    now_flash:     now_flash
  )]) }

  describe '#update' do
    it 'saves form and responds with successful update' do
      form.should_receive(:save).and_return(true)
      response
        .should_receive(:respond_with)
        .with(success_response, form)

      injector.dispatch described_class.new.method(:update)
    end

    it 'render edit page when save fails' do
      form.should_receive(:save).and_return(false)
      response
        .should_receive(:render)
        .with(action: 'edit', ivars: {registration: form})

      injector.dispatch described_class.new.method(:update)

      now_flash[:message].length.should > 0
    end
  end

  describe success_response do
    describe '#html' do
      it 'redirects to registration' do
        response.should_receive(:redirect_to).with(:registrations, event)

        injector.dispatch success_response.new(form).method(:html)
      end

      it 'includes name in flash message' do
        form.stub(:name).and_return("Don")

        injector.dispatch success_response.new(form).method(:html)

        flash[:message].should include(form.name)
      end
    end
  end
end

registrations_controller.rb

class RegistrationsController < ApplicationController
  def update
    injector = Injector.new([
      ControllerSource.new(self),
      Controller::RegistrationSource.new
    ])
    injector.dispatch Controller::Registration.new.method(:update)
  end
end