Allow testing of locals when rendering Controllers

assign_locals_helper.rb

def assigns_local(key)
   controller.instance_variable_get('@patched_locals')[key]
end

def patch_controller_render_to_support_locals(controller)
  def controller.render(options = nil, extra_options = {}, &block)
    [options, extra_options].select { |o| o.kind_of?(Hash) }.each do |option_hash|
      if option_hash.include?(:locals)
        @patched_locals = option_hash[:locals]
      end
    end
    super(options, extra_options, &block)
  end
end

example_controller.rb

class FooController < ApplicationController
  def index
    render locals: {
      foos: Foo.all
    }
  end
end

example_controller_spec.rb

require 'spec_helper'

describe FooController do
  describe :index do
    it 'should render locals' do
      expect(response).to render_template('index')
      expect(assigns_local(:foos)).to eq(Foo.all)
    end
  end
end

spec_helper.rb

require 'support/assigns_locals_helper.rb'

config.before(:each, type: :controller) do
  patch_controller_render_to_support_locals controller
end

assert_template 'index', locals: { foos: Foo.all } seems to work too