Injecting controller helper methods inside the helper context in helper methods tests (RSpec)

application_controller.rb

# frozen_string_literal: true

class ApplicationController < ActionController::Base
  helper_method :is_happy?

  private

  def is_happy? = true
end

application_helper.rb

# frozen_string_literal: true

module ApplicationHelper
  def show_mood_text = is_happy? ? 'Happy' : 'Sad'
end

application_helper_spec.rb

# frozen_string_literal: true

require 'rails_helper'

RSpec.describe ApplicationHelper do
  describe '#show_mood_text' do
    subject { helper.show_mood_text }
    before { inject_controller_helper_methods(ApplicationController) }

    context 'when is_happy? returns true' do
      before { allow(helper).to receive(:is_happy?).and_return(true) }

      it { is_expected.to eq('Happy') }
    end

    context 'when is_happy? returns false' do
      before { allow(helper).to receive(:is_happy?).and_return(false) }

      it { is_expected.to eq('Sad') }
    end
  end

  def inject_controller_helper_methods(controller_class)
    helper_module = (controller = controller_class.new)._helpers
    helper_methods = helper_module.instance_methods(false).sort
    helper_method = ->(method) { helper_module.instance_method(method) }

    helper_methods.each do |method|
      helper.class.define_method(method, helper_method[method])
    end

    helper.controller = controller
  end
end