app_controller.rb

class ApplicationController < ActionController::Base
  @@page_title = nil
  
  def self.set_title(page_title)
    @@page_title = page_title
  end
  
  
  def page_title
    ignore_for = %w{sessions devise}
    if !ignore_for.include?(params[:controller]) && @@page_title.nil?
      raise "set_title must be set for #{params[:controller]}##{params[:action]}"
    end
    @@page_title
  end
  helper_method :page_title
end

shopping_list_controller.rb

class ShoppingListController < ApplicationController
  before_action :authenticate_user!
  set_title "Shopping List"
  
  def index
    ....
  end
  ...
end