Configuration concept

gistfile1.rb

module MyEngine
  class Engine < ::Rails::Engine

    # Autoload from the lib directory
    config.autoload_paths << File.expand_path('../../', __FILE__)

    config.my_engine = MyEngine::Configuration.new

    # default configuration
    config.my_engine.foo.class_for_bar = MyEngine::Bar
    
    # ...
  end
end


module MyEngine
  class Configuration

    FooConfig = Struct.new(:class_for_bar, :proc_baz)

    attr_accessor :value_0, :value_1

    attr_reader :foo

    def initialize
      @foo = FooConfig.new
    end

    #...
  end
end


module MyEngine
  class Foo

    def calculate_blah_blub
      obj = bar_class.new
      obj.calculate
    end

    private

    def bar_class
      Rails.application.config.my_engine.foo.class_for_bar
    end
  end
end



# host_app/config/initializers/my_engine.rb

HostApp::Application.configure do
  
  config.my_engine.foo = MyOtherBetterEngine::TheOtherBar

  # ...
  
end