configurable_superclass.rb

class Render
  def self.[](engine:)
    Class.new(self) do
      define_singleton_method(:engine) do
        engine
      end

      define_singleton_method(:with_engine) do
        engine
      end

      def self.inherited(subclass)
        puts "subclass: #{subclass}"
        puts "engine: #{subclass.engine}"
      end
    end
  end

  def render
    self.class.engine.render(self.to_h)
  end
end

class Render::JSON
  def self.render(h)
    require 'json'
    JSON.generate(h)
  end
end

class Memo < Render[engine: Render::JSON]
  attr_accessor :title
  attr_accessor :body

  def to_h
    { title: title, body: body }
  end
end

memo = Memo.new
memo.title = 'my title'
memo.body = 'my body'

puts memo.render