Ruby Virtual Proxy Pattern

virtual_proxy.rb

class VirtualProxy < BasicObject
  def initialize(&loader)
    @loader = loader
    @object = nil
  end

  def method_missing(name, *args, &block)
    __load__
    @object.public_send(name, *args, &block)
  end

  def inspect
    "VirtualProxy(#{@object ? @object.inspect : ''})"
  end

  def __object__
    @object
  end

  def __load__
    @object ||= @loader.call
  end
end

virtual_proxy_spec.rb

require 'spec_helper'

require_relative '../../lib/virtual_proxy'

RSpec.describe VirtualProxy, type: :unit do
  before :context do
    Movie = Struct.new(:title, :year, :director)
    Director = Struct.new(:name)
  end

  let(:movie1) { Movie.new('This Is Spinal Tap', 1984) }

  it "is a virtual proxy class" do
    vp = VirtualProxy.new { movie1 }

    expect(vp.inspect).to eq("VirtualProxy()")
    expect(vp.__object__).to eq(nil)

    vp.__load__
    expect(vp.inspect).to eq("VirtualProxy(#{movie1.inspect})")
    expect(vp.__object__).to eq(movie1)
  end

  it "calls the loading block only the first time" do
    vp = VirtualProxy.new {
      puts 'Loading!'
      movie1
    }

    expect{ vp.title }.to output("Loading!\n").to_stdout
    expect{ vp.title }.to_not output("Loading!\n").to_stdout
  end

  it "forwards the method calls to the target object" do
    vp = VirtualProxy.new { movie1 }

    expect(vp.title).to eq('This Is Spinal Tap')
    expect(vp.year).to eq(1984)
  end

  context "When used in place of an association" do
    it "keeps forwarding messages and stays as a proxy" do
      movie_1 = create_movie_example_1

      expect(movie_1.director.class).to eq(Director)
      expect(Director === movie_1.director).to be false
    end

    it "forwards the first message and replaces itself with the real object" do
      movie_2 = create_movie_example_2

      expect(movie_2.director.class).to eq(Director)
      expect(Director === movie_2.director).to be true
    end
  end

  private
  def create_movie_example_1
    vp = VirtualProxy.new { Director.new('Rob Reiner') }
    Movie.new('This Is Spinal Tap', 1984, vp)
  end

  def create_movie_example_2
    movie = Movie.new('This Is Spinal Tap', 1984)
    movie.director = VirtualProxy.new {
      movie.director = Director.new('Rob Reiner')
    }
    movie
  end
end