Dependency injection example

get_posts_from_fake_medium.rb

posts = Posts.new
posts.source = PostsSources::FakeMedium
posts.user_posts

get_posts_from_medium.rb

posts = Posts.new
posts.source = PostsSources::Medium
posts.user_posts

medium.rb

class Medium
  cattr_accessor :adapter
  def initialize(name = nil)
    @client = adapter.new(name)
  end
  def posts
    @client.user_posts
  end
end

posts.rb

class Posts
  cattr_accessor :source
  def initialize
    @client = source.new
  end
  def posts(args)
    @client.user_posts(args)
  end
end

posts_adapter_fake.rb

module PostsAdapter
  class Fake
    def initialize(username)
      @client = OpenStruct.new(
        posts: [
          {
            title: 'Signal v Noise exits Medium[Fake adapter]',
            ...
          }
        ]
      )
    end
    def user_posts
      @client.posts
    end
  end
end

posts_adapter_medium.rb

module PostsAdapter
  class Medium
    def initialize(username)
      @client = MediumAPI.new(username)
    end
    def user_posts
      @client.posts
    end
  end
end

rspec_settings.rb

RSpec.configure do |config|
  config.around do |example|
    # keep old adapter to use after test runs
    default_source = PostsSources::Medium
    # set our in-memory adapter
    Posts.adapter = PostsSources::Fake
    # run the test
    example.run
    # put back the previous adapter
    Posts.adapter = default_source
  end
end

set_default_adapter.rb

class Posts
  cattr_accessor :source
  self.adapter = PostsSources::Medium
  …
end