observer.rb

class Artist
  attr_reader :subscribers, :name, :label

  def initialize(name, label)
    @name = name
    @label = label
    @subscribers = []
  end

  def add_subscriber(subscriber)
    @subscribers << subscriber
  end

  def name=(name)
    @name = name
    update
  end

  def label=(label)
    @label = label
    update
  end

  private

  def update
    subscribers.each do |subscriber|
      subscriber.notify(self)
    end
  end
end

class Album
  attr_reader :artist, :name

  def initialize(name, artist)
    @name = name
    @artist = artist
  end

  def notify(observed)
    @artist = observed
  end
end

class Genre
  attr_reader :artist, :name

  def initialize(name, artist)
    @name = name
    @artist = artist
  end

  def notify(observed)
    @artist = observed
  end
end

snoop = Artist.new('Snoop Dogg', "RCA Records")
tdf = Album.new("Tha Doggfather", snoop)
west_side = Genre.new("West Coast Rap", snoop)

snoop.add_subscriber(tdf)
snoop.add_subscriber(west_side)

p "Before any changes"
p tdf.artist.name
p tdf.artist.label

snoop.name = "Snoop Lion"
p "After name change"
p tdf.artist.name
p tdf.artist.label

snoop.label = "Aftermath"
p "After label change"
p west_side.artist.name
p west_side.artist.label