observer.rb

# Define a one-to-many dependency between objects so that when
# one object changes state, all its dependents are notified
# and updated automatically

module Observable
  attr_reader :observers

  def initialize(attrs = {})
    @observers = []
  end

  def add_observer(observer)
    @observers << observer
  end

  def notify_observers
    @observers.each{ |observer| observer.update }
  end
end

class Tile
  include Observable

  def initialize(attrs = {})
    super
    @cursed = attrs.fetch(:cursed, false)
  end

  def cursed?
    @cursed
  end

  def activate_curse
    notify_observers
  end
end

class Hero
  attr_reader :health

  def initialize
    @cursed = false
    @health = 10
  end

  def damage(hit)
    @health -= hit
  end

  def cursed?
    @cursed
  end

  def discover(tile)
    if tile.cursed?
      @cursed = true
      tile.add_observer(self)
    end
  end

  def update
    damage(4)
  end
end

# Usage
hero = Hero.new
tile = Tile.new cursed: true
hero.discover(tile)
tile.activate_curse
puts hero.health
# => 6
puts hero.cursed?
# => true