observer2.cr

module NumObserver
  abstract def on_num(num)
end

module NumObservable
  def add_observer(observer : NumObserver)
    num_observers = @num_observers ||= [] of NumObserver
    num_observers << observer
  end

  def notify_on_num(num)
    @num_observers.try &.each &.on_num(num)
  end
end

class Foo
  include NumObservable
end

class Bar
  include NumObserver

  def on_num(num)
    puts self
    puts num
  end
end

foo = Foo.new
foo.add_observer(Bar.new)
foo.add_observer(Bar.new)
foo.notify_on_num 1