observer.cr

abstract class Observer(T)
  abstract def notify(obj : T)
end

module Observable(T)
  def add_observer(observer : Observer(T))
    observers = @observers ||= [] of Observer(T)
    observers << observer
  end

  def notify_observers(obj : T)
    @observers.try &.each &.notify(obj)
  end
end

class Foo
  include Observable(Int32)
end

class Bar < Observer(Int32)
  def notify(num)
    puts self
    puts num
  end
end

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