observer3.cr

macro def_observer_pattern(name)
  {% up = name.id %}
  {% down = name.stringify.downcase.id %}

  module {{up}}Observer
    abstract def on_{{down}}(obj)
  end

  module {{up}}Observable
    def add_{{down}}_observer(observer : {{up}}Observer)
      observers = @{{down}}_observers ||= [] of {{up}}Observer
      observers << observer
    end

    def notify_on_{{down}}(obj)
      @{{down}}_observers.try &.each &.on_{{down}}(obj)
    end
  end
end

def_observer_pattern Num
def_observer_pattern String

class Foo
  include NumObservable
  include StringObservable
end

class Bar
  include NumObserver
  include StringObserver

  def on_num(num)
    puts "#{self} (Num): #{num}"
  end

  def on_string(str)
    puts "#{self} (String): #{str}"
  end
end

foo = Foo.new
foo.add_num_observer(Bar.new)
foo.add_num_observer(Bar.new)
foo.add_string_observer(Bar.new)
foo.notify_on_num 1
foo.notify_on_string "Hello!"