event_02.rb

require_relative './event_emitter_core'

# class for users
class User
  attr_reader :first_name

  def initialize(first_name)
    @first_name = first_name
  end
end

# service object for notification about new messages
class Notificator
  def self.call(message, user)
    # notify user about new message
    puts "new message '#{message}' for user #{user.first_name}"
  end
end

# class for chats
class Chat
  include EventEmitterCore
  def initialize(title, creator)
    @title = title
    @messages = []
    on(:new_message)

    add_subscriber(creator)
  end

  # to add a subscriber to the chat
  def add_subscriber(user)
    notify_user = lambda do |event_name, chat_object|
      Notificator.call(chat_object.last_message, user)
    end
    subscribe(:new_message, notify_user)
  end

  # to delete a subscriber from the chat
  def del_subscriber(user, hander_id)
    unsubscribe(:new_message, hander_id)
    new_message("#{user.first_name} left the chat", nil)
  end

  # to add new messages to the chat
  def new_message(message, user)
    @messages << { text: message, from: user&.first_name }
    emit(:new_message)
  end

  # to get the last message
  def last_message
    @messages.last[:text]
  end
end

# user Sasha create new chat
sasha = User.new('sasha')
chat = Chat.new("invitation for Sasha's birthday", sasha)
chat.new_message('test', sasha)

# users Ivan and Vova subscribe to the chat
ivan = User.new('ivan')
vova = User.new('vova')
ivan_handler_id = chat.add_subscriber(ivan)
vova_handler_id = chat.add_subscriber(vova)

# Sasha write a message
chat.new_message('hello, everyone. I invite you to my birthday party', sasha)

# unfortunately, Vova leave the chat
chat.del_subscriber(vova, vova_handler_id)
chat.new_message('Ohhh, Vova left the chat :(', sasha)

#### output is
# new message 'test' for user sasha
# new message 'hello, everyone. I invite you to my birthday party' for user sasha
# new message 'hello, everyone. I invite you to my birthday party' for user ivan
# new message 'hello, everyone. I invite you to my birthday party' for user vova
# new message 'vova left the chat' for user sasha
# new message 'vova left the chat' for user ivan
# new message 'Ohhh, Vova left the chat :(' for user sasha
# new message 'Ohhh, Vova left the chat :(' for user ivan