Message Bus Blog Example

gistfile1.rb

module MessageBus
  class MessageBus
    @handlers = {}

    def self.subscribe message, handler, method
      @handlers[message] ||= []
      @handlers[message] << { :handler => handler, :method => method }
    end

    def self.send message, args
      raise "There are no handlers for #{message}" unless @handlers.include? message
      raise "There is more than one handler for #{message}" if @handlers[message].size > 1

      handler = @handlers[message].first
      handler[:handler].send handler[:method], args
    end

    def self.publish message, args
      @handlers[message] ||= []
      @handlers[message].each do |handler|
        handler[:handler].send handler[:method], args
      end
    end
  end

  module MessageHandler
    def handles message, method = message
      MessageBus.subscribe message, self, method
    end

    def subscribes message, method = message
      handles message, method
    end
  end

  module EventHandler
    include MessageHandler
  end

  module CommandHandler
    include MessageHandler
  end

  def self.send message, args
    MessageBus.send message, args
  end
	
  def self.publish message, args
    MessageBus.publish message, args
  end
end

class UpdateCustomerName
  extend MessageBus::CommandHandler

  handles :update_customer_name, :handle # override default method for command

  def self.handle args
	id = args.fetch(:id)
	name = args.fetch(:name)
	
    customer = Customer.find(id)
	customer.update_name name
	Customer.save(customer)
  end
end

class Customer
  
  @customers = {}
  def self.find id
    # some fake persistence ...
    customer = @customers[id] ||= Customer.new(id)
  end
  
  def self.save customer
    @customers[customer.id] = customer
  end
  
  attr_accessor :id, :name
  
  def initialize id, name = ""
    @id = id
	@name = name
  end
  
  def update_name name
    @name = name
	MessageBus.publish :customer_name_updated, { :id => @id, :name => @name }
  end
end

class Logger
  extend MessageBus::EventHandler
  
  subscribes :customer_name_updated

  def self.customer_name_updated args
    id = args.fetch(:id)
	name = args.fetch(:name)
	puts "Customer #{id} changed their name to #{name}"
  end
end

MessageBus.send :update_customer_name, {:id => 1, :name => "Acme LLC" }