Simple event emitter

event_emitter.rb

# Returns an event emitter class with given event names
# @return [Object] the event emitter class
# @param [String, Symbol] event_names
# @example Simple event emitter
#    class CrawlEvent < EventEmitter(:found_url, :page_title)
#    end
#    
#    event = CrawlEvent.new do |on|
#      on.every_found_url { {|url| puts "Found URL: #{url}" }
#      on.every_page_title { {|title| puts "Page title: #{title}" }
#      # you can add multiple blocks
#      on.every_page_title { |title| puts "[WARNING] Title too long!: #{title}" if title.length > 155 }
#    end
#
#    event.emit_page_title(page_title)
#    event.emmit_found_url(url)
def self.EventEmitter(*event_names)
  Class.new do
    def initialize
      yield(self) if block_given?
    end

    event_names.each do |name|
      define_method("emit_#{name}") do |*args|
        blocks_for(name).each { |block| block.call(*args) }
      end

      define_method("every_#{name}") do |*args, &block|
        raise(ArgumentError, 'block must be given!') unless block
        blocks_for(name) << block
      end

      define_method(:blocks_for) do |name|
        variable_name = "@#{name}_blocks"
        if value = instance_variable_get(variable_name)
          value
        else
          instance_variable_set(variable_name, [])
        end
      end
    end
  end
end

event_emitter_spec.rb

require "spec_helper"
require "event_emitter"

RSpec.describe EventEmitter(:data_point) do
  describe "#initialize" do
    it "yields self to block" do
      event = nil

      event_handler = described_class.new do |handler|
        event = handler
      end

      expect(event).to eq(event_handler)
    end
  end


  describe "#data_point event" do
    it "can register and emit event" do
      result = nil

      handler = described_class.new do |on|
        on.every_data_point { |data| result = data }
      end

      data = 2
      handler.emit_data_point(data)

      expect(result).to eq(data)
    end
  end
end