Some defer for that "A"

completions.rb

require 'eventmachine'

class HardWorker

  def initialize(name)
    @name = name
    @completion = EM::Completion.new
  end

  def work
    operation = Proc.new {
      10.times do |i|
        puts "starting job #{@name} - ##{i}"
        sleep rand(4)
      end
    }

    callback = Proc.new { @completion.succeed(Time.now) }

    EM.defer operation, callback
    @completion
  end
end

class CompletionCollection

  def initialize
    @worker_count = 3
    @completions  = []
  end

  def run
    @worker_count.times do |i|
      worker = HardWorker.new "Job#{i}"
      @completions << completion = worker.work

      completion.callback { |time|
        @completions.pop
        time_str = time.strftime('%e %b %Y %H:%m:%S%p')
        puts "Job#{i} is finished at (#{time_str})."
        EM.stop if @completions.empty?
      }

      completion.errback  { puts("Job#{i} failed.") }
    end
  end
end

EM.run do
  CompletionCollection.new.run
end