Use Queue to keep synchronous interface while performing work in parallel. Queue is thread-safe, and extremely easy to use!

gistfile1.txt

require 'thread'

def provider()
  threads = []
  queue = Queue.new
  1.upto(10).each do |n|
    threads << Thread.new {
      sleep rand(0.1..2)
      queue << n
    }
  end
  left = 10
  while left > 0
    val = queue.pop
    yield val
    left -= 1
  end
end

provider { |s| puts s }