A very simple event loop written in Crystal. (crystal-lang.org)

reactor.cr

module Reactor

  class Task
    property :block
    def initialize(timestamp=Time.now, milliseconds=0, &block : Loop ->)
      @block = block
      @timestamp = timestamp.epoch_ms
      @milliseconds = milliseconds
    end
    def run reactor
      @block.call reactor
    end
  end

  class Timeout < Task
    def run *args
      if Time.now.epoch_ms >= (@timestamp + @milliseconds)
        super
      else
        args[0].add_to_stack self
      end
    end
  end

  class PeriodicTimer < Task
    def run *args
      if Time.now.epoch_ms >= (@timestamp + @milliseconds)
        super
        @timestamp = Time.now.epoch_ms
      end
      args[0].add_to_stack self
    end
  end

  class Loop
    property :stack
    def initialize(&block : Loop ->)
      @stack = Array(Task|Timeout).new
      @stopped = false
      next_tick &block
      react
    end
    def next_tick(&block : Loop ->)
      @stack << Task.new(Time.now, 0, &block)
    end
    def set_timeout(milliseconds, &block : Loop ->)
      @stack << Timeout.new(Time.now, milliseconds, &block)
    end
    def stop
      @stopped = true
    end
    def add_to_stack obj
      @stack << obj
    end
    def set_interval(milliseconds, &block: Loop ->)
      @stack << PeriodicTimer.new(Time.now, milliseconds, &block)
    end
    private def react
      loop do
        unless @stopped
          some_process = @stack.pop if @stack.length > 0
          some_process.run(self) if some_process
        else
          break
        end
      end
    end
  end
end

Reactor::Loop.new do |reactor|
  puts "I like eggs"
  reactor.set_timeout 3000 do
    puts "I like flowers"
    reactor.set_interval 5000 do
      puts ["foo", "bar", "baz"].sample
    end
  end
  puts "I like trees"
end

Sweet, @fernandes!