ruby readline while printing

prompt.rb

# encoding: UTF-8
require "EventMachine"
require "readline"

prompt = '⚡ '

def mutex
    @mutex || Mutex.new
end

def sync(&block)
    mutex.synchronize do
        block.call
    end
end

def output
    @num ||= 0
    @num = (@num+1) % 5
    STDOUT.print "\e[0G\e[K"
    puts @num
    Readline.refresh_line
end

def stop
    EM.stop_event_loop
end

EM.run do
    Thread.start do
        while buf = Readline.readline(prompt, true)
            unless Readline::HISTORY.count == 1
                Readline::HISTORY.pop if buf.empty? || Readline::HISTORY[-1] == Readline::HISTORY[-2]
            end
            sync {
                puts buf
            }
        end
        stop
    end

    EM.add_periodic_timer(3) do
        if Readline.line_buffer.nil? || Readline.line_buffer.empty?
            sync {
                output
            }
        end
    end

    trap('INT') { stop }

end