Unblocking the Keyboard and Readline in EventMachine

Posted: 2012-02-12 in Ruby » EventMachine

Tags: ruby, eventmachine, readline, keyboard

Reading from the keyboard without blocking under EventMachine is pretty simple. Just use the builtin EM.open_keyboard and give it a connection handler.

The Code Snippits Wiki for EventMachine gives an easy to follow example:

1
2
3
4
5
6
7
8
9
10
11
 
require 'eventmachine'

module MyKeyboardHandler
  def receive_data keystrokes
    puts "I received the following data from the keyboard: #{keystrokes}"
  end
end

EM.run {
   EM.open_keyboard(MyKeyboardHandler)
}

If we want niceties like tab-completion, history, emacs edit mode, etc.,. you're out of luck. Normally you'd just use readline, but it will block. Maybe that's a good thing. Maybe you want the world to stop when asking the human questions. In some situations you don't want to stop the reactor loop though.

The best solution I've come up with is to replace $stdin with an EventMachine keyboard connection and use RbReadline.

RbReadline defaults to using $stdin for input, so when the NbKeyboard starts it makes $stdin point to itself. When RbReadline calls #read(1) on $stdin, NbKeyboard gives it back one character if it's available. Otherwise it pauses the current Fiber until the human types something in.

The Stlib Readline isn't supported because I haven't found a good way to get it to read from NbKeyboard rather than STDIN.

To verify that this works just run the demo: 

caleb@kiff % ruby demo.rb
enter something ........ab.cd.ef.g.h.k.li..
you entered: "abcdefghkli"
..
caleb@kiff %

STDIN Blocking Note

Normally calling STDIN.read_nonblock(1) or STDIN.read(1) will not return until the user hits enter. The RbReadline and even standard Readline reset the terminal to not wait for the enter key to be pressed.

If you use EM.open_keyboard without RbReadline attached to it, your keyboard handler will not receive data until the user presses enter. You can get around that by using the stty program, or the rb-termios gem to reconfigure the terminal manually.