X のウィンドウのクラスとタイトルを取得する

win_class_title.rb

require 'ffi'

module FFI
  typedef :pointer, :Display
  typedef :ulong, :XID
  typedef :XID, :Window
  typedef :int, :Status
  typedef :ulong, :Atom
end

class XClassHint < FFI::Struct
  layout(
    :name, :pointer,
    :class, :pointer,
  )
end

class XTextProperty < FFI::Struct
  layout(
    :value, :pointer,
    :encoding, :Atom,
    :format, :int,
    :nitems, :ulong,
  )
end

module X11
  extend FFI::Library
  ffi_lib 'X11'
  attach_function :XOpenDisplay, [:string], :Display
  attach_function :XGetInputFocus, [:Display, :pointer, :pointer], :int
  attach_function :XGetClassHint, [:Display, :Window, XClassHint.by_ref], :int
  attach_function :XQueryTree, [:Display, :Window, :pointer, :pointer, :pointer, :pointer], :Status
  attach_function :XGetWMName, [:Display, :Window, XTextProperty.by_ref], :void
  attach_function :Xutf8TextPropertyToTextList, [:Display, XTextProperty.by_ref, :pointer, :pointer], :int
  attach_function :XFree, [:pointer], :int
  attach_function :XFreeStringList, [:pointer], :void
end

display = X11.XOpenDisplay(nil)
raise "Cannot open display: #{ENV['DISPLAY']}" if display.null?

root = FFI::MemoryPointer.new(:Window)
parent = FFI::MemoryPointer.new(:Window)
children = FFI::MemoryPointer.new(:pointer)
nchildren = FFI::MemoryPointer.new(:int)
focus_win = FFI::MemoryPointer.new(:Window)
revert_to = FFI::MemoryPointer.new(:int)
text_list = FFI::MemoryPointer.new(:pointer)
count = FFI::MemoryPointer.new(:int)
loop do
  X11.XGetInputFocus(display, focus_win, revert_to)
  window = focus_win.read(:Window)
  class_hint = XClassHint.new
  while window > 0
    X11.XGetClassHint(display, window, class_hint)
    X11.XQueryTree(display, window, root, parent, children, nchildren)
    X11.XFree(children.read_pointer)
    break unless class_hint[:name].null? && class_hint[:class].null?
    window = parent.read(:Window)
  end
  prop = XTextProperty.new
  prop[:encoding] = 1
  X11.XGetWMName(display, window, prop)
  X11.Xutf8TextPropertyToTextList(display, prop, text_list, count)
  win_name = class_hint[:name].read_string
  win_class = class_hint[:class].read_string
  title = text_list.read(:pointer).read(:pointer).read_string.force_encoding('utf-8')
  p [win_class, title]
  X11.XFree(class_hint[:name]) unless class_hint[:name].null?
  X11.XFree(class_hint[:class]) unless class_hint[:class].null?
  X11.XFreeStringList(text_list.read(:pointer))
  sleep 0.1
end

Fiddle バージョンはこちら https://gist.github.com/tmtm/8a68a34d1380e14a0fb29b56c96f9753