libuing.rb

require 'ffi'

module UI
  extend FFI::Library

  class InitOptions < FFI::Struct
    layout(
      :Size, :size_t
    )
  end

  @@libuing_import_done = false

  def self.load_lib(libpath = Dir.pwd + '/libui.dll')
    ffi_lib_flags :now, :global
    ffi_lib libpath
    import_symbols() unless @@libuing_import_done
  end

  def self.import_symbols()
    callback :WindowOnClosingCallback, [:pointer, :pointer], :int

    attach_function :Init, :uiInit, [:pointer], :pointer
    attach_function :Uninit, :uiUninit, [], :void
    attach_function :NewWindow, :uiNewWindow, [:pointer, :int, :int, :int], :pointer
    attach_function :Main, :uiMain, [], :void
    attach_function :ControlShow, :uiControlShow, [:pointer], :void
    attach_function :WindowOnClosing, :uiWindowOnClosing, [:pointer, :WindowOnClosingCallback, :pointer], :void
    attach_function :Quit, :uiQuit, [], :void
    attach_function :NewLabel, :uiNewLabel, [:pointer], :pointer
    attach_function :WindowSetChild, :uiWindowSetChild, [:pointer, :pointer], :void

    @@libuing_import_done = true
  end
end

main.rb

require_relative 'libuing'

onClosing = Proc.new do |sender, sender_data|
  puts 'Closing ...'
  UI.Quit()
  return 1
end

if __FILE__ == $PROGRAM_NAME
  UI.load_lib()
  init_opts = UI::InitOptions.new
  UI.Init(init_opts)

  w = UI.NewWindow('Hello', 200, 50, 0)
  UI.WindowOnClosing(w, onClosing, nil)

  l = UI.NewLabel('World')
  UI.WindowSetChild(w, l)

  UI.ControlShow(w)
  UI.Main()
  UI.Uninit()
end