[Ruby + GTK 3] Simple GTK app

App.rb

require "gtk3"

app = Gtk::Application.new "com.example.ruby-gtk3", :FLAGS_NONE

app.signal_connect :activate do |app|
  head = Gtk::HeaderBar.new
  head.visible = true
  head.title = "Ruby GTK+ 3 example"
  head.show_close_button = true

  win = Gtk::ApplicationWindow.new app
  win.default_width  = 600
  win.default_height = 400
  win.set_titlebar head
  win.present
end

puts app.run