customwidget.rb

require 'gtk3'

Gtk.init

class MyWidget < Gtk::Widget
  type_register

  def initialize
    super

    self.has_window = true
    self.redraw_on_allocate = true
    style_context.add_class 'mywidget'
  end

  def do_get_request_mode
    puts "#{self.class.name}#do_get_request_mode"
    Gtk::SizeRequestMode::HEIGHT_FOR_WIDTH
  end

  def do_get_preferred_width
    puts "#{self.class.name}#do_get_preferred_width"
    [100, 300]
  end

  def do_get_preferred_height
    puts "#{self.class.name}#do_get_preferred_height"
    get_preferred_height_for_width(preferred_width[0])
  end

  def do_get_preferred_height_for_width(width)
    puts "#{self.class.name}#do_get_preferred_heigth_for_width(width=#{width})"
    min = 100
    h = width.zero? ? min : 9000 / width
    [min, h]
  end

  def signal_do_size_allocate(allocation)
    self.allocation = allocation

    x, y, w, h = allocation.x, allocation.y, allocation.width, allocation.height
    realized? and window.move_resize x, y, w, h
  end

  def signal_do_realize
    x, y, w, h = allocation.x, allocation.y, allocation.width, allocation.height
    attr = Gdk::WindowAttr.new w, h, :input_output, :child
    attr.x = x
    attr.y = y
    attr.visual = visual
    attr.event_mask = events | Gdk::EventMask::EXPOSURE_MASK
    attr.wclass = :input_output

    wat = Gdk::WindowAttributesType
    mask = wat::X | wat::Y | wat::VISUAL

    self.window = Gdk::Window.new parent_window, attr, mask
    register_window window

    self.realized = true
  end

  def signal_do_unrealize
    unregister_window window
    window.destroy
    self.realized = false
  end

  def singla_do_map
    super

    window.show
  end

  def signal_do_unmap
    super

    window.hide
  end

  def signal_do_draw(cr)
    x, y, w, h = allocation.x, allocation.y, allocation.width, allocation.height

    Gtk.render_frame style_context, cr, x, y, w, h
    Gtk.render_background style_context, cr, x, y, w, h
  end
end

provider = Gtk::CssProvider.new
provider.load_from_data <<EOF
.mywidget { border: 1px solid red; }
EOF
screen = Gdk::Screen.default
Gtk::StyleContext.add_provider_for_screen screen, provider, :application

mywidget = MyWidget.new

p mywidget.request_mode
p mywidget.preferred_width
p mywidget.get_preferred_height_for_width(100)

box = Gtk::Box.new :vertical
box.add mywidget
box.show_all

window = Gtk::Window.new
window.add box
window.set_size_request 300, 300
window.present

Gtk.main