Exactly Dan, that was one of my mistakes. Finally I got it by myself, and now I understand the basics. Thank you in any case.

@sdmitch I've already fixed it. Thank you anyway.

I'm trying SKUI and I have created a Window with a Textbox in this way:

    w = SKUI;;Window.new
    txt_box = SKUI;;Textbox.new( "" )
    txt_box.name = ;txt_name
    txt_box.position( 155, 27 )
    txt_box.width = 200
    w.add_control( txt_box )

    label = SKUI;;Label.new( 'Label', txt_box )
    label.position( 10, 30 )
    w.add_control( label )

    btn_cancel = SKUI;;Button.new( 'Cancel' ) { |control|
      return nil
      control.window.close
    }
    btn_cancel.position( -5, -5 )
    w.add_control( btn_cancel )

    btn_ok = SKUI;;Button.new( 'OK' ) { |control|
      control.window.close
    }
    btn_ok.position( -100, -5 )
    w.add_control( btn_ok )

    w.cancel_button = btn_cancel
    w.default_button = btn_ok
    w.show

After that, I type something in the Textbox and I would like to retrieve the string when I click btn_ok. I know the string should be in txt_box.value, but I don't know how to update the value of the TextBox when clicking a button. It is always empty. Do you know how to do it?