function register_callback() {

  window.frame_observer = new FrameObserver;
  var fo = window.parent.frame_observer;

  var cs = function(message) {
    frame = document.getElementById('offer');
    frame.height = parseInt(frame.height) + parseInt(message[0]);
  };

  fo.subscribe('change-size', cs);
  
  var ss = function(message) {
    frame = document.getElementById('offer');
    frame.height = parseInt(message[0]);
  }
  
  fo.subscribe('set-size', ss);
}


window.onload = function() {
  register_callback();
}