<?php
require_once 'Subject.php';

abstract class Observer {
  protected $subject = NULL;

  public function attach(Subject $subject) {
    $this->subject = $subject;
    $this->subject->attach($this);
  }

  public function detach() {
    if ($this->subject !== NULL) {
      $this->subject->detach($this);
    }
  }

  public abstract function update();
}
?>