Implementing observer pattern in PHP >= 5.4

demo.php

<?php
require_once 'observer.php';
require_once 'subject.php';

use Common\Observer\Observer;
use Common\Observer\Subject;

class Notice
{
    use Subject;

    /**
     * The title
     * @var string
     */
    private $title = '';

    /**
     * The notice content
     * @var unknown_type
     */
    private $content = '';

    /**
     * The last modified time
     * @var unknown_type
     */
    private $lastModified = null;

    /**
     * The constructor
     */
    public function __construct($title, $content)
    {
        $this->title = $title;
        $this->content = $content;
        $this->lastModified = new DateTime();
    }
    
    public function __get($attribute)
    {
        return $this->$attribute;
    }
    
    public function __set($attribute, $value)
    {
        $this->lastModified = new DateTime();
        $this->$attribute = $value;
        $this->notify();  // notify all observers have be registered to this instance
    }
}


class NoticeBoard implements Observer
{
    private function drawSingleLine($char='-', $repeat=80)
    {
        while($repeat--) {
            echo $char;
        }
        echo '<br />';
    }
    
    public function display($notice)
    {
        $this->drawSingleLine();
        echo "Title: {$notice->title}" . '<br />';
        echo "Content: {$notice->content}" . '<br />';
        echo "Last Modified At: {$notice->lastModified->format('Y-m-d H:i:s')}" . '<br />';
        $this->drawSingleLine();
    }
    
    public function update($subject)
    {
        $this->drawSingleLine('=');
        echo '<strong>' . '[A notice has been updated]' . '</strong>';
        echo "The notice '{$subject->title}' has been modified." . '<br />';
        echo "New Content: {$subject->content}" . '<br />';
        echo "Last Modified At: {$subject->lastModified->format('Y-m-d H:i:s')}" . '<br />';
        $this->drawSingleLine('=');
    }
}


/**
 * Let the notice be displayed again while it being modified.
 * 
 * @author TonySeek
 *
 */
class NoticeDisplayAgainWhileModifying implements Observer
{
    public function __construct($board)
    {
        $this->board = $board;
    }
    
    public function update($subject)
    {
        $this->board->display($subject);
    }
}


$notice = new Notice('Be careful', 'This is a dangerous behavior.');

$board = new NoticeBoard();
$displayAgain = new NoticeDisplayAgainWhileModifying($board);

$notice->attachObserver($board);
$notice->attachObserver($displayAgain);  // try to remove this line

// the first display
$board->display($notice);

// modified the content
echo "<br />Now, we try to modify the notice's content." . '<br />' . '<br />';
$notice->content = 'The behavior is safty but you should be careful still.';

observer.php

<?php
namespace Common\Observer;

interface Observer
{
    /**
     * @param \Common\Observer\Subject $subject
     */
    function update($subject);
}

subject.php

<?php
namespace Common\Observer;

use SplObjectStorage;

/**
 * The subject implement in the observer pattern.
 * 
 * @author TonySeek
 *
 */
trait Subject
{
    private $observers = null;

    /**
     * Create or get existed observer object container.
     * 
     * @return \SplObjectStorage;
     */
    private function getObserverStorage()
    {
        if (!$this->observers) {
            $this->observers = new SplObjectStorage();
        }
        return $this->observers;
    }
    
    /**
     * @return \Common\Observer\Subject
     */
    public function attachObserver(Observer $observer)
    {
        $storage = $this->getObserverStorage();
        $storage->attach($observer);
        return $this;
    }
    
    /**
     * @return \Common\Observer\Subject
     */
    public function detachObserver(Observer $observer)
    {
        $storage = $this->getObserverStorage();
        $storage->detach($observer);
        return $this;
    }
    
    /**
     * @see SplSubject::notify()
     */
    public function notify()
    {
        $storage = $this->getObserverStorage();
        foreach($storage as $observer) {
            $observer->update($this);
        }
    }
}