3.7. Observer

3.7.1. Purpose

To implement a publish/subscribe behaviour to an object, whenever a“Subject” object changes its state, the attached “Observers” will benotified. It is used to shorten the amount of coupled objects and usesloose coupling instead.

3.7.2. Examples

• a message queue system is observed to show the progress of a job in aGUI

3.7.3. Note

PHP already defines two interfaces that can help to implement thispattern: SplObserver and SplSubject.

3.7.4. UML Diagram

3.7.5. Code

You can also find this code on GitHub

User.php

<?php
 
namespace DesignPatterns\Behavioral\Observer;
 
/**
 * User implements the observed object (called Subject), it maintains a list of observers and sends notifications to
 * them in case changes are made on the User object
 */
class User implements \SplSubject
{
    /**
     * @var string
     */
    private $email;
 
    /**
     * @var \SplObjectStorage
     */
    private $observers;
 
    public function __construct()
    {
        $this->observers = new \SplObjectStorage();
    }
 
    public function attach(\SplObserver $observer)
    {
        $this->observers->attach($observer);
    }
 
    public function detach(\SplObserver $observer)
    {
        $this->observers->detach($observer);
    }
 
    public function changeEmail(string $email)
    {
        $this->email = $email;
        $this->notify();
    }
 
    public function notify()
    {
        /** @var \SplObserver $observer */
        foreach ($this->observers as $observer) {
            $observer->update($this);
        }
    }
}

UserObserver.php

<?php
 
namespace DesignPatterns\Behavioral\Observer;
 
class UserObserver implements \SplObserver
{
    /**
     * @var User[]
     */
    private $changedUsers = [];
 
    /**
     * It is called by the Subject, usually by SplSubject::notify()
     *
     * @param \SplSubject $subject
     */
    public function update(\SplSubject $subject)
    {
        $this->changedUsers[] = clone $subject;
    }
 
    /**
     * @return User[]
     */
    public function getChangedUsers(): array
    {
        return $this->changedUsers;
    }
}

3.7.6. Test

Tests/ObserverTest.php

<?php
 
namespace DesignPatterns\Behavioral\Observer\Tests;
 
use DesignPatterns\Behavioral\Observer\User;
use DesignPatterns\Behavioral\Observer\UserObserver;
use PHPUnit\Framework\TestCase;
 
class ObserverTest extends TestCase
{
    public function testChangeInUserLeadsToUserObserverBeingNotified()
    {
        $observer = new UserObserver();
 
        $user = new User();
        $user->attach($observer);
 
        $user->changeEmail('foo@bar.com');
        $this->assertCount(1, $observer->getChangedUsers());
    }
}