目的

要實(shí)現(xiàn)對象的發(fā)布/訂閱行為，只要[Subject]對象改變它的狀態(tài)，就會(huì)通知附加的[observer]。它用于減輕對象的耦合度，并使用松耦合代替。

例子

• 通過觀察消息隊(duì)列系統(tǒng)，可以在GUI中顯示作業(yè)的進(jìn)度

注意

PHP已經(jīng)定義了兩個(gè)接口來幫助實(shí)現(xiàn)這個(gè)模式:SplObserver和SplSubject。

UML 圖

代碼

User.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Behavioral\Observer;

use SplSubject;
use SplObjectStorage;
use SplObserver;

/**
 * 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
{
    private SplObjectStorage $observers;
    private $email;

    public function __construct()
    {
        $this->observers = new SplObjectStorage();
    }

    public function attach(SplObserver $observer): void
    {
        $this->observers->attach($observer);
    }

    public function detach(SplObserver $observer): void
    {
        $this->observers->detach($observer);
    }

    public function changeEmail(string $email): void
    {
        $this->email = $email;
        $this->notify();
    }

    public function notify(): void
    {
        /** @var SplObserver $observer */
        foreach ($this->observers as $observer) {
            $observer->update($this);
        }
    }
}

UserObserver.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Behavioral\Observer;

use SplObserver;
use SplSubject;

class UserObserver implements SplObserver
{
    /**
     * @var SplSubject[]
     */
    private array $changedUsers = [];

    /**
     * It is called by the Subject, usually by SplSubject::notify()
     */
    public function update(SplSubject $subject): void
    {
        $this->changedUsers[] = clone $subject;
    }

    /**
     * @return SplSubject[]
     */
    public function getChangedUsers(): array
    {
        return $this->changedUsers;
    }
}

測試

Tests/ObserverTest.php

<?php

declare(strict_types=1);

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());
    }
}