1. 设计模式
  2. 行为型模式
  3. 观察者模式

观察者模式

目的

观察者模式为对象实现“发布/订阅”功能，每当“主题”状态改变的时候，将通知所有被绑定的“观察者”。观察者模式用于减少耦合的对象数量，使用松散耦合来代替。

使用场景

  • 在 GUI 程序中，通过观察消息队列来显式作业执行进度

说明

PHP 中提供了两个可以帮助我们实现观察者模式的接口：SplObserver 和 SplSubject。

UML 类图

观察者模式类图

代码

User.php

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<?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 string $email;
    private SplObjectStorage $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

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<?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)
    {
        $this->changedUsers[] = clone $subject;
    }

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

测试

Tests/ObserverTest.php

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<?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());
    }
}
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