コードを記述して PHP8 設計パターンを学習して適用する方法

class Database
{
    private static ?Database $instance = null;
    
    private function __construct()
    {
        // 私有构造方法
    }
    
    public static function getInstance(): Database
    {
        if (self::$instance === null) {
            self::$instance = new self();
        }
        
        return self::$instance;
    }
    
    // 其他方法
}

$db = Database::getInstance();

interface Product
{
    public function getName(): string;
}

class ConcreteProductA implements Product
{
    public function getName(): string {
        return 'Product A';
    }
}

class ConcreteProductB implements Product
{
    public function getName(): string {
        return 'Product B';
    }
}

class ProductFactory
{
    public function createProduct(string $type): ?Product
    {
        if ($type === 'A') {
            return new ConcreteProductA();
        } elseif ($type === 'B') {
            return new ConcreteProductB();
        }
        
        return null;
    }
}

$factory = new ProductFactory();
$productA = $factory->createProduct('A');
$productB = $factory->createProduct('B');

class Subject implements SplSubject
{
    private array $observers = [];
    private string $state;
    
    public function attach(SplObserver $observer): void
    {
        $this->observers[] = $observer;
    }
    
    public function detach(SplObserver $observer): void
    {
        $index = array_search($observer, $this->observers);
        
        if ($index !== false) {
            unset($this->observers[$index]);
        }
    }
    
    public function notify(): void
    {
        foreach ($this->observers as $observer) {
            $observer->update($this);
        }
    }
    
    // 其他方法
}

class Observer implements SplObserver
{
    public function update(SplSubject $subject): void
    {
        $state = $subject->getState();
        // 更新操作
    }
}

$subject = new Subject();
$observer1 = new Observer();
$observer2 = new Observer();

$subject->attach($observer1);
$subject->attach($observer2);

$subject->setState('new state');
$subject->notify();