의존성 역전 실전

역전과 주입

원칙을 정확히 이해하기

핵심부에서 추상화 정의하기

<?php
// src/Billing/Application/Port/PaymentGateway.php
interface PaymentGateway
{
    public function charge(Money $amount, CardToken $token): ChargeId;
}

final class Money {
    public function __construct(public readonly int $cents, public readonly string $currency) {}
}
final class CardToken { public function __construct(public readonly string $value) {} }
final class ChargeId { public function __construct(public readonly string $value) {} }

어댑터를 추상화에 맞추기

<?php
final class StripePaymentGateway implements PaymentGateway
{
    public function __construct(private \Stripe\StripeClient $stripe) {}

    public function charge(Money $amount, CardToken $token): ChargeId {
        $intent = $this->stripe->paymentIntents->create([
            'amount'   => $amount->cents,
            'currency' => strtolower($amount->currency),
            'payment_method' => $token->value,
            'confirm'  => true,
        ]);
        return new ChargeId($intent->id);
    }
}

생성자 주입을 기본으로 사용하기

<?php
final class CheckoutService
{
    public function __construct(
        private PaymentGateway $payments,   // abstraction, not StripeClient
        private OrderRepository $orders,
    ) {}

    public function pay(OrderId $id, CardToken $token): ChargeId {
        $order  = $this->orders->get($id);
        $charge = $this->payments->charge($order->total(), $token);
        $order->markPaid($charge);
        $this->orders->save($order);
        return $charge;
    }
}

구성 루트

<?php
// public/index.php — composition root
$stripe   = new \Stripe\StripeClient(getenv('STRIPE_SECRET'));
$gateway  = new StripePaymentGateway($stripe);
$orders   = new PdoOrderRepository(new PDO(getenv('DB_DSN')));
$checkout = new CheckoutService($gateway, $orders);

// Everything below depends only on abstractions
$controller = new CheckoutController($checkout);

DI 컨테이너로 연결하기

<?php
use function DI\autowire;
use function DI\get;

return [
    PaymentGateway::class => autowire(StripePaymentGateway::class),
    OrderRepository::class => autowire(PdoOrderRepository::class),
    \Stripe\StripeClient::class => fn() => new \Stripe\StripeClient(getenv('STRIPE_SECRET')),
    PDO::class => fn() => new PDO(getenv('DB_DSN')),
];

어댑터 교체로 원리 입증하기

<?php
final class FakeGateway implements PaymentGateway {
    public array $charges = [];
    public function charge(Money $a, CardToken $t): ChargeId {
        $this->charges[] = $a;
        return new ChargeId('ch_test_' . count($this->charges));
    }
}

$fake = new FakeGateway();
$id = $fake->charge(new Money(2500, 'EUR'), new CardToken('tok_visa'));
echo $id->value, ' charges=', count($fake->charges), PHP_EOL; // ch_test_1 charges=1

서비스 로케이터 함정 피하기

<?php
// ANTI-PATTERN: hidden dependencies, container leaks everywhere
final class BadCheckout {
    public function __construct(private ContainerInterface $c) {}
    public function pay($id, $token) {
        $gateway = $this->c->get(PaymentGateway::class); // hidden!
        // ...
    }
}

컨테이너가 나타나도 되는 곳

서비스 로케이션 없이 지연 연결하기

<?php
final class ReportService
{
    /** @param Closure():PaymentGateway $gatewayFactory */
    public function __construct(private Closure $gatewayFactory) {}

    public function refundIfNeeded(bool $needed): void {
        if (!$needed) return;
        $gateway = ($this->gatewayFactory)(); // built only when required
        // $gateway->charge(...) etc.
    }
}

// Composition root supplies the factory, not the container
$svc = new ReportService(fn() => new StripePaymentGateway($stripe ?? null));
echo 'lazy dependency wired', PHP_EOL;

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