GoFのデザインパターンとは？ 生成・構造・振る舞いの23パターンを体系的に理解する

interface ShippingStrategy {
  fee(subtotal: number): number;
}
class StandardShipping implements ShippingStrategy {
  fee(subtotal: number) { return subtotal >= 5000 ? 0 : 550; }
}
class ExpressShipping implements ShippingStrategy {
  fee(subtotal: number) { return subtotal >= 10000 ? 450 : 1100; }
}
function checkout(subtotal: number, shipping: ShippingStrategy) {
  return subtotal + shipping.fee(subtotal);
}
checkout(4200, new StandardShipping()); // 4750
checkout(4200, new ExpressShipping());  // 5300

type Listener = (orderId: string) => void;
class OrderEvents {
  private listeners = new Set<Listener>();
  subscribe(listener: Listener) {
    this.listeners.add(listener);
    return () => this.listeners.delete(listener);
  }
  confirmed(orderId: string) {
    this.listeners.forEach(listener => listener(orderId));
  }
}
const events = new OrderEvents();
const unsubscribe = events.subscribe(id => console.log(id));
events.confirmed("order-1042");
unsubscribe();

interface Payment {
  pay(amount: number): string;
}
class CardPayment implements Payment {
  pay(amount: number) { return "カード決済 " + amount; }
}
class BankPayment implements Payment {
  pay(amount: number) { return "銀行振込 " + amount; }
}
abstract class PaymentFlow {
  protected abstract createPayment(): Payment;
  run(amount: number) { return this.createPayment().pay(amount); }
}
class CardFlow extends PaymentFlow {
  protected createPayment() { return new CardPayment(); }
}
class BankFlow extends PaymentFlow {
  protected createPayment() { return new BankPayment(); }
}
new CardFlow().run(5000);

class LegacyOrderApi {
  read(): string { return "ORDER-1042"; }
}
interface OrderGateway {
  load(): { orderId: string; source: string };
}
class LegacyOrderAdapter implements OrderGateway {
  constructor(private legacy: LegacyOrderApi) {}
  load() {
    return { orderId: this.legacy.read(), source: "legacy" };
  }
}
const gateway: OrderGateway =
  new LegacyOrderAdapter(new LegacyOrderApi());
gateway.load();