Domain Probe

domain_probe.ts

// DOMAIN
// ------

interface ShoppingCartDomainProbe {
  applyDiscountCode(): void;
  discountCodeLookupFailed(error: string): void;
  discountCodeLookupSucceeded(): void;
  discountApplied(amountDiscounted: number): void;
}

interface DiscountService {
  lookup(discountCode: string): number | undefined;
}

type ShoppingCartDomainProbeFactory = (discountCode: string) => ShoppingCartDomainProbe;

class ShoppingCart {
  private _probeFactory: ShoppingCartDomainProbeFactory;
  private _discountService: DiscountService;

  constructor(discountService: DiscountService, probeFactory: ShoppingCartDomainProbeFactory) {
    this._probeFactory = probeFactory;
    this._discountService = discountService;
  }

  public applyDiscount(discountCode: string) {
    const probe = this._probeFactory(discountCode);

    probe.applyDiscountCode();
    const amountDiscounted = this._discountService.lookup(discountCode);
    if (amountDiscounted) {
      probe.discountCodeLookupSucceeded();
      probe.discountApplied(amountDiscounted);
    } else {
      probe.discountCodeLookupFailed(`no discount for code ${discountCode}`);
    }
  }
}

// INFRASTRUCTURE
// --------------

class DummyDiscountService implements DiscountService {
  public lookup(discountCode: string): number | undefined {
    if (discountCode === "HAPPY2020") {
      return 10;
    } else {
      return undefined;
    }
  }
}

interface ObservationMetadata {
  userId: string;
  discountCode: string;
}

class ShoppingCartDomainProbeImpl implements ShoppingCartDomainProbe {
  constructor(public metadata: ObservationMetadata) { }

  public applyDiscountCode(): void {
    console.log(`${(new Date()).toISOString()} ${this.metadata.userId} Applying discount code ${this.metadata.discountCode}`);
  }
  public discountCodeLookupFailed(error: string): void {
    console.log(`${(new Date()).toISOString()} ${this.metadata.userId} Failed to lookup discount code ${this.metadata.discountCode}: ${error}`);
  }
  public discountCodeLookupSucceeded(): void {
    console.log(`${(new Date()).toISOString()} ${this.metadata.userId} Found discount code ${this.metadata.discountCode}`);
  }
  public discountApplied(amountDiscounted: number): void {
    console.log(`${(new Date()).toISOString()} ${this.metadata.userId} Discount code ${this.metadata.discountCode} grants ${amountDiscounted} discount`);
  }
}

class ObservationContext {
  constructor(public userId: string) {}

  public createShoppingCartDomainProbe(discountCode: string): ShoppingCartDomainProbe {
    return new ShoppingCartDomainProbeImpl({ userId: this.userId, discountCode });
  }
}

// APPLICATION
// -----------

const run = (): void => {
  const discountService = new DummyDiscountService();
  const ctxt = new ObservationContext("sylvain");

  const cart = new ShoppingCart(discountService, ctxt.createShoppingCartDomainProbe.bind(ctxt));

  cart.applyDiscount("YEAH");
  cart.applyDiscount("HAPPY2020");
}

run();