Structural Design Patterns

Structural patterns explain how to assemble objects and classes into larger structures.

Adapter Pattern

// Legacy system
class LegacyPaymentGateway {
  makePayment(amount: number, account: string): boolean {
    console.log(`Legacy: Processing $${amount}`);
    return true;
  }
}

// Modern interface
interface PaymentProcessor {
  processPayment(payment: PaymentRequest): Promise;
}

// Adapter makes legacy compatible with modern interface
class LegacyPaymentAdapter implements PaymentProcessor {
  private legacyGateway = new LegacyPaymentGateway();

  async processPayment(payment: PaymentRequest): Promise {
    const success = this.legacyGateway.makePayment(payment.amount, payment.account);
    return { success, transactionId: success ? 'TXN_' + Date.now() : '' };
  }
}

Decorator Pattern

interface Coffee {
  getDescription(): string;
  getCost(): number;
}

class SimpleCoffee implements Coffee {
  getDescription(): string { return 'Simple Coffee'; }
  getCost(): number { return 2.00; }
}

class MilkDecorator implements Coffee {
  constructor(private coffee: Coffee) {}
  getDescription(): string { return this.coffee.getDescription() + ', Milk'; }
  getCost(): number { return this.coffee.getCost() + 0.50; }
}

class WhipDecorator implements Coffee {
  constructor(private coffee: Coffee) {}
  getDescription(): string { return this.coffee.getDescription() + ', Whip'; }
  getCost(): number { return this.coffee.getCost() + 0.75; }
}

// Decorators can be stacked
let coffee: Coffee = new SimpleCoffee();
coffee = new MilkDecorator(coffee);
coffee = new WhipDecorator(coffee);
console.log(coffee.getDescription()); // Simple Coffee, Milk, Whip
console.log(coffee.getCost());        // 3.25

Facade Pattern

// Complex subsystems
class VideoDecoder { decode(file: string): VideoStream { /* ... */ } }
class AudioDecoder { decode(file: string): AudioStream { /* ... */ } }
class VideoRenderer { render(video: VideoStream, audio: AudioStream): void { /* ... */ } }

// Facade simplifies the complex interface
class MediaPlayerFacade {
  private videoDecoder = new VideoDecoder();
  private audioDecoder = new AudioDecoder();
  private renderer = new VideoRenderer();

  play(videoFile: string): void {
    const video = this.videoDecoder.decode(videoFile);
    const audio = this.audioDecoder.decode(videoFile);
    this.renderer.render(video, audio);
  }
}

// Client uses simple facade interface
const player = new MediaPlayerFacade();
player.play('movie.mp4');

Structural Patterns

Structural Patterns

Structural Design Patterns

Adapter Pattern

Decorator Pattern

Facade Pattern