Gof Abstract Factory

# GOF Abstract Factory

> Create families of related objects through factory interfaces without coupling to concrete types.

## When to Use

- You need to create multiple related objects that must work together (a UI theme, a cloud provider SDK, a DB + cache pair)
- You want to swap an entire product family at runtime without changing client code
- You have multiple variants of a product suite that must remain internally consistent
- A simple factory method is insufficient because you're producing more than one related type

## Instructions

**Define the abstract factory and product interfaces first:**

```typescript
// Abstract product interfaces
interface Button {
  render(): string;
  onClick(handler: () => void): void;
}

interface TextField {
  render(): string;
  getValue(): string;
}

// Abstract factory interface
interface UIFactory {
  createButton(label: string): Button;
  createTextField(placeholder: string): TextField;
}
```

**Implement concrete families:**

```typescript
// --- Dark theme family ---
class DarkButton implements Button {
  constructor(private label: string) {}
  render() {
    return `<button class="dark">${this.label}</button>`;
  }
  onClick(handler: () => void) {
    /* attach event */
  }
}

class DarkTextField implements TextField {
  private value = '';
  constructor(private placeholder: string) {}
  render() {
    return `<input class="dark" placeholder="${this.placeholder}" />`;
  }
  getValue() {
    return this.value;
  }
}

class DarkThemeFactory implements UIFactory {
  createButton(label: string): Button {
    return new DarkButton(label);
  }
  createTextField(placeholder: string): TextField {
    return new DarkTextField(placeholder);
  }
}

// --- Light theme family ---
class LightButton implements Button {
  constructor(private label: string) {}
  render() {
    return `<button class="light">${this.label}</button>`;
  }
  onClick(handler: () => void) {
    /* attach event */
  }
}

class LightTextField implements TextField {
  private value = '';
  constructor(private placeholder: string) {}
  render() {
    return `<input class="light" placeholder="${this.placeholder}" />`;
  }
  getValue() {
    return this.value;
  }
}

class LightThemeFactory implements UIFactory {
  createButton(label: string): Button {
    return new LightButton(label);
  }
  createTextField(placeholder: string): TextField {
    return new LightTextField(placeholder);
  }
}
```

**Client code depends only on the abstract factory:**

```typescript
class LoginForm {
  private submitButton: Button;
  private emailField: TextField;

  constructor(factory: UIFactory) {
    this.submitButton = factory.createButton('Log In');
    this.emailField = factory.createTextField('Email address');
  }

  render(): string {
    return `<form>${this.emailField.render()}${this.submitButton.render()}</form>`;
  }
}

// Swap families by changing the factory
const theme = process.env.THEME === 'dark' ? new DarkThemeFactory() : new LightThemeFactory();
const form = new LoginForm(theme);
console.log(form.render());
```

**Cloud provider example (more realistic for backend):**

```typescript
interface StorageProvider {
  upload(key: string, data: Buffer): Promise<string>;
  download(key: string): Promise<Buffer>;
}

interface QueueProvider {
  publish(topic: string, message: object): Promise<void>;
  subscribe(topic: string, handler: (msg: object) => void): void;
}

interface CloudFactory {
  createStorage(bucket: string): StorageProvider;
  createQueue(): QueueProvider;
}

class AWSFactory implements CloudFactory {
  createStorage(bucket: string): StorageProvider {
    return new S3Storage(bucket);
  }
  createQueue(): QueueProvider {
    return new SQSQueue();
  }
}

class GCPFactory implements CloudFactory {
  createStorage(bucket: string): StorageProvider {
    return new GCSStorage(bucket);
  }
  createQueue(): QueueProvider {
    return new PubSubQueue();
  }
}
```

## Details

**Abstract Factory vs. Factory Method:** Factory Method uses inheritance — one factory method per subclass. Abstract Factory uses composition — inject a factory object that produces a whole family. Abstract Factory is the better choice when you have multiple products that vary together.

**Adding a new product to an existing family breaks the Open/Closed Principle** — every concrete factory must implement the new method. This is the main drawback. Mitigate by providing a default implementation in an abstract base factory.

**Anti-patterns:**

- Abstract Factory that creates unrelated objects — the products must be a coherent family
- Overusing when only one product variant exists — use a simple factory function instead
- Mixing factory selection logic into the factory itself — the factory should only create, not decide

**Testing tip:** Inject a `MockFactory` that returns stub implementations for every product type. This decouples all component tests from real infrastructure.

## Source

refactoring.guru/design-patterns/abstract-factory

## Process

1. Read the instructions and examples in this document.
2. Apply the patterns to your implementation, adapting to your specific context.
3. Verify your implementation against the details and edge cases listed above.

## Harness Integration

- **Type:** knowledge — this skill is a reference document, not a procedural workflow.
- **No tools or state** — consumed as context by other skills and agents.

## Success Criteria

- The patterns described in this document are applied correctly in the implementation.
- Edge cases and anti-patterns listed in this document are avoided.