DDD UseCase Generator

Generate application layer use cases and event listeners with proper dependency injection, domain event emission, and orchestration logic.

What This Skill Does

Creates application layer components that orchestrate domain logic:

• Use Cases: Single-responsibility classes with execute() method
• Event Listeners: Handlers for domain events
• Proper DI: tsyringe decorators and injection patterns
• Event Emission: Domain event publishing after operations
• Error Handling: Domain error propagation

When to Use This Skill

Use when you need to:

• Add new business operations to existing context
• Create complex workflows orchestrating multiple entities
• Implement event-driven logic
• Add CRUD operations for entities

Examples:

• "Create a RegisterUser use case with email verification"
• "Generate a PlaceOrder use case with inventory check"
• "Add a ProcessPayment use case with transaction handling"

UseCase Pattern

import { inject, injectable } from 'tsyringe';
import type { IEntityRepository } from '@/contexts/context/domain';
import { EntityNotFoundError } from '@/contexts/context/domain';
import { eventBus } from '@/global/events/event-bus';
import { EntityCreated } from '@/contexts/context/domain';

export interface ExecuteInput {
  field1: string;
  field2: number;
}

export interface ExecuteOutput {
  id: string;
  field1: string;
  field2: number;
}

@injectable()
export class ActionEntityUseCase {
  constructor(
    @inject('IEntityRepository')
    private readonly entityRepo: IEntityRepository,

    @inject(OtherUseCase)
    private readonly otherUseCase: OtherUseCase
  ) {}

  async execute(input: ExecuteInput): Promise<ExecuteOutput> {
    this.validateInput(input);

    const entity = await this.entityRepo.findById(input.id);
    if (!entity) {
      throw new EntityNotFoundError(input.id);
    }

    entity.performBusinessLogic(input.field1);

    await this.entityRepo.save(entity);

    await eventBus.emit('EntityCreated', new EntityCreated(
      entity.getId(),
      entity.getField1()
    ));

    return this.toOutput(entity);
  }

  private validateInput(input: ExecuteInput): void {
    if (!input.field1) {
      throw new InvalidDataError('Field1 required');
    }
  }

  private toOutput(entity: Entity): ExecuteOutput {
    return {
      id: entity.getId(),
      field1: entity.getField1(),
      field2: entity.getField2(),
    };
  }
}

Event Listener Pattern

import { inject, injectable } from 'tsyringe';
import type { EntityCreated } from '@/contexts/context/domain';
import type { IRelatedRepository } from '@/contexts/related/domain';

@injectable()
export class EntityCreatedListener {
  constructor(
    @inject('IRelatedRepository')
    private readonly relatedRepo: IRelatedRepository
  ) {}

  async handle(event: EntityCreated): Promise<void> {
    try {
      await this.performAction(event);
    } catch (error) {
      console.error('Error handling EntityCreated:', error);
    }
  }

  private async performAction(event: EntityCreated): Promise<void> {
    // Listener logic
  }
}

Dependency Injection Rules

Repositories: Inject by string token

constructor(
  @inject('IUserRepository')
  private readonly userRepo: IUserRepository
) {}

Use Cases: Inject by class

constructor(
  @inject(OtherUseCase)
  private readonly otherUseCase: OtherUseCase
) {}

Services: Inject by token or class depending on type

Common Use Case Types

Create Use Case

@injectable()
export class CreateEntityUseCase {
  constructor(
    @inject('IEntityRepository')
    private readonly repo: IEntityRepository
  ) {}

  async execute(input: CreateInput): Promise<CreateOutput> {
    this.validateInput(input);

    const entity = Entity.create({
      field1: input.field1,
      field2: input.field2,
    });

    await this.repo.save(entity);

    await eventBus.emit('EntityCreated', new EntityCreated(
      entity.getId()
    ));

    return this.toOutput(entity);
  }
}

Find Use Case

@injectable()
export class FindEntityUseCase {
  constructor(
    @inject('IEntityRepository')
    private readonly repo: IEntityRepository
  ) {}

  async execute(id: string): Promise<FindOutput> {
    const entity = await this.repo.findById(id);

    if (!entity) {
      throw new EntityNotFoundError(id);
    }

    return this.toOutput(entity);
  }
}

Update Use Case

@injectable()
export class UpdateEntityUseCase {
  constructor(
    @inject('IEntityRepository')
    private readonly repo: IEntityRepository
  ) {}

  async execute(input: UpdateInput): Promise<UpdateOutput> {
    const entity = await this.repo.findById(input.id);
    if (!entity) {
      throw new EntityNotFoundError(input.id);
    }

    entity.updateField(input.field);

    await this.repo.save(entity);

    await eventBus.emit('EntityUpdated', new EntityUpdated(
      entity.getId()
    ));

    return this.toOutput(entity);
  }
}

Delete Use Case

@injectable()
export class DeleteEntityUseCase {
  constructor(
    @inject('IEntityRepository')
    private readonly repo: IEntityRepository
  ) {}

  async execute(id: string): Promise<void> {
    const exists = await this.repo.exists(id);
    if (!exists) {
      throw new EntityNotFoundError(id);
    }

    await this.repo.delete(id);

    await eventBus.emit('EntityDeleted', new EntityDeleted(id));
  }
}

Critical Rules

MUST DO:

• Add @injectable() decorator
• Inject repositories with string tokens
• Inject use cases by class
• Single public execute() method
• Define input/output interfaces
• Validate input
• Emit domain events after success
• Throw domain errors (not HTTP exceptions)
• Return plain objects (not entities)

MUST NOT:

• Forget @injectable() decorator
• Mix injection patterns (token vs class)
• Include HTTP concerns (status codes)
• Put business logic in use case (belongs in entity)
• Return domain entities directly
• Skip input validation
• Throw HTTP exceptions

Generated Files

/src/contexts/{Context}/application/
├── usecases/
│   ├── create-{entity}.usecase.ts
│   ├── find-{entity}.usecase.ts
│   ├── find-all-{entity}.usecase.ts
│   ├── update-{entity}.usecase.ts
│   └── delete-{entity}.usecase.ts
├── listeners/
│   └── {event}.listener.ts
└── index.ts

Integration

Update Application Barrel Export

// application/index.ts
export * from './usecases/create-entity.usecase';
export * from './usecases/find-entity.usecase';
export * from './usecases/update-entity.usecase';
export * from './usecases/delete-entity.usecase';
export * from './listeners/entity-created.listener';

Register Event Listener

Add to /src/main.ts:

import { EntityCreatedListener } from '@/contexts/entity/application';

const listener = container.resolve(EntityCreatedListener);
eventBus.on('EntityCreated', listener.handle.bind(listener));

Validation Checklist

After generation, verify:

• All use cases have @injectable()
• Repositories injected with string tokens
• Use cases injected by class
• Single execute() method present
• Input/output interfaces defined
• Input validation implemented
• Domain events emitted after success
• Domain errors thrown (not HTTP)
• Returns plain objects
• Event listeners have @injectable()
• Listeners have handle() method
• Listeners catch and log errors

Related Skills

• ddd-context-generator: Generate complete context
• ddd-entity-generator: Generate entities used by use cases
• ddd-api-generator: Generate controllers that call use cases
• di-helper: Check DI registration