LazyServiceIdentifier

Overview

The LazyServiceIdentifier class is a utility that delays the resolution of service identifiers until they're actually needed during dependency injection. Its primary purpose is to prevent accessing class service identifiers before they're fully initialized.

Problem: Accessing Class Service Identifiers Prematurely

In TypeScript/JavaScript applications, when you directly reference a class as a service identifier before the class is fully defined, you might encounter initialization issues:

// ServiceModule.ts
import { injectable, inject } from 'inversify';

@injectable()
export class Service {
  // Potential issue - Service class used before fully defined
  constructor(@inject(AnotherService) private dependency: AnotherService) {}
}

class AnotherService {
  // Implementation
}

This can lead to several issues:

1. TypeScript errors about using variables before they're declared
2. Runtime errors as the class might not be fully initialized when used as a service identifier

Solution: LazyServiceIdentifier

LazyServiceIdentifier solves this problem by deferring the service identifier resolution until the injection process actually requires it:

import { inject, injectable, LazyServiceIdentifier } from 'inversify';

@injectable()
export class Service {
  // Using LazyServiceIdentifier to safely reference the class itself
  constructor(
    @inject(new LazyServiceIdentifier(() => AnotherService))
    public readonly dependency: unknown,
  ) {}
}

@injectable()
export class AnotherService {
  // Implementation
}

API Reference

Constructor

constructor(buildServiceId: () => ServiceIdentifier<TInstance>)

• buildServiceId: A function that returns a ServiceIdentifier. This function is called only when the service identifier is actually needed.

Methods

unwrap(): ServiceIdentifier<TInstance>

Returns the resolved service identifier by calling the function provided in the constructor.

static is<TInstance>(value: unknown): value is LazyServiceIdentifier<TInstance>

Checks if a value is an instance of LazyServiceIdentifier.