Use DIOD library for enabling dependency injection

[NickSeagull]

If we switch to a class based service approach we can easily benefit from automatic dependency injection, instead of having to pass services around manually.

I don't like the usage of dependency injection frameworks as they do a lot of magic under the hoods in runtime.

Yet, there's the diod library, that is a very thin layer that helps constructing the instances of the services based on the metadata generated by the TypeScript compiler (or metadata-booster, which is compatible).

The tl;dr is something like:

1. Create the service, which must be decorated. The decorator can be anything, it is just used to trigger the metadata generation by TypeScript. The dependencies to other services are added in the constructor.

@Service()
export class SignUpUseCase {
  constructor(
    readonly userRepository: UserRepository,
    readonly mailer: Mailer
  ) {}

  // define any method you want
  public async execute(...){...}
}

2. Initialize the services in a single file for the project (usually named `diod.config.ts` but it doesn't matter)

```ts
import { ContainerBuilder } from 'diod'
// Other imports...

const builder = new ContainerBuilder()

// Register interfaces and implementations with .register(...).use(...)
builder.register(Mailer).use(AcmeMailer)
builder.register(UserRepository).use(SqliteUserRepository)

// Or if you've got only one interface and one implementation in the same class, do so
// with only one call
builder.registerAndUse(SignUpUseCase)

// Initialize the services container, it will check for circular dependencies and other
// things that might make it go wrong
const container = builder.build()


// Get the service for your main use case
const signUpUseCase = container.get(SignUpUseCase)

// Run the method normally.
signUpUseCase.execute({
  /* ... */
})

The good part about this is that we can make the decorator add stuff like automatic tracing to all of our services for free (#1383)

Related discussions

• Use class-based services instead of functions #1381
• Automatic Tracing Decorator #1383