1.0 Dependency injection

Injector: The injector object that exposes APIs to us to create instances of dependencies.

Provider: A provider is like a recipe that tells the injector how to create an instance of a dependency. A provider is a Function token or a IProvider object.

Dependency: A dependency is the type of which an object should be created.

export interface IProvider {
 provide: any;
 useValue?: any;
 useClass?: Function;
 useFactory?: Function;
 deps?: Array<Function|IProvider>;
}

@Injectable() - is an empty @Provider

@Provider([ServiceA, ServiceB]) - provide list of dependencies required by class.

@Inject(Token) - is used to inject

// Constructor
@Injectable()
export class Car {
  constructor(
    @Inject(Engine) engine,
    @Inject(Tires) tires
  ) {
    // access to tiers and engine
  }
}


// Constructor without @Inject
@Injectable()
export class Car {
  constructor(
    engine: Engine,
    tires: Tires
  ) {}
}


// Injection point via @Inject after object is initialized
@Injectable()
export class Car  implements IAfterConstruct  {
   @Inject(Engine) engine: Engine;
   @Inject(Tires) tires: Tires;

   afterConstruct() { 
     // access to this.engine, this.tires
   }   

   getEngine() {
      return this.engine;
   }
}

How to create an instance of Car via Injector:

import { Injector } from “typeix”;

var injector = Injector.createAndResolve(Car, [
  Engine,
  Tires
]);

var car: Car = injector.get(Car);

Type of injection providers:

Token:

var injector = Injector.createAndResolve(Car, [
  Engine
]);

Is equal to:

var injector = Injector.createAndResolve(Car, [
 {
   provide: Engine,
   useClass: Engine
 }
]);

This is useful for providing custom implementations or mocks for testing purposes.

Providing custom value:

var injector = Injector.createAndResolve(Car, [
 {
   provide: Engine,
   useValue: new Engine()
 }
]);

Providing factory:

var injector = Injector.createAndResolve(Car, [
 {
   provide: Engine,
   useFactory: (a, b) => new Engine(a, b),
   deps: [ServiceA, ServiceB]
 }
]);

More about dependency injection.