Dependency Injection in Angular 2

Dependency injection

Dependency injection is a way to supply a new instance of a class with the fully-formed dependencies it requires. Most dependencies are services. Angular uses dependency injection to provide new components with the services they need.

Angular can tell which services a component needs by looking at the types of its constructor parameters. For example, the constructor of your HeroListComponent needs a HeroService:

Sample Code-

When Angular creates a component, it first asks an injector for the services that the component requires.

An injector maintains a container of service instances that it has previously created. If a requested service instance is not in the container, the injector makes one and adds it to the container before returning the service to Angular. When all requested services have been resolved and returned, Angular can call the component's constructor with those services as arguments. This is dependency injection.

The process of HeroService injection looks a bit like this:

If the injector doesn't have a HeroService, how does it know how to make one?

In brief, you must have previously registered a provider of the HeroService with the injector. A provider is something that can create or return a service, typically the service class itself.

You can register providers in modules or in components.

In general, add providers to the root module so that the same instance of a service is available everywhere.

Alternatively, register at a component level in the providers property of the @Component metadata:

  selector:    'hero-list',
  templateUrl: './hero-list.component.html',
  providers:  [ HeroService ]
})

Registering at a component level means you get a new instance of the service with each new instance of that component.

Points to remember about dependency injection:

• Dependency injection is wired into the Angular framework and used everywhere.

• The injector is the main mechanism.

  • An injector maintains a container of service instances that it created.
  • An injector can create a new service instance from a provider.

• A provider is a recipe for creating a service.

• Register providers with injectors.