为什么Angular2不自动检测组件定义中使用的指令和提供程序？

Question

For custom directives used inside the template, I guess it might make sense for Angular to not know the new html elements/attributes until we tell it in the "directive" property of the Component function.

However, what I don't understand why Angular doesn't figure out what providers we want for dependency injection even though we pass those services names via parameters in the Component constructor function. Was this a design decision, or is Angular unable to figure out the providers needed just from the constructor parameters?

In essence, why do we need providers: [HeroService], when we already have constructor(heroService: HeroService) {...} ?

Answer 1

Providers define two things

1. the scope
   Where you define a provider defines what instance will be injected. Providers added at the root share a single instance with the whole application.
   Providers on a component override root providers for themselves and their subtree.

2. the concrete value
   You can have a provider like

{ provide: HeroService, useClass: SuperHeroService }
{ provide: 'SomeConfig', useValue: 'someValue' }
{ provide: Foo, useFactory: () => new Bar() }

Answer 2

Already taken care of actually. RC5 removed a LOT of boilerplate code.

So you no longer need the 'directives' or 'providers' syntax in your @Component or @Directive declarations.

Link: http://angularjs.blogspot.com/2016/08/angular-2-rc5-ngmodules-lazy-loading.html

Answer 3

This was definitely a design decision, because in some cases you might want a singleton service across your entire application and in another case you maybe need more instances of the same service.

And regarding to the answer by @cDecker32 you still need to register the providers, only now you do it in @NgModules take a look at this documentation:

https://angular.io/docs/ts/latest/guide/ngmodule.html