Controlling service instance creation and injection with NgModule

In a stark departure from Angular 1.x, Angular 2 features a hierarchical injection scheme. This has a substantial number of implications, and one of the more prominent one is the ability to control when, and how many, services are created.

Note

The code, links, and a live example of this are available at http://ngcookbook.herokuapp.com/2102/.

Getting ready

Suppose you begin with the following simple application:

[app/root.component.ts] 
 
import {Component} from '@angular/core'; 
 
@Component({ 
  selector: 'root', 
  template: ` 
    <h1>root component!</h1> 
    <article></article> 
    <article></article> 
  ` 
}) 
export class RootComponent {} 
[app/article.component.ts] 
 
import {Component} from '@angular/core'; 
 
@Component({ 
  selector: 'article', 
  template: ` 
    <p>Article...