如果将服务放入提供者数组，则它们仅被实例化一次.他们基本上是单身.您可以像这样在应用模块中注册服务.

If you put service into providers array they are instantiated only once. They are basically singleton. You register services in app module like this.

providers: [ArticleService, Category1Service, Category2Service],

下一步，我们将在类声明的顶部使用@Injectable()装饰器，以使angular知道可以注入类.

In next step we will use @Injectable() decorator on top of class declaration to let angular know that class can be injected.

然后使用@Inject()将以下服务注入到另一个服务中，例如下面的示例.

And then inject following service to another service using @Inject() as in example bellow.

import { Injectable, Inject } from '@angular/core';

@Injectable()
export class ArticleService {
    public name: string;

    constructor() {
        this.name = "Awesome article";
    };
}

@Injectable()
export class Category1Service {
    constructor(@Inject(ArticleService) public articleService: ArticleService) { }
}

@Injectable()
export class Category2Service {
    constructor(@Inject(ArticleService) public articleService: ArticleService) { }
}

因为我们的Article服务已注册为单例，所以是正确的.

Because our Article service is registered as singleton following is true.

export class FooComponent {
    constructor(
        @Inject(ArticleService) private articleService: ArticleService,
        @Inject(Category1Service) private category1Service: Category1Service,
        @Inject(Category2Service) private category2Service: Category2Service,) {

        // both will print "Awesome article" 
        console.log(category1Service.articleService.name);
        console.log(category2Service.articleService.name);

        // change name in article service
        articleService.name = "Bad article";

        // both will print "Bad article"   
        console.log(category1Service.articleService.name);
        console.log(category2Service.articleService.name);    

    }
}