例子：

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

let count = 0;

@Injectable()

export class MyService {

 seed: number;

 _count: number;

 constructor(){

   this.seed = Number((Math.random() * 100).toFixed(0));

   this._count = count++;

   console.log('diablo constructor called: ' + this.seed);

 }

}

这个单例行为在Angular DI帮助文档里有提到：

https://angular.io/guide/glossary#injector