Angular 2 http获得可观察的两次调用

这是我的Plunker：http://plnkr.co/edit/SqAiY3j7ZDlFc8q3I212?p=preview

这是服务代码：

import {Injectable} from '@angular/core';
import {Http,Response} from '@angular/http';
import {Observable} from 'rxjs/Rx';
import 'rxjs/add/operator/map';
import 'rxjs/add/operator/do';

@Injectable()
export class DemoService {


  constructor(private http:Http) { }

  // Uses http.get() to load a single JSON file
  getData() {
    return this.http.get('./src/data.json')
      .map((res:Response) => res.json())
      .do(data => console.log(data))
      .subscribe(data => {
        return <PageContent[]>data;
      },error => console.log("there was an error!"));
  }
}

export class PageContent {
  constructor(public _id: string,public tag: string,public title: string,public body?:string,public image?: string) {}
}

……以及使用它的简单组件.

//our root app component
import {Component,NgModule,OnInit } from '@angular/core'
import {BrowserModule} from '@angular/platform-browser'
import { DemoService,PageContent } from './service';
import { HttpModule } from '@angular/http';

@Component({
  selector: 'my-app',template: `
    <div>
      <h2>Hello {{name}}</h2>
    </div>
    <div *ngFor="let page of pages">
      {{ page.title }}
    </div>
  `
})
export class App implements OnInit {
  name:string;
  pages: PageContent[] = [];

  constructor(private _service: DemoService) {
    this.name = 'Angular2'
    this.loadData();  // <-- this is called once
  }

  ngOnInit() {
    //this.loadData();  // <-- this is called twice 
  }

  loadData(){
    this.pages = this._service.getData();
    console.log(this.pages);
  }
}

@NgModule({
  imports: [ BrowserModule,HttpModule ],declarations: [ App ],providers: [DemoService],bootstrap: [ App ]
})
export class AppModule {}

免责声明：它是错误的,但是当从构造??函数调用服务方法时,您可以看到它被提供一次,但是当它在ngOnInit钩子内时被调用两次.

我的问题是,为什么从OnInit函数调用两次？

更新：所有答案的解决方案：

这是新的服务方法：

getData() {
    return this.http.get('./src/data.json')
        .map((res:Response) => res.json() as PageContent[]);
}

…这是新的组件方法：

loadData(){
    this._service.getData()
        .subscribe(data => this.pages = data);
}