Angular 2 http得到可观察的两次调用

Question

在Angular 2 v2.0.1中,onInit被调用两次.(显然我在做一次调用时也做错了,但现在不是问题)

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);
}

Answer 1

您subscribe应该放在组件而不是服务中.作为您的组件的原因是订阅从服务返回的数据,稍后您可以根据需要取消订阅或添加更多控制(例如谴责).更改后代码将如下所示.

在您的组件中:

  ngOnInit() {
    this.loadData();
  }



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

在你的服务:

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