angular - HTTP 请求未触发

Question

我是 Angular 的新手，正在观看视频教程来发出 HTTP 请求并获取响应数据。出于某种原因，我使用的代码没有触发 http.get。

import { Component, Injectable } from '@angular/core';
import { Http } from '@angular/http';
import 'rxjs/add/operator/map';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})

@Injectable()
export class AppComponent {

  constructor( private http: Http ){
    console.log('Until here works');
    this.http.get('https://jsonplaceholder.typicode.com/posts').map(data => {
      console.log('Finished');
      console.log( data );
    });
  }

}

Answer 1

这是来自文档:

The http.get does not send the request just yet. This Observable is cold, which means that the request won't go out until something subscribes to the Observable.

有关更多信息和详细示例，请参阅此链接:https://angular.io/docs/ts/latest/guide/server-communication.html

这是一个具体的例子:

getProducts(): Observable<IProduct[]> {
    return this._http.get(this._productUrl)
        .map((response: Response) => <IProduct[]> response.json())
        .do(data => console.log('All: ' +  JSON.stringify(data)))
        .catch(this.handleError);
}

然后这样订阅:

ngOnInit(): void {
    this._productService.getProducts()
            .subscribe(products => this.products = products,
                       error => this.errorMessage = <any>error);
}