Angular 2 Http Get Response Example

问题:

What is the correct way to get json data from an http get in Angular 2. I am working on testing some local data with a mocked endpoint, and I can see the result in the http.get() but I cannot assign it locally or there is some timing issue. Here is my simple service:

import {Injectable} from '@angular/core';
import {Http} from '@angular/http';
import 'rxjs/add/operator/map';  // we need to import this now

    @Injectable()
    export class MyHttpService {
    constructor(private _http:Http) {}

    getDataObservable(url:string) {
        return this._http.get(url)
            .map(data => {
                data.json();
                console.log("I CAN SEE DATA HERE: ", data.json());
        });
    }
}

And here is how i'm trying to assign the data:

import {Component, ViewChild} from "@angular/core";

import { MyHttpService } from '../../services/http.service';

@Component({
    selector: "app",
    template: require<any>("./app.component.html"),
    styles: [
        require<any>("./app.component.less")
    ],
    providers: []
})
export class AppComponent implements OnInit, AfterViewInit {
    private dataUrl = 'http://localhost:3000/testData';  // URL to web api
    testResponse: any;

    constructor(private myHttp: MyHttpService) {
    }

    ngOnInit() {
        this.myHttp.getDataObservable(this.dataUrl).subscribe(
            data => this.testResponse = data
        );
        console.log("I CANT SEE DATA HERE: ", this.testResponse);
    }
}

I can see the data I want in the get() call but I don't seem to have access to it after that call...please tell me what i'm doing wrong!

回答1:

That's not supposed to work this way. When the data arrives the callback passed to the observable is called. Code that needs to access this data has to be inside the callback.

ngOnInit() {
    this.myHttp.getDataObservable(this.dataUrl).subscribe(
        data => {
          this.testResponse = data;
          console.log("I CANT SEE DATA HERE: ", this.testResponse);
        }
    );
}

update

getDataObservable(url:string) {
    return this._http.get(url)
        .map(data => {
            data.json();
            // the console.log(...) line prevents your code from working 
            // either remove it or add the line below (return ...)
            console.log("I CAN SEE DATA HERE: ", data.json());
            return data.json();
    });
}

回答2:

Because http call is async. You need to make assignments in the subscribe function. Try like this:

this.myHttp.getDataObservable(this.dataUrl).subscribe(
            data => {
                      this.testResponse = data ;
                      console.log("I SEE DATA HERE: ", this.testResponse);
                     }
        );

回答3:

Here is an easy to use sample that allows you to use promises.

import { Injectable } from '@angular/core';
import { Http } from '@angular/http';
import { Config } from '../Config';
import { Observable } from 'rxjs/Observable';
import 'rxjs/Rx';

@Injectable()
export class Request {

    constructor(public http: Http)
    {

    }

    get(url): Promise<any>
    {
        return this.http.get(Config.baseUrl + url).map(response => {
            return response.json() || {success: false, message: "No response from server"};
        }).catch((error: Response | any) => {
            return Observable.throw(error.json());
        }).toPromise();
    }

    post(url, data): Promise<any>
    {
        return this.http.post(Config.baseUrl + url, data).map(response => {
            return response.json() || {success: false, message: "No response from server"};
        }).catch((error: Response | any) => {
            return Observable.throw(error.json());
        }).toPromise();
    }
}