ng2 从 HttpGet 服务中获取对象 [英] ng2 Get Object from HttpGet Service

问题描述

我正在尝试从我的 WebAPI 获取 JSON 数据，但它不起作用.每当我尝试在我的组件中使用该返回对象时，我总是得到 undefined.

I am trying to get JSON data from my WebAPI and it's not working. I always get undefined whenever I try to use that return object in my component.

我想在我网站的主页登录页面上显示学生和课程数据.目前，我的控制器/api 正在返回硬编码数据.

I would like to display the student and course data on the home landing page of my website. At the moment, my controller / api is returning the hardcoded data.

student.service.ts

import { Injectable } from '@angular/core';
import { Observable } from 'rxjs/Observable';
import { Http } from '@angular/http';
import 'rxjs/add/operator/map';
import 'rxjs/add/observable/of';

@Injectable()
export class StudentService {
    private http: Http;            

    constructor(http: Http) {
        this.http = http;
    }

    getStudentCourse(): Observable<IStudentCourse> {
        var model: IStudentCourse;

        this.http.get('/api/student/getstudentcourse').subscribe(result => {
            model = result.json() as IStudentCourse;

            console.log(model);            

        }, error => console.log('Could not load data.'));

        console.log("model: outside of httpget: " + model);

        return Observable.of(model);        
    }
}

export interface IStudentCourse {
    refNo: string;
    student: number;
    offeringName: number;
    offeringID: string;
}

我可以确认我的服务正在返回 JSON 数据，我可以在网络流量中看到它，并且可以在我的控制台中看到它.

I can confirm that my service is returning JSON data and I can see it in Network Traffic and can see it my console.

home.component.ts

import { Component, OnInit } from '@angular/core';
import { StudentService, IStudentCourse } from './student.service';

@Component({
    selector: 'home',
    templateUrl: './home.component.html'
})
export class HomeComponent implements OnInit {    

    studentCourse: IStudentCourse;
    Message: string;

    constructor(
        private _studentService: StudentService) {        
    }

    ngOnInit(): void {
        this.getStudentCourse();
        console.log("fromngOnInit");
        console.log("Init: " + this.studentCourse.offeringName);
    }

    getStudentCourse() {        
        this._studentService.getStudentCourse().subscribe(
            item => this.studentCourse = Object.assign({}, item),
            error => this.Message = <any>error);
    }
}

您可以在我的屏幕截图中看到，studentCourse 在 ngOnInit 中始终为 null，我无法绑定它.

You can see in my screenshot that, studentCourse is always null in ngOnInit and I couldn't manage to bind it.

你能帮我解决这个错误吗?谢谢.

Could you please help me with this error? Thanks.

更新: plnkr 链接

我更喜欢将这个 HttpGet 服务放在单独的服务文件中，因为我也需要在其他组件中使用它.

I prefer to put this HttpGet service in the separate service file because I need to use it in other components too.

推荐答案

参见 plunker : plunker

在你的模板中使用 ?:

<h2>Hello {{studentCourse?.student}}</h2>

不要订阅服务，你可以这样做:

don't subscribe in the service , you can do this:

 getStudentCourse(): Observable<IStudentCourse> {
        let model: IStudentCourse;

        return this.http.get('/api/student/getstudentcourse').map(result => {
            model = result.json() as IStudentCourse;
            return model;
        }).catch( error => console.log('Could not load data.'));

    }

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