我正在使用Angular 5并尝试从JsonPlaceholder获取一些数据。
首先,我创建了服务,然后添加了:
import { HttpClientModule } from '@angular/common/http';
这是服务代码:
import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
@Injectable()
export class DataService {
private ROOT_URL = 'http://jsonplaceholder.typicode.com';
constructor(private http: HttpClient) {}
getPosts() {
this.http.get(`${this.ROOT_URL}/posts`).subscribe(data => {
return data;
});
}
}
最后,在我的app.component.ts上:
import { Component, OnInit } from '@angular/core';
import { DataService } from '../../services/data.service';
@Component({
selector: 'app-root',
templateUrl: './parent.component.html',
styleUrls: ['./parent.component.css']
})
export class AppComponent implements OnInit {
data;
constructor(private dataService: DataService) {
this.data = dataService.getPosts();
console.log(this.data;
}
ngOnInit() {
}
}
在控制台上,它只是返回"未定义">
我做错了什么?
不要订阅服务,返回可观察量并在组件上订阅它。因为它是异步的,所以组件上的数据变量将未定义,因为你在 http 请求可以解析值之前分配了。
在服务上:
getPosts() {
return this.http.get(`${this.ROOT_URL}/posts`);
}
在收件人上:
ngOnInit() {
this.dataService.getPosts().subscribe(posts => this.posts = posts);
}
尝试以下代码片段。您之所以undefined
,是因为您在http
请求可以解析值之前分配数据。从服务中删除订阅并将其移动到组件。
import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
@Injectable()
export class DataService {
private ROOT_URL = 'https://jsonplaceholder.typicode.com';
constructor(private http: HttpClient) {}
getPosts() {
return this.http.get(`${this.ROOT_URL}/posts`);
}
}
AppComponent.ts
import { Component } from '@angular/core';
import {DataService} from './data.service';
@Component({
selector: 'my-app',
templateUrl: './app.component.html',
styleUrls: [ './app.component.css' ]
})
export class AppComponent {
constructor(public data:DataService){
this.data.getPosts().subscribe(data=>{
console.log(data);
})
}
}
在此处查看演示
import { HttpClientModule } from '@angular/common/http';
//This is the service code:
import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
@Injectable()
export class DataService {
private ROOT_URL = 'http://jsonplaceholder.typicode.com';
constructor(private http: HttpClient) {}
getPosts() {
//if you want to use the observable which returns from .get()
//.. you need to do "return"
return this.http.get(`${this.ROOT_URL}/posts`);
}
}
//app.component.ts:
import { Component, OnInit } from '@angular/core';
import { DataService } from '../../services/data.service';
@Component({
selector: 'app-root',
templateUrl: './parent.component.html',
styleUrls: ['./parent.component.css']
})
export class AppComponent implements OnInit {
data;
constructor(private dataService: DataService) {
this.data = dataService.getPosts();
//so if you want to use the value of your http call outside..
//..of the service here is a good place where to do subscribe()
this.data.subscribe(data => {
console.log(this.data;
});
}
ngOnInit() {
}
}
您期望返回值,但不返回任何内容。您的 return 语句放置在嵌套 lambda 函数中,因此在您使用它的位置使用 "return" 会导致内部函数返回一个值,而不是您需要的外部函数。
我建议你阅读异步编程,特别是Angular中的Observable(它与Promise的概念相同(。
我基本上同意@Eduardo Vargas 的答案,但在解析器中执行此操作也可能是一个好主意,解析器将调用 api,并将数据放入路由快照中。因此,它不会在空页面上等待在构造函数中订阅时加载数据。更多信息在这里:
https://blog.thoughtram.io/angular/2016/10/10/resolving-route-data-in-angular-2.html