（未知 url）Angular 4 的 Http 失败响应

Question

我在系统上有两台服务器。一个端口服务于 Angular 项目，另一个端口服务于 Laravel。我正在使用 Laravel 制作的后端以 json 格式给出响应。我已经制作了 Angular 项目，在该项目中我实现了一项服务来使用来自端口的数据并显示列表。正如我在浏览器中开发人员工具的网络选项卡中看到的那样，Angular 正在从后端服务器检索文本，但是没有显示列表。它只是显示该错误。这是代码

blog-list.component.html

<div>
<mat-list role="list">
<mat-list-item role="listitem" *ngFor="let blog of blogs">{{blog.id}}</mat-list-item>
</mat-list>
</div>

blog-list.component.ts

import { Component, OnInit } from '@angular/core';
import {NetworkOperationsService} from '../network-operations.service';
import {Blog} from '../blog';
import {Category} from '../category';

@Component({
selector: 'app-blog-list',
templateUrl: './blog-list.component.html',
styleUrls: ['./blog-list.component.css']
})
export class BlogListComponent implements OnInit
{
blogs : Blog[];
constructor(private networkOperationsService : NetworkOperationsService)
{}
ngOnInit()
{
this.getBlogs();
}
getBlogs() : void
  {
  this.networkOperationsService.getBlogs()
  .subscribe(blogs=>this.blogs = blogs);
  }
}

network-operations.service.ts

import { Injectable } from '@angular/core';
import { Observable } from 'rxjs/Observable';
import { of } from 'rxjs/observable/of';
import { HttpClient, HttpHeaders } from '@angular/common/http';
import {Blog} from './blog';
import {Category} from './category';
import {ApiConfigurations} from './api-configurations';


@Injectable()
export class NetworkOperationsService
{
 constructor(private http: HttpClient) { }

 getBlogs() : Observable<Blog[]>
  {
  return this.http.get<Blog[]>('http://localhost:8000/api/blogs');
  }
}

JSON 响应

[{"id":1,"title":"My Second Blog","body":"Here is the body of my first blogger app","categories":"[1]","created_at":"2017-12-22 11:28:02","updated_at":"2017-12-22 11:32:15"},{"id":2,"title":"Is Android a right choice for career","body":"A lot of people are trying to make android as their primary motive of their lives so please people stop there and watch for more choices and make your choices more sophesticatedly.","categories":"[1]","created_at":"2017-12-22 17:12:26","updated_at":"2017-12-22 17:12:26"},{"id":3,"title":"Is Android a right choice for career","body":"A lot of people are trying to make android as their primary motive of their lives so please people stop there and watch for more choices and make your choices more sophesticatedly.","categories":"[]","created_at":"2017-12-22 17:12:40","updated_at":"2017-12-22 17:12:40"},{"id":4,"title":"Is Android a right choice for career","body":"A lot of people are trying to make android as their primary motive of their lives so please people stop there and watch for more choices and make your choices more sophesticatedly.","categories":"[5]","created_at":"2017-12-22 17:12:45","updated_at":"2017-12-22 17:12:45"},{"id":5,"title":"Is Android a right choice for career","body":"A lot of people are trying to make android as their primary motive of their lives so please people stop there and watch for more choices and make your choices more sophesticatedly.","categories":"[6]","created_at":"2017-12-22 17:12:51","updated_at":"2017-12-22 17:12:51"},{"id":6,"title":"Is Android a right choice for career","body":"A lot of people are trying to make android as their primary motive of their lives so please people stop there and watch for more choices and make your choices more sophesticatedly.","categories":"[8]","created_at":"2017-12-22 17:12:55","updated_at":"2017-12-22 17:12:55"},{"id":7,"title":"Dushyant SUthar","body":"Dushyant SUthar","categories":"Dushyant SUthar","created_at":"2017-12-25 11:20:06","updated_at":"2017-12-25 11:20:06"}]

错误文字

Angular is running in the development mode. Call enableProdMode() to enable the production mode.
core.js:3660
ERROR 
{…}
error: error
bubbles: false
cancelBubble: false
cancelable: false
composed: false
currentTarget: null
defaultPrevented: false
eventPhase: 0
explicitOriginalTarget: XMLHttpRequest { __zone_symbol__xhrSync: false, __zone_symbol__xhrURL: "http://localhost:8000/api/blogs", readyState: 4, … }
isTrusted: true
lengthComputable: false
loaded: 0
originalTarget: XMLHttpRequest { __zone_symbol__xhrSync: false, __zone_symbol__xhrURL: "http://localhost:8000/api/blogs", readyState: 4, … }
target: XMLHttpRequest { __zone_symbol__xhrSync: false, __zone_symbol__xhrURL: "http://localhost:8000/api/blogs", readyState: 4, … }
timeStamp: 25351.271703
total: 0
type: "error"
__proto__: ProgressEventPrototype { lengthComputable: Getter, loaded: Getter, total: Getter, … }
headers: Object { normalizedNames: Map, lazyUpdate: null, headers: Map }
message: "Http failure response for (unknown url): 0 Unknown Error"
name: "HttpErrorResponse"
ok: false
status: 0
statusText: "Unknown Error"
url: null
__proto__: Object { constructor: HttpErrorResponse() }
core.js:1427
Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://localhost:8000/api/blogs. (Reason: CORS header ‘Access-Control-Allow-Origin’ missing). 

Answer 1

在 public/index.php 文件中添加这些行

header("Access-Control-Allow-Origin: *");
header('Access-Control-Allow-Methods: GET, POST, DELETE, PUT, OPTIONS');
header('Access-Control-Allow-Headers: Origin, Content-Type, Accept, Authorization, X-Request-With');
header('Access-Control-Allow-Credentials: true');

并检查此this 和this