angularjs – TypeError：无法读取[null]中未定义的属性’post’

在我的Angular 2项目中,ListingService无法通过post从服务器获取数据.我收到这个错误：

EXCEPTION: TypeError: Cannot read property 'post' of undefined in [null]
ORIGINAL EXCEPTION: TypeError: Cannot read property 'post' of undefined
ORIGINAL STACKTRACE:
TypeError: Cannot read property 'post' of undefined
at ListingService.getListings (http://localhost:3000/app/listing.service.js:30:30)
at ListingsComponent.getListings (http://localhost:3000/app/listings.component.js:45:41)
at ListingsComponent.ngOnInit (http://localhost:3000/app/listings.component.js:41:26)
at AbstractChangeDetector.ChangeDetector_HostListingsComponent_0.detectChangesInRecordsInternal (eval at <anonymous> (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:10897:14),<anonymous>:22:99)
at AbstractChangeDetector.detectChangesInRecords (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8824:14)
at AbstractChangeDetector.runDetectChanges (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8807:12)
at AbstractChangeDetector._detectChangesContentChildren (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8871:14)
at AbstractChangeDetector.runDetectChanges (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8808:12)
at AbstractChangeDetector._detectChangesInViewChildren (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8877:14)
at AbstractChangeDetector.runDetectChanges (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8811:12)

我的ListingService.ts看起来像这样：

import {Injectable,Injector} from 'angular2/core';
import {HTTP_PROVIDERS,Http,Headers} from 'angular2/http';
import {Listing} from './listing';
import 'rxjs/add/operator/map';
@Injectable()
export class ListingService {
 http: Http;
 listings: Array<Listing>;
 getListings() {
    var headers = new Headers();
    headers.append('Content-Type','application/json');
    this.http.post('http://144.376.29.134:and-so-on',JSON.stringify(
    {"id":1,"title": "Title","description": "Проводите","discount": "21%","imageUrl": "http://lorempixel.com/400/200/sports","tags": [{"tag": "Еда"}]
    }),{headers:headers}
    ).map(res => res.json())
    .subscribe((res:Array<Listing>) => this.listings = res);
    return Promise.resolve(this.listings);
 }
 getListing(id: number) {
    return Promise.resolve(this.listings)
    .then(listings => listings.filter(l => l.id === id)[0]);
 }
}

使用ListingService的ListingsComponent如下所示：

import {Component,OnInit} from 'angular2/core';
import {HTTP_PROVIDERS,Headers} from 'angular2/http';
import {Listing} from './listing';
import {ListingService} from './listing.service';
import 'rxjs/add/operator/map';
@Component({
  ....
})
export class ListingsComponent implements OnInit {
 listings: Array<Listing>;
 constructor(private listingService:ListingService,private _router: Router) {}
 ngOnInit() {
  this.getListings();
 }
 getListings() {
  this.listingService.getListings().then(listings => this.listings = listings);
 }
 getListing(id: number) {
  return Promise.resolve(this.listings)
  .then(listings => listings.filter(l => l.id === id)[0]);
 }
 gotoDetail(listing: Listing) {
  this._router.navigate(['ListingDetail',{ id: listing.id }]);
 }
}

这有什么问题？

您必须将HTTP_PROVIDERS添加到组件提供程序数组,如下所示：

providers: [HTTP_PROVIDERS]

或者最好是在这样的引导程序中：

bootstrap(AppComponent,[HTTP_PROVIDERS]);

而且你在ListingService中缺少构造函数http注入：

export class ListingService {
   constructor(private http : Http){}
}

附录

您没有收到任何列表的原因是因为您使用的是Promise而不是Observable：

在ListingService中的getListings()中返回：

return this.http.post("bla bla").map(res => res.json).map((res) => {
    return this.listings = res;
});

然后在ListingsComponent的getListings()中订阅这个：

getListings() {
  this.listingService.getListings().subscribe(listings => this.listings = listings);
}

angularjs – TypeError：无法读取[null]中未定义的属性’post’

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