Observable type error: cannot read property of undefined

Question

在我的 Angular 2 应用程序中，出现错误：

Cannot read property 'title' of undefined.

这是一个非常简单的组件，只是试图让最低限度的组件在这里工作。它击中了我的 API 控制器（奇怪的是多次），并且它似乎在返回对象后击中了回调。我的 console.log 输出我期望的对象。这是完整的错误：

TypeError: Cannot read property 'title' of undefined
    at AbstractChangeDetector.ChangeDetector_About_0.detectChangesInRecordsInternal (eval at <anonymous> (http://localhost:55707/lib/angular2/bundles/angular2.dev.js:10897:14), <anonymous>:31:26)
    at AbstractChangeDetector.detectChangesInRecords (http://localhost:55707/lib/angular2/bundles/angular2.dev.js:8824:14)
    at AbstractChangeDetector.runDetectChanges (http://localhost:55707/lib/angular2/bundles/angular2.dev.js:8807:12)
    at AbstractChangeDetector._detectChangesInViewChildren (http://localhost:55707/lib/angular2/bundles/angular2.dev.js:8877:14)
    at AbstractChangeDetector.runDetectChanges (http://localhost:55707/lib/angular2/bundles/angular2.dev.js:8811:12)
    at AbstractChangeDetector._detectChangesContentChildren (http://localhost:55707/lib/angular2/bundles/angular2.dev.js:8871:14)
    at AbstractChangeDetector.runDetectChanges (http://localhost:55707/lib/angular2/bundles/angular2.dev.js:8808:12)
    at AbstractChangeDetector._detectChangesInViewChildren (http://localhost:55707/lib/angular2/bundles/angular2.dev.js:8877:14)
    at AbstractChangeDetector.runDetectChanges (http://localhost:55707/lib/angular2/bundles/angular2.dev.js:8811:12)
    at AbstractChangeDetector.detectChanges (http://localhost:55707/lib/angular2/bundles/angular2.dev.js:8796:12)

服务（about.service.ts）：

import {Http} from 'angular2/http';
import {Injectable} from 'angular2/core';
import {AboutModel} from './about.model';
import 'rxjs/add/operator/map';

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

    get() {
        return this._http.get('/api/about').map(res => {
            console.log(res.json()); // I get the error on the line above but this code is still hit.
            return <AboutModel>res.json();
        });
    }
}

组件（about.component.ts）：

import {Component, View, OnInit} from 'angular2/core';
import {AboutModel} from './about.model';
import {AboutService} from './about.service';
import {HTTP_PROVIDERS} from 'angular2/http';

@Component({
    selector: 'about',
    providers: [HTTP_PROVIDERS, AboutService],
    templateUrl: 'app/about/about.html'
})

export class About implements IAboutViewModel, OnInit {
    public about: AboutModel;

    constructor(private _aboutService: AboutService) {}

    ngOnInit() {    
        this._aboutService.get().subscribe((data: AboutModel) => {
            this.about = data;
        });
    }
}

export interface IAboutViewModel {
    about: AboutModel;
}

index.html

<script src="~/lib/systemjs/dist/system.src.js"></script>
<script src="~/lib/angular2/bundles/router.js"></script>
<script src="~/lib/angular2/bundles/http.js"></script>
<script src="~/lib/angular2/bundles/angular2-polyfills.js"></script>
<script src="~/lib/angular2/bundles/angular2.dev.js"></script>
<script src="~/lib/es6-shim/es6-shim.js"></script>
<script>
    System.config({
        packages: {
            app: {
                format: 'register',
                defaultExtension: 'js'
            },
            rxjs: {
                defaultExtension: 'js'
            }
        },
        map: {
            rxjs: "lib/rxjs"
        }
    });
    System.import('app/boot')
            .then(null, console.error.bind(console));
</script>

Answer 1

您似乎在视图 about.html 中引用了 about.title，但 about 变量仅在 http 请求完成后才实例化。为避免此错误，您可以将 about.html 换成 <div *ngIf="about"> ... </div>

Answer 2

下次请附上您的观点和模型（app/about/about。html 和 about.model）。

如果您要返回数组，您可以使用 asyncPipe，"subscribes to an Observable or Promise and returns the latest value it has emitted. When a new value is emitted, the async pipe marks the component to be checked for changes" 因此视图将更新为新值。

如果要返回 原始类型（字符串、数字、布尔值），您还可以使用 asyncPipe。

如果您要返回一个对象，~~我不知道有什么方法可以使用 asyncPipe~~，我们可以使用异步管道, 连同 safe navigation operator ?. 如下：

{{(objectData$ | async)?.name}}

但这看起来有点复杂，我们必须对要显示的每个对象重复此操作属性。

正如@pixelbits 在评论中提到的，您可以 subscribe() 到控制器中的可观察对象并将包含的对象存储到组件中属性。然后在模板中使用安全导航操作符或NgIf：

service.ts

import {Injectable} from 'angular2/core';
import {Http} from 'angular2/http';
import 'rxjs/add/operator/map';  // we need to import this now

@Injectable()
export class MyService {
  constructor(private _http:Http) {}
  getArrayData() {
    return this._http.get('./data/array.json')
      .map(data => data.json());
  }
  getPrimitiveData() {
    return this._http.get('./data/primitive.txt')
      .map(data => data.text());   // note .text() here
  }
  getObjectData() {
    return this._http.get('./data/object.json')
      .map(data => data.json());
  }
}

app.ts

@Component({
  selector: 'my-app',
  template: `
    <div>array data using '| async':
      <div *ngFor="let item of arrayData$ | async">{{item}}</div>
    </div>
    <div>primitive data using '| async': {{primitiveData$ | async}}</div>
    <div>object data using .?: {{objectData?.name}}</div>
    <div *ngIf="objectData">object data using NgIf: {{objectData.name}}</div>`
  providers: [HTTP_PROVIDERS, MyService]
})
export class AppComponent {
  constructor(private _myService:MyService) {}
  ngOnInit() {
    this.arrayData$     = this._myService.getArrayData();
    this.primitiveData$ = this._myService.getPrimitiveData();
    this._myService.getObjectData()
      .subscribe(data => this.objectData = data);
  }
}

data/array.json

[ 1,2,3 ]

data/primitive.json

Greetings SO friends!

data/object.json

{ "name": "Mark" }

输出：

array data using '| async':
1
2
3
primitive data using '| async': Greetings SO friends!
object data using .?: Mark
object data using NgIf: Mark

Plunker

Answer 3

前面的回答是正确的。在模板中使用变量之前，您需要检查变量是否已定义。使用 HTTP 请求需要时间来定义它。使用 *ngIf 检查。示例由 angular 和 https://angular.io/docs/ts/latest/tutorial/toh-pt5.html 提供例如 http://plnkr.co/edit/?p=preview

<div *ngIf="hero">
  <h2>{{hero.name}} details!</h2>
<div>

你可以查看app/hero-detail.component [ts and html]

Observable type error: cannot read property of undefined

Observable type error: cannot read property of undefined

observable

rxjs

angular