Question

在我的Angular 2应用程序中，出现错误：

无法读取undefined的属性'title'。

这是一个非常简单的组件，只是试图在这里工作。它击中我的API控制器（奇怪地多次），它似乎在返回一个对象后点击回调。我的console.log输出了我期望的对象。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;
}

的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

请在下次包含您的观点和型号（app / about / about.html和about.model）。

如果要返回数组，可以使用asyncPipe，它“订阅Observable或Promise并返回它发出的最新值。当发出新值时，异步管道标记要检查更改的组件“因此视图将使用新值进行更新。

如果要返回基本类型（字符串，数字，布尔值），还可以使用asyncPipe。

如果你要返回对象，~~我不知道有什么方法可以使用asyncPipe~~ ，我们可以使用异步管道和{{3 } ?.如下：

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

但这看起来有点复杂，我们必须为我们想要显示的每个对象属性重复这一点。

作为注释中提到的@pixelbits，您可以subscribe()到控制器中的observable并将包含的对象存储到组件属性中。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);
  }
}

数据/ array.json

[ 1,2,3 ]

数据/ primitive.json

Greetings SO friends!

数据/ 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

safe navigation operator

Answer 2

看起来您在视图about.title中引用了about.html，但about变量仅在http请求完成后才会实例化。要避免此错误，您可以使用about.html

打包<div *ngIf="about"> ... </div>

Answer 3

之前的答案是正确的。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来检查。示例由角度https://angular.io/docs/ts/latest/tutorial/toh-pt5.html提供示例是http://plnkr.co/edit/?p=preview

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

您可以查看app / hero-detail.component [ts和html]

可观察类型错误：无法读取未定义的属性

3 个答案: