rest - Angular2 模板试图在从 http.get 返回的值之前呈现值

Question

我有一个angular2项目如下:

package.json:

{
  "name": "self-assessment",
  "version": "1.0.0",
  "scripts": {
    "postinstall": "npm run typings install",
    "lite": "lite-server",
    "gulp": "gulp",
    "start": "concurrent \"npm run gulp\" \"npm run lite\" ",
    "typings" : "typings"
  },
  "license": "ISC",
  "dependencies": {
    "angular2": "2.0.0-beta.3",
    "systemjs": "0.19.6",
    "es6-promise": "^3.0.2",
    "es6-shim": "^0.33.3",
    "reflect-metadata": "0.1.2",
    "rxjs": "5.0.0-beta.0",
    "zone.js": "0.5.11"
  },
  "devDependencies": {
    "autoprefixer": "^6.2.1",
    "cssnano": "^3.4.0",
    "concurrently": "^1.0.0",
    "gulp": "^3.9.0",
    "gulp-ext-replace": "^0.2.0",
    "gulp-imagemin": "^2.4.0",
    "gulp-postcss": "^6.0.1",
    "gulp-sourcemaps": "^1.6.0",
    "gulp-typescript": "^2.10.0",
    "gulp-uglify": "^1.5.1",
    "lite-server": "^2.0.1",
    "postcss": "^5.0.13",
    "postcss-scss": "^0.1.3",
    "precss": "^1.3.0",
    "typings":"^0.6.8",
    "tsd": "^0.6.5-beta"
  }
}

app.component.ts:

import {Component} from 'angular2/core';
import {AssessmentListComponent} from "./assessment-list.component";
import {HomeComponent} from "./home.component";
import {ApiService} from "../services/api.service";
import {RouteConfig, ROUTER_DIRECTIVES, ROUTER_PROVIDERS} from 'angular2/router';
import {AssessmentComponent} from "./assessment.component";

@Component({
    selector: 'my-app',
    templateUrl: 'app/components/app.component.html',
    directives: [
        ROUTER_DIRECTIVES,
    ],
    providers: [
        ApiService,
        ROUTER_PROVIDERS,
    ]
})
@RouteConfig([
    {path: '/home', name: 'Home', component: HomeComponent, useAsDefault: true},
    {path: '/assessments', name: 'Assessments', component: AssessmentListComponent},
    {path: '/assessment/:id', name: 'Assessment', component: AssessmentComponent},

])
export class AppComponent {

    title = "Welcome to Self-Assessment"

}

assessment.component.ts:

import {Component, OnInit} from 'angular2/core';
import {AssessmentModel} from '../models/assessment.model';
import {RouteParams} from 'angular2/router';
import {ApiService} from '../services/api.service';

@Component({
    selector: 'assessment-component',
    templateUrl: 'app/components/assessment.component.html',
})
export class AssessmentComponent implements OnInit{
    assessment: AssessmentModel;
    errorMessage: any;

    constructor(
        private _apiService: ApiService,
        private _routeParams: RouteParams
    ) {}


    ngOnInit():any {
        let id = +this._routeParams.get('id');
        this.getAssessment(id);
    }

    private getAssessment(id: number) {
        this._apiService.getAssessment(id)
            .subscribe(
                assessment => this.assessment = assessment,
                error => this.errorMessage = <any>error
            );
    }

    goBack() {
        console.log(this.assessment);
    }
}

api.service.ts:

import {Injectable} from 'angular2/core';
import {Http, Response} from 'angular2/http';
import {Observable} from 'rxjs/Observable';

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

    private _baseApiUrl = 'http://localhost:8000/';

    getAssessments() {
        return this.getObjectList('assessments', 'json');
    }

    getAssessment(id: number) {
        return this.getObject('assessments', id, 'json');
    }

    getObjectList(listName:string, listExtension: string) {

        var url =  this._baseApiUrl + listName + '.' + listExtension;

        return this._http.get(url)
            .map(response => response.json())
            .catch(this.handleError);
    }

    getObject(objectName:string, objectId:number, objectExtension: string) {

        var url =  this._baseApiUrl + objectName + '/' + objectId + '.' + objectExtension;

        return this._http.get(url)
            .map(response => response.json())
            .catch(this.handleError);
    }

    private handleError(error: Response) {
        console.error(error);
        return Observable.throw(error.json().error || 'Server error');
    }
}

最后 assessment.component.html:

<h3>{{ assessment.id }}</h3>
<button (click)="goBack()">Go Back 2</button>

当使用 url 调用页面时:http://localhost:3000/assessment/4它对后端进行适当的调用两次，并在控制台中生成以下错误:

Uncaught EXCEPTION: TypeError: Cannot read property 'id' of undefined in [{{ assessment.id }} in AssessmentComponent@0:4]
ORIGINAL EXCEPTION: TypeError: Cannot read property 'id' of undefined
ORIGINAL STACKTRACE:
TypeError: Cannot read property 'id' of undefined
    at AbstractChangeDetector.ChangeDetector_AssessmentComponent_0.detectChangesInRecordsInternal (viewFactory_AssessmentComponent:30:28)
    at AbstractChangeDetector.detectChangesInRecords (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8116:14)
    at AbstractChangeDetector.runDetectChanges (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8099:12)
    at AbstractChangeDetector._detectChangesInViewChildren (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8184:14)
    at AbstractChangeDetector.runDetectChanges (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8103:12)
    at AbstractChangeDetector._detectChangesContentChildren (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8178:14)
    at AbstractChangeDetector.runDetectChanges (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8100:12)
    at AbstractChangeDetector._detectChangesInViewChildren (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8184:14)
    at AbstractChangeDetector.runDetectChanges (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8103:12)
    at AbstractChangeDetector.detectChanges (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8088:12)
ERROR CONTEXT:
[object Object]ExceptionHandler.call @ angular2.dev.js:1206(anonymous function) @ angular2.dev.js:12591NgZone._notifyOnError @ angular2.dev.js:13635collection_1.StringMapWrapper.merge.onError @ angular2.dev.js:13539Zone.run @ angular2-polyfills.js:1247NgZone._notifyOnTurnDone @ angular2.dev.js:13450(anonymous function) @ angular2.dev.js:13565zoneBoundFn @ angular2-polyfills.js:1220lib$es6$promise$asap$$flush @ angular2-polyfills.js:262

所以它看起来好像 {{ assessment.id }} 试图在对象从后端返回之前呈现。如果我随后单击“Go Back 2”按钮，它将控制台适本地记录对象:

{"id":4,"title":"Assessment Title here!","description":"Here is a description","questions":["http://localhost:8000/questions/1.json","http://localhost:8000/questions/2.json","http://localhost:8000/questions/3.json","http://localhost:8000/questions/4.json","http://localhost:8000/questions/5.json","http://localhost:8000/questions/6.json","http://localhost:8000/questions/7.json","http://localhost:8000/questions/8.json","http://localhost:8000/questions/9.json","http://localhost:8000/questions/10.json","http://localhost:8000/questions/11.json","http://localhost:8000/questions/12.json","http://localhost:8000/questions/13.json"],"responses":[]}

“有趣的是”我没有在应用程序的其他地方看到问题(即可以很好地加载和显示评估列表等)。此外，我在昨晚作为学习项目使用的 Ionic2 应用程序中遇到了完全相同的问题。

用谷歌搜索了我的屁股，希望有人知道这个谜语的答案。

谢谢!

Answer 1

你有两个选择:

1. 使用safe navigation operator (以前称为 Elvis 运算符)，{{ assessment?.id }} ，防止空值和未定义值
2. 使用 NgIf，<h3 *ngIf="assessment">{{assessment.id}}</h3> ，如果您不希望在数据可用/填充之前将元素添加到 DOM 中，这会更合适

您可能不会遇到列表问题，因为 NgFor 会为您处理未定义、空或空数组的情况。然后当数据进来时，NgFor 被重新评估。