在Angular中使用httpclient服务加载本地json异步

Question

我正在尝试使用 Angular 服务调用 load Json，而不是在 ngOnInit 上，而是在用户请求的 Button 上。它接缝确实会加载它，但不会异步执行。我收到此错误：错误错误：未捕获（承诺中）：类型错误：

我是 Angular 的新手，所以请不要评判。

core.js:15724 ERROR Error: Uncaught (in promise): TypeError: Cannot read property 'length' of undefined
TypeError: Cannot read property 'length' of undefined
    at PollComponent.push../src/app/components/poll/poll.component.ts.PollComponent.shuffle (poll.component.ts:23)
    at PollComponent.<anonymous> (poll.component.ts:44)
    at step (tslib.es6.js:97)
    at Object.next (tslib.es6.js:78)
    at fulfilled (tslib.es6.js:68)
    at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invoke (zone.js:391)
    at Object.onInvoke (core.js:17299)
    at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invoke (zone.js:390)
    at Zone.push../node_modules/zone.js/dist/zone.js.Zone.run (zone.js:150)
    at zone.js:889
    at resolvePromise (zone.js:831)
    at zone.js:741
    at fulfilled (tslib.es6.js:68)
    at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invoke (zone.js:391)
    at Object.onInvoke (core.js:17299)
    at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invoke (zone.js:390)
    at Zone.push../node_modules/zone.js/dist/zone.js.Zone.run (zone.js:150)
    at zone.js:889
    at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invokeTask (zone.js:423)
    at Object.onInvokeTask (core.js:17290)
defaultErrorLogger @ core.js:15724
push../node_modules/@angular/core/fesm5/core.js.ErrorHandler.handleError @ core.js:15772
next @ core.js:17771
schedulerFn @ core.js:13515
push../node_modules/rxjs/_esm5/internal/Subscriber.js.SafeSubscriber.__tryOrUnsub @ Subscriber.js:196
push../node_modules/rxjs/_esm5/internal/Subscriber.js.SafeSubscriber.next @ Subscriber.js:134
push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber._next @ Subscriber.js:77
push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber.next @ Subscriber.js:54
push../node_modules/rxjs/_esm5/internal/Subject.js.Subject.next @ Subject.js:47
push../node_modules/@angular/core/fesm5/core.js.EventEmitter.emit @ core.js:13499
(anonymous) @ core.js:17321
push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invoke @ zone.js:391
push../node_modules/zone.js/dist/zone.js.Zone.run @ zone.js:150
push../node_modules/@angular/core/fesm5/core.js.NgZone.runOutsideAngular @ core.js:17258
onHandleError @ core.js:17321
push../node_modules/zone.js/dist/zone.js.ZoneDelegate.handleError @ zone.js:395
push../node_modules/zone.js/dist/zone.js.Zone.runGuarded @ zone.js:164
_loop_1 @ zone.js:694
api.microtaskDrainDone @ zone.js:703
drainMicroTaskQueue @ zone.js:608
push../node_modules/zone.js/dist/zone.js.ZoneTask.invokeTask @ zone.js:502
invokeTask @ zone.js:1744
globalZoneAwareCallback @ zone.js:1770

这是我的服务：

import { Injectable } from '@angular/core';
import { Observable } from 'rxjs';
import { HttpClient } from '@angular/common/http';
import { IQuestion } from '../../assets/IData';

@Injectable({
  providedIn: 'root'
})
export class PollServiceService {

  private _url: string = "assets/data/TEST_me_50_Pyetje.json";
  constructor(private http: HttpClient) {

   }

  getQuestions() : Observable<IQuestion[]> {
    return this.http.get<IQuestion[]>(this._url);
   }
}

这是我的方法。它在 HTML 中的按钮上调用：

  ngOnInit() {

  }
  async startTestClick(){

      var promise = await this.pollService.getQuestions().subscribe(data => this.que = data);
      this.nrSakte = 0;
      this.testQuestions = this.shuffle(this.que).slice(0,50);
      for (let entry of this.testQuestions) {
        entry.picture ="A";
        entry.color = "black";
      }


      let a = await document.getElementsByClassName("alternativa");
      for(let j in a){
        a[j].setAttribute("style", "background-color:#fafafa;");
      }
  }

我真的希望它等到整个 Json 加载完毕，以便我可以选择在 HTML 上显示哪些元素。它确实显示了它们，但它显示了这个错误。如果我想动态加载其他一些 json，它就不再起作用了。

提前致谢！

Answer 1

我认为您需要在对startTestClick 的调用中将subscribe 中的一些逻辑移动。

类似这样的：

async startTestClick() {

    var promise = await this.pollService.getQuestions().subscribe(data => {
        this.que = data;
        this.nrSakte = 0;
        this.testQuestions = this.shuffle(this.que).slice(0,50);
        for (let entry of this.testQuestions) {
            entry.picture ="A";
            entry.color = "black";
        }
    });
    ...
}

没有这个，您的其余代码将在 subscribe 等待结果时运行，而 this.que 还没有任何结果。

我不认为你真的需要async/await。所以，这也可以。

startTestClick() {

    this.pollService.getQuestions().subscribe(data => {
        this.que = data;
        this.nrSakte = 0;
        this.testQuestions = this.shuffle(this.que).slice(0,50);
        for (let entry of this.testQuestions) {
            entry.picture ="A";
            entry.color = "black";
        }
    });
    ...
}

再一次，你在document.getElementsByClassName 调用中使用了await，所以我可能是错的。该调用可能也需要在 subscribe 内。

尝试两种方法，看看哪种方法有效。如果这不能解决您的问题，我可以删除此答案。根据帖子中的内容，这有点猜测，但方式太多了，无法发表评论。