角5-可观察到的返回错误无法读取未定义的属性

Question

I'm starting to implement a simple pagination using Spring Rest and Angular 5 inside my angular service when I call my web service using httpClient a get a correct response with the requested page it's display data on my page correctly but there an error the console web browser that there an error inside my *ngFor loop cannot read property of undefined although that the template display results correctly :

The problem is in the Service method : getPageClient() and the *ngFor directive This the log of my error :

This my service where I used the observable :

import { Injectable } from '@angular/core';
import {Client} from '../models/Client'
import {PageClient} from '../models/PageClient'
import { Observable, of } from 'rxjs';
import { HttpClient,HttpHeaders } from '@angular/common/http';
import {catchError,map,tap} from 'rxjs/operators';

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

  private url = 'http://localhost:8099/api/clients';

  private urlPage = 'http://localhost:8099/api/clients/get?page=0&size=3';

  getClient(): Observable<Client[]>{
     return this.http.get<Client[]>(this.url)
      .pipe(
           catchError(this.handleError('getClient', []))
      );
  }

 getPageClient(): Observable<PageClient>{
  return this.http.get<PageClient>(this.urlPage)
  .pipe(
    map(response => {
      const data = response;
      console.log(data.content);
      return data ;
    }));
}
  constructor(private http : HttpClient) { }

  private handleError<T> (operation = 'operation', result?: T) {
    return (error: any): Observable<T> => {

      // TODO: send the error to remote logging infrastructure
      console.error(error); // log to console instead

      // TODO: better job of transforming error for user consumption
      console.log(`${operation} failed: ${error.message}`);

      // Let the app keep running by returning an empty result.
      return of(result as T);
    };
  }
}

This my model Class :

import {Client} from '../models/Client' ;  

export class PageClient {
    content : Client[];
    totalPages : number;
    totalElements : number;
    last : boolean;
    size : number ;
    first : boolean ;
    sort : string ;
    numberOfElements : number ;
}

This my Component code :

import { Component, OnInit } from '@angular/core';
import {Client} from '../models/Client' ;
import {PageClient} from '../models/PageClient'
import {ClientService} from '../services/client.service' ;

@Component({
  selector: 'app-clients',
  templateUrl: './clients.component.html',
  styleUrls: ['./clients.component.css']
})
export class ClientsComponent implements OnInit {

  clients : Client[];
  pageClient : PageClient ;

  getClient(): void {
    this.clientService.getClient()
        .subscribe(clients => this.clients = clients);
  }

  getPageClient(): void {
    this.clientService.getPageClient()
        .subscribe(page => this.pageClient = page);

  }
  constructor(private clientService : ClientService) { }

  ngOnInit() {
     this.getClient();
     this.getPageClient();
  }

}

And this my template part that implement the pagination :

<nav aria-label="...">
  <ul class="pagination"  *ngIf="pageClient.content" >
    <li class="page-item disabled">
      <a class="page-link" href="#" tabindex="-1">Previous</a>
    </li>
    <li *ngFor="let page of pageClient.content ; let i=index " class="page-item"><a class="page-link" href="#">{{i}}</a></li>
   <!-- <li class="page-item active">
      <a class="page-link" href="#">2 <span class="sr-only">(current)</span></a>
    </li>-->
    <li class="page-item">
      <a class="page-link" href="#">Next</a>
    </li>
  </ul>
</nav>

Can someone helpe me find the problem with my Observable Thanks in advance

Answer 1

Because the getPageClient function is asynchronous, pageClient is undefined when the page first loads. That means when doing pageClient.content , you will get an error.

Thankfully, Angular provides a useful bit of syntax you can use in the template to avoid this. You should use *ngIf="pageClient?.content" > instead.

The ? tells Angular to only read content if pageClient is not null / undefined.

More info here