原因是当ngFor在DOM中进行渲染时，订阅值仍未返回，因此ngFor没有可用的数据列表。可以使用RxJS的async管道，它会自动订阅并取消订阅。示例代码：

在组件.ts：

import { Component, OnInit, OnDestroy } from '@angular/core';
import { Subscription } from 'rxjs';
import { DataService } from './data.service';

@Component({
  selector: 'app-component',
  template: `
    

      {{ item.name }}
    
  `,
})
export class MyComponent implements OnInit, OnDestroy {
  data$: Observable;
  subscription: Subscription;

  constructor(private dataService: DataService) {}

  ngOnInit() {
    this.data$ = this.dataService.getData();
  }

  ngOnDestroy() {
    this.subscription.unsubscribe();
  }
}

在服务.ts：

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

@Injectable({
  providedIn: 'root',
})
export class DataService {
  constructor(private http: HttpClient) {}

  getData(): Observable {
    return this.http.get('/api/data');
  }
}