子组件无法通过服务订阅EventEmitter

import { Subject } from 'rxjs';

@Injectable()
export class DataService {
  private dataSubject = new Subject<string>();
  data$ = this.dataSubject.asObservable();

  sendData(data: string) {
    this.dataSubject.next(data);
  }
}

import { Component, OnInit } from '@angular/core';
import { DataService } from 'path-to-data-service';

@Component({
  selector: 'app-child',
  template: `
    <div>{{ receivedData }}</div>
  `,
})
export class ChildComponent implements OnInit {
  receivedData: string;

  constructor(private dataService: DataService) {}

  ngOnInit() {
    this.dataService.data$.subscribe(data => {
      this.receivedData = data;
    });
  }
}