如何在Angular2中取消订阅Firebase列表？

import { Component, OnInit, OnDestroy } from '@angular/core';
import { AngularFireDatabase, AngularFireList } from '@angular/fire/database';
import { Subscription } from 'rxjs';

export class YourComponent implements OnInit, OnDestroy {
  itemsRef: AngularFireList<any>;
  items: Observable<any[]>;
  subscription: Subscription;
  
  constructor(private db: AngularFireDatabase) { }
  
  ngOnInit() {
    this.itemsRef = this.db.list('your-firebase-list');
    this.items = this.itemsRef.valueChanges();
    
    this.subscription = this.items.subscribe();
  }
  
  ngOnDestroy() {
    this.subscription.unsubscribe();
  }
}