在ionic2中的Firebase响应中调用了两次警报

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

@Component({
  selector: 'app-my-component',
  templateUrl: './my-component.component.html',
  styleUrls: ['./my-component.component.scss']
})
export class MyComponent implements OnInit, OnDestroy {
  private subscription: Subscription;

  ngOnInit() {
    this.subscription = firebase.database().ref('path/to/data').on('value', (snapshot) => {
      alert('Data changed!');
    });
  }

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

import { Component, OnInit, OnDestroy } from '@angular/core';
import { Subject } from 'rxjs';
import { takeUntil } from 'rxjs/operators';

@Component({
  selector: 'app-my-component',
  templateUrl: './my-component.component.html',
  styleUrls: ['./my-component.component.scss']
})
export class MyComponent implements OnInit, OnDestroy {
  private unsubscribe$ = new Subject<void>();

  ngOnInit() {
    firebase.database().ref('path/to/data').on('value', (snapshot) => {
      alert('Data changed!');
    }).pipe(takeUntil(this.unsubscribe$)).subscribe();

  }

  ngOnDestroy() {
    this.unsubscribe$.next();
    this.unsubscribe$.complete();
  }
}