Ionic with Cordova-Plugin-Firebase

app.component.ts

import { Nav, Platform, ToastController } from 'ionic-angular';
import { Firebase } from '@ionic-native/firebase';
constructor(
...
private firebase: Firebase,
public platform: Platform,
//When the app's open, we'll show them as Toasts, but feel free to use an Alert instead
public toastCtrl: ToastController
){
    this.platform.ready().then(() => {
      try{
        this.initializeFirebase();
      } catch (error) {
        this.firebase.logError(error);
      }
    });
  }
initializeFirebase() {
    if(!this.platform.is("core")) {
      this.firebase.subscribe("all");
      this.platform.is('android') ? this.initializeFirebaseAndroid() : this.initializeFirebaseIOS();
    }
  }
initializeFirebaseAndroid() {
    this.firebase.getToken().then(token => {});
    this.firebase.onTokenRefresh().subscribe(token => {})
    this.subscribeToPushNotifications();
  }
initializeFirebaseIOS() {
    this.firebase.grantPermission()
    .then(() => {
      this.firebase.getToken().then(token => {});
      this.firebase.onTokenRefresh().subscribe(token => {})
      this.subscribeToPushNotifications();
    })
    .catch((error) => {
      this.firebase.logError(error);
    });
  }
subscribeToPushNotifications() {
    this.firebase.onNotificationOpen().subscribe((response) => {
      if(response.tap){
        //Received while app in background (this should be the callback when a system notification is tapped)
        //This is empty for our app since we just needed the notification to open the app
      }else{
        //received while app in foreground (show a toast)
        let toast = this.toastCtrl.create({
          message: response.body,
          duration: 3000
        });
        toast.present();
      }
    });
  }