subscribe and unsubscribe to topic for push notification

login.js

import React, { Component } from 'react';
import { View, StyleSheet, Text } from 'react-native';
import { Actions } from 'react-native-router-flux';
import FCM from 'react-native-fcm';

.....

_onLoginSuccess = () => {
    const topic = '/topics/list'
    FCM.subscribeToTopic(topic); // subscribe to topic
    Actions.main();
  }

_onLoginPress = () => {
    const { email, password } = this.state;
    this.setState({
      error: '',
      loading: true,
    });

    firebase.auth().signInWithEmailAndPassword(email, password)
      .then(this._onLoginSuccess)
      .catch(error => this._onLoginFailure(error));
  }

.....

logout.js

import React, { Component } from 'react';
import { View, StyleSheet, Text } from 'react-native';
import { Actions } from 'react-native-router-flux';
import FCM from 'react-native-fcm';

.....

_onLogoutPress = () => {
    const topic = '/topics/list'
    FCM.unsubscribeFromTopic(userTopic); // unsubscribe to topic
  
    const ref = firebase.database().ref('subjects');
    ref.off();

    firebase.auth().signOut();
    Actions.auth();
  };
.....