Simple Firestore/State Subscription

App.js

// https://github.com/invertase/react-native-firebase/issues/568#issuecomment-389733298

import firebase from './Firebase';

const db = firebase.firestore();

db.collection("messages")
		.where("chatId", "==", this.props.chatId)
		.orderBy("createdAt", "desc")
		.limit(5)
		.onSnapshot(querySnapshot => {
			let messages = [];
			querySnapshot.forEach(snapshot => {
				messages.push(snapshot.data());
			});
			this.setState({
				isLoading: false,
				messages: messages
			})
		})

Complete.js

import React, { Component } from 'react';
import { Button, Text, View, StyleSheet } from 'react-native';
import { Constants } from 'expo';
import * as firebase from 'firebase'; // 4.3.1

// *** 1. ADD YOUR FIREBASE CREDENTIALS
// - Go to firebase
// - Sign in to your Google account, go to Console
// - Add a new project
// - When you see "Welcome to Firebase! Get started here." click "Add Firebase to your web app"
// - You will see a modal that contains the config data for your app. Copy and paste it in
//   on top of the config below.

const config = {
  apiKey: 'AIzaSyCOsaB6-a0GwYtVnTy1dhwdzij3BgHhMrk',
  authDomain: 'fir-expo-firebase.firebaseapp.com',
  databaseURL: 'https://fir-expo-firebase.firebaseio.com',
  projectId: 'fir-expo-firebase',
  storageBucket: 'fir-expo-firebase.appspot.com',
  messagingSenderId: '706774196990',
};

try {
  firebase.initializeApp(config);
} catch (e) {
  console.log('App reloaded, so firebase did not re-initialize');
}

// *** 2. UPDATE FIREBASE RULES
// - Configure the rules for your database like in this screenshot: http://url.brentvatne.ca/t4H6md.png
// - Note that this makes it so authentication is not required to make changes. Of course this makes
//   not much sense for a real app, but for a hackathon, this doesn't matter -- it makes you faster.
// - When you're done, run the app, then delete all of these silly comments.
// - You can read more about Firebase integration with authentication, *if you care*, here:
//   https://docs.expo.io/versions/latest/guides/using-firebase.html

// *** Troubleshooting
// - Value stuck at zero? Double check that you set the rules correctly!

export default class App extends Component {
  render() {
    return (
      <View style={styles.container}>
        <CounterContainer />
      </View>
    );
  }
}

class CounterContainer extends Component {
  state = {
    count: null,
  };

  componentWillMount() {
    firebase.database().ref('counter').on('value', snapshot => {
      let count = snapshot.val().count;
      this.setState({ count });
    });
  }

  render() {
    return <Counter count={this.state.count} />;
  }
}

class Counter extends Component {
  render() {
    let { count } = this.props;

    return (
      <View>
        <Text style={styles.counterText}>
          Current count: {count === null ? 'Zero' : count}
        </Text>
        <Button title="Add one" onPress={this._increaseCount} />
      </View>
    );
  }

  _increaseCount = () => {
    firebase.database().ref('counter').set({
      count: (this.props.count || 0) + 1,
    });
  };
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    paddingTop: Constants.statusBarHeight,
    backgroundColor: '#fff',
    alignItems: 'center',
    justifyContent: 'center',
  },
  counterText: {
    fontSize: 20,
    marginBottom: 10,
  },
});

Firebase.js

const firebase = require("firebase");
require("firebase/firestore");

firebase.initializeApp({
    apiKey: "XXXXX",
    authDomain: "XXXX.firebaseapp.com",
    databaseURL: "https://XXXXX.firebaseio.com",
    projectId: "XXXXX",
    storageBucket: "XXXXX.appspot.com",
    messagingSenderId: "XXXXX"
});

export default firebase;