App.js

import React, { useState, useEffect } from 'react';
import { View, Text, Button, StatusBar } from 'react-native';

import auth from '@react-native-firebase/auth';
import { GoogleSignin } from '@react-native-community/google-signin';

GoogleSignin.configure({
	webClientId: '',
});

async function onGoogleButtonPress() {
  // Get the users ID token
  const { idToken } = await GoogleSignin.signIn();
  
  // Create a Google credential with the token
  const googleCredential = auth.GoogleAuthProvider.credential(idToken);

  // Sign-in the user with the credential
  return auth().signInWithCredential(googleCredential);
}

// Sign In Button
function GoogleSignIn() {
  return (
    <Button
      title="Google Sign-In"
      onPress={() => onGoogleButtonPress().then(() => console.log('Signed in with Google!')).catch(err => console.log(err))}
    />
  );
}

// Sign Out Button
function GoogleSignOut() {
	return (
	  <Button
		  title="Google Sign-Out"
		  onPress={() => auth().signOut().then(() => console.log('Signed Out From Google!')).catch(err => console.log(err))}
	  />
	);
}

function LoginApp() {
  // Set an initializing state whilst Firebase connects
  const [initializing, setInitializing] = useState(true);
  const [user, setUser] = useState();

  // Handle user state changes
  function onAuthStateChanged(user) {
	  console.log(user)
    setUser(user);
    if (initializing){
      setInitializing(false);
    }
  }

  // useEffect is a react Hook.
  useEffect(() => {
    const subscriber = auth().onAuthStateChanged(onAuthStateChanged);
    return subscriber; // unsubscribe on unmount
  }, []);

  if (initializing) return null;

  if (!user) {
    return (
      <View>
        <Text>Login</Text>
      </View>
    );
  }

  // If login is successfull we will display the user email
  return (
    <View>
      <Text>Welcome {user.email}</Text>
	    <GoogleSignOut />
    </View>
  );
}

function App(){
  return (
    <>
      <StatusBar barStyle="dark-content" />
      <View style={{paddingTop: 60, paddingLeft: 15}}>
        <GoogleSignIn />
        <LoginApp />
      </View>
    </>
  );
};

export default App;