FirebaseAuthentication demo using Google Sign-in in Flutter.

flutter_firebase_authentication_with_google_sign_in.dart

import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import 'package:flutter_signin_button/button_view.dart';
import 'package:flutter_signin_button/flutter_signin_button.dart';
import 'package:google_sign_in/google_sign_in.dart';

class SignInDemoApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Google Sign-In Demo',
      theme: ThemeData(
        primarySwatch: Colors.deepOrange,
      ),
      home: SignInDemo(),
    );
  }
}

class SignInDemo extends StatefulWidget {
  @override
  State createState() => SignInDemoState();
}

class SignInDemoState extends State<SignInDemo> {
  final _auth = FirebaseAuth.instance;

  @override
  void initState() {
    super.initState();
    _auth.authStateChanges().listen((User user) {
      if (user == null) {
        print('User is currently signed out!');
      } else {
        print('User is signed in!');
      }
    });
  }

  // Googleを使ってサインイン
  Future<UserCredential> signInWithGoogle() async {
    // 認証フローのトリガー
    final googleUser = await GoogleSignIn(scopes: [
      'email',
    ]).signIn();
    // リクエストから、認証情報を取得
    final googleAuth = await googleUser.authentication;
    // クレデンシャルを新しく作成
    final credential = GoogleAuthProvider.credential(
      accessToken: googleAuth.accessToken,
      idToken: googleAuth.idToken,
    );
    // サインインしたら、UserCredentialを返す
    return FirebaseAuth.instance.signInWithCredential(credential);
  }

  @override
  Widget build(BuildContext context) {
    FirebaseAuth.instance.authStateChanges().listen((User user) {
      if (user == null) {
        print('User is currently signed out!');
      } else {
        print('User is signed in!');
      }
    });
    return Scaffold(
      appBar: AppBar(
        title: const Text(
          'Google Sign-In Demo',
        ),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            SignInButton(
              Buttons.Google,
              onPressed: () async {
                try {
                  final userCredential = await signInWithGoogle();
                } on FirebaseAuthException catch (e) {
                  print('FirebaseAuthException');
                  print('${e.code}');
                } on Exception catch (e) {
                  print('Exception');
                  print('${e.toString()}');
                }
              },
            ),
          ],
        ),
      ),
    );
  }
}