【Flutter】FirebaseでFacebook認証（Auth）のやり方まとめ【スクショ・ソースあり】

dependencies:
  firebase_auth: ^0.14.0+5
  flutter_facebook_login: ^3.0.0

$ flutter pub get

import 'package:flutter/material.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter_facebook_login/flutter_facebook_login.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'auth sample',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: AuthPage(
        title: 'Auth Sample with Firebase',
      ),
    );
  }
}

class AuthPage extends StatefulWidget {
  AuthPage({
    Key key,
    this.title,
  }) : super(
          key: key,
        );

  final String title;

  @override
  _AuthPageState createState() => _AuthPageState();
}

class _AuthPageState extends State {
  final FirebaseAuth _auth = FirebaseAuth.instance;
  final FacebookLogin facebookSignIn = new FacebookLogin();

  Future signInWithFacebook() async {
    final facebookLogin = FacebookLogin();
    // Facebookの認証画面が開く
    final facebookLoginResult = await facebookLogin.logIn((['email']));

    // Firebaseのユーザー情報と連携
    final AuthCredential credential = FacebookAuthProvider.getCredential(
      accessToken: facebookLoginResult.accessToken.token,
    );

    // Firebaseのユーザー情報を取得
    final FirebaseUser user =
        (await _auth.signInWithCredential(credential)).user;

    assert(!user.isAnonymous);
    assert(await user.getIdToken() != null);

    final FirebaseUser currentUser = await _auth.currentUser();
    assert(user.uid == currentUser.uid);

    login();
  }

  bool logined = false;

  void login() {
    setState(() {
      logined = true;
    });
  }

  void logout() {
    setState(() {
      logined = false;
    });
  }

  void signOutFacebook() async {
    await facebookSignIn.logOut();
    logout();
    print("User Sign Out Facebook");
  }

  @override
  Widget build(BuildContext context) {
    Widget logoutText = Text("ログアウト中");
    Widget loginText = Text("ログイン中");


    Widget loginBtnFb = RaisedButton(
      child: Text("Sign in with Facebook"),
      color: Color(0xFF3B5998),
      textColor: Colors.white,
      onPressed: signInWithFacebook,
    );

    Widget logoutBtnFb = RaisedButton(
      child: Text("Sign out with Facebook"),
      color: Color(0xFF3B5998),
      textColor: Colors.white,
      onPressed: signOutFacebook,
    );

    return Scaffold(
      appBar: AppBar(
        centerTitle: true,
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            logined ? loginText : logoutText,
            logined ? logoutBtnFb : loginBtnFb,
          ],
        ),
      ),
    );
  }
}