Firebase + Flutter:使用StreamBuilder()登录

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

class LoginPage extends StatelessWidget {
  final FirebaseAuth _auth = FirebaseAuth.instance;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Login'),
      ),
      body: Center(
        child: StreamBuilder<User>(
          stream: _auth.authStateChanges(),
          builder: (BuildContext context, AsyncSnapshot<User> snapshot) {
            if (snapshot.connectionState == ConnectionState.waiting) {
              return CircularProgressIndicator();
            } else {
              if (snapshot.hasData) {
                // 用户已登录
                return Text('已登录');
              } else {
                // 用户未登录
                return RaisedButton(
                  child: Text('登录'),
                  onPressed: () {
                    _signIn(context);
                  },
                );
              }
            }
          },
        ),
      ),
    );
  }

  Future<void> _signIn(BuildContext context) async {
    try {
      UserCredential userCredential = await _auth.signInWithEmailAndPassword(
        email: 'user@example.com',
        password: 'password',
      );
      // 登录成功
    } catch (e) {
      // 登录失败
    }
  }
}