主屏幕显示 firebaseAuth Flutter 中的空未来构建器

Question

我目前正在尝试从 FirebaseAuth 获取当前用户状态，并在用户未注销时在主屏幕之间切换，如果用户已注销，则在启动屏幕之间切换。用户注册和登录都成功并在firestore数据库中注册。但是每次我从手机关闭应用程序并重新打开时，MediaQuery.of(context) 宽度都有一个空值并显示错误。

但底部导航栏仍在显示。当我重新构建应用程序并再次登录时它可以工作，它会将我带到主屏幕。仅当我关闭并重新打开应用程序时才会发生。

import 'package:country_code_picker/country_localizations.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:firebase_core/firebase_core.dart';
import 'package:flutter/material.dart';
import 'routes.dart';
import 'package:screens/home/home_screen.dart';
import 'package:screens/splash/splash_screen.dart';

import 'constants.dart';

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      supportedLocales: [Locale('en', 'US')],
      localizationsDelegates: [CountryLocalizations.delegate],
      title: 'App',
      theme: theme(),
      routes: routes,
      home: LandingPage(),
    );
  }
}

class LandingPage extends StatelessWidget {
  const LandingPage({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final Future<FirebaseApp> _init = Firebase.initializeApp();
    return FutureBuilder(
      future: _init,
      builder: (context, snapshot) {
        if (snapshot.hasError) {
          return Scaffold(
            body: Center(
              child: Text("Error: ${snapshot.error}"),
            ),
          );
        }

    if (snapshot.connectionState == ConnectionState.done) {
      return StreamBuilder(
        stream: FirebaseAuth.instance.authStateChanges(),
        builder: (context, streamSnapshot) {
          if (streamSnapshot.hasError) {
            return Scaffold(
              body: Center(
                child: Text("Error: ${streamSnapshot.error}"),
              ),
            );
          }

          if (streamSnapshot.connectionState == ConnectionState.active) {
            print(streamSnapshot.data.toString());
            if (streamSnapshot.data == null) {
              return SplashScreen();
            } else {
              return HomeScreen();
            }
          }

          return Scaffold(
            body: Center(
              child: CircularProgressIndicator(
                backgroundColor: Colors.white,
                color: kPrimaryColor,
                strokeWidth: 3,
              ),
            ),
          );
        },
      );
    }

    return Scaffold(
      body: Center(
        child: CircularProgressIndicator(
          backgroundColor: Colors.white,
          color: kPrimaryColor,
          strokeWidth: 3,
        ),
      ),
    );
  },
);
}
}

也许我认为这是未来构建器或流构建器的问题？你们中的任何人都有解决此问题的方法或其他实现方法。

Launching lib/main.dart on STK L21 in debug mode...
✓  Built build/app/outputs/flutter-apk/app-debug.apk.
Connecting to VM Service at ws://127.0.0.1:33019/yIPGCqY2gwY=/ws
I/flutter ( 4249): locale.languageCode: en
D/AwareBitmapCacher( 4249): handleInit switch not opened pid=4249
I/flutter ( 4249): User(displayName: , email: rohanbhautoo@gmail.com, emailVerified: false, isAnonymous: false, metadata: UserMetadata(creationTime: 2021-06-10 08:20:44.583, lastSignInTime: 2021-06-10 15:50:23.573), phoneNumber: , photoURL: null, providerData, [UserInfo(displayName: , email: rohanbhautoo@gmail.com, phoneNumber: , photoURL: null, providerId: password, uid: rohanbhautoo@gmail.com)], refreshToken: , tenantId: null, uid: yviaYOJ5ziayFQhvhvemB3MjkPr1)

════════ Exception caught by widgets library ═══════════════════════════════════
The following _CastError was thrown building Body(dirty):
Null check operator used on a null value

The relevant error-causing widget was
Body
When the exception was thrown, this was the stack
#0      getProportionateScreenWidth
#1      Body.build
#2      StatelessElement.build
#3      ComponentElement.performRebuild
#4      Element.rebuild
...
════════════════════════════════════════════════════════════════════════════════

Size_config.dart

import 'package:flutter/material.dart';

class SizeConfig {
  static MediaQueryData? _mediaQueryData;
  static double? screenWidth;
  static double? screenHeight;
  static double? defaultSize;
  static Orientation? orientation;

  void init(BuildContext context) {
    _mediaQueryData = MediaQuery.of(context);
    screenWidth = _mediaQueryData!.size.width;
    screenHeight = _mediaQueryData!.size.height;
    orientation = _mediaQueryData!.orientation;
  }
}

// Proportionate height per screen size
double getProportionateScreenHeight(double inputHeight) {
  double? screenHeight = SizeConfig.screenHeight;
  return (inputHeight / 812.0) * screenHeight!;
}

// Proportionate width per screen size
double getProportionateScreenWidth(double inputWidth) {
  double? screenWidth = SizeConfig.screenWidth;
  return (inputWidth / 375.0) * screenWidth!;
}

#0 的堆栈跟踪行是 return (inputWidth / 375.0) * screenWidth!;它显示我在 screenWidth 上使用空检查！。

Answer 1

我没有看到您实际调用 SizeConfig().init(context) 的位置？ 在您的项目开始之前，什么都不会为您定义。

Answer 2

我猜罪魁祸首是这一行：

print(streamSnapshot.data.toString());

您实际上是在检查下一行的 streamSnapshot.data 是否为空：

            print(streamSnapshot.data.toString());
            if (streamSnapshot.data == null) {
              return SplashScreen();
            } else {
              return HomeScreen();
            }