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我想做什么？

像往常一样，我试图将数据从一个屏幕发送到另一个屏幕，onGenerateRoute()但我的屏幕不接受参数类型并显示错误参数类型“对象？” 不能分配给参数类型“Map<String, dynamic>”。

我还尝试更改Object?接收器屏幕上的参数类型，但现在我无法从Objectto中提取数据Map<String, dynamic>。我认为两者都是相同的数据类型，但 null 安全版本的处理方式不同。我认为这是一个错误。

我已经看到了有关使用参数导航的颤振官方文档，当我在交互式示例部分切换到 null-safety 时，它也显示错误。请参阅此屏幕截图或自己尝试。

截图 2021-03-30 at 20 13 56

它在颤振的非空安全版本中正常工作

这是片段

路由生成器类

class RouteGenerator {
  static Route<dynamic> generateRoute(RouteSettings settings) {
    // Getting arguments passed while calling Navigator.pushNamed
    final args = settings.arguments;
    switch (settings.name) {
      case HomeScreen.routeName:
        return MaterialPageRoute(
          builder: (context) => HomeScreen(),
        );
      case LoginScreen.routeName:
        return MaterialPageRoute(
          builder: (context) => LoginScreen(),
        );
      case VerifyFirebaseOtpScreen.routeName:
        return MaterialPageRoute(
          builder: (context) => VerifyFirebaseOtpScreen(data: args), // Here is the error: The argument type 'Object?' can't be assigned to the parameter type 'Map<String, dynamic>'.
        );
      case AboutScreen.routeName:
        return MaterialPageRoute(
          builder: (context) => AboutScreen(),
        );
      default:
        return MaterialPageRoute(
          builder: (context) => Scaffold(
            body: SafeArea(
              child: Center(
                child: Text('No route defined for ${settings.name}'),
              ),
            ),
          ),
        );
    }
  }
}

验证FirebaseOtpScreen

class VerifyFirebaseOtpScreen extends StatelessWidget {
  static const String routeName = '/verify_firebase_otp_screen';

  final Map<String, dynamic> data;

  const VerifyFirebaseOtpScreen({
    Key? key,
    required this.data,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: () {
        FocusScope.of(context).unfocus();
      },
      child: Scaffold(
        body: Center(
          child: SingleChildScrollView(
            padding: const EdgeInsets.all(8.0),
            child: Container(
              width: double.infinity,
              child: VerifyFirebaseOtpScreenDataSection(
                mobile: '${data['mobile']}',
              ),
            ),
          ),
        ),
      ),
    );
  }
}
日志 
abhishekkumar@Abhisheks-MacBook-Air ~ % flutter doctor -v
[✓] Flutter (Channel beta, 2.1.0-12.2.pre, on macOS 11.2.3 20D91 darwin-x64, locale en-IN)
    • Flutter version 2.1.0-12.2.pre at /Users/abhishekkumar/flutter
    • Framework revision 5bedb7b1d5 (13 days ago), 2021-03-17 17:06:30 -0700
    • Engine revision 711ab3fda0
    • Dart version 2.13.0 (build 2.13.0-116.0.dev)

[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
    • Android SDK at /Users/abhishekkumar/Library/Android/sdk
    • Platform android-30, build-tools 30.0.2
    • ANDROID_HOME = /Users/abhishekkumar/Library/Android/sdk
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6915495)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 12.4, Build version 12D4e
    • CocoaPods version 1.10.1

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 4.1)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin can be installed from:
       https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
       https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6915495)

[✓] VS Code (version 1.51.1)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.17.0

[✓] Connected device (3 available)
    • iPhone SE (1st generation) (mobile) • 035FA189-09FF-46B5-96AC-C34E8D068C21 • ios            • com.apple.CoreSimulator.SimRuntime.iOS-14-4 (simulator)
    • macOS (desktop)                     • macos                                • darwin-x64     • macOS 11.2.3 20D91 darwin-x64
    • Chrome (web)                        • chrome                               • web-javascript • Google Chrome 89.0.4389.90

• No issues found!
4

1 回答 1

1

简短地回答

使用类型转换运算符as ，所以上面问题的答案就像final args = settings.arguments as Map<String, dynamic>;

答案说明

我还在GitHub 上将此问题作为问题提交，并感谢 Goderbauer（Flutter 团队成员）正确识别此问题并通过提供适当的解决方案将其关闭。

戈德鲍尔 说

在您的示例settings.arguments中输入为Object?，并且您将其传递给VerifyFirebaseOtpScreen.data输入为Map<String, dynamic>. 在 null-safety 之前，这是合法的，被称为隐式向下转换。但是使用 null 安全性，Dart 完全删除了隐式向下转换（您可以在此处阅读更多相关信息https://dart.dev/null-safety/understanding-null-safety，只需在页面上搜索“隐式向下转换”）。因此，现在，如果您确定确实settings.arguments是类型，则Map<String, dynamic>需要进行显式转换，例如：settings.arguments as Map<String, dynamic>.

他还说

（一旦我们将它们迁移到空安全，页面上的示例也必须更新）

dart.dev 解释和示例

参考文档页面了解 null-safety & using-nullable-types解释也涵盖了这一点。

他们下面的例子已经足够解释了

// Without null safety:
requireStringNotObject(String definitelyString) {
  print(definitelyString.length);
}

main() {
  Object maybeString = 'it is';
  requireStringNotObject(maybeString);
}

// Using null safety:
requireStringNotObject(String definitelyString) {
  print(definitelyString.length);
}

main() {
  Object maybeString = 'it is';
  requireStringNotObject(maybeString as String);
}
于 2021-04-01T13:11:09.527 回答