无法在 `null-safety` 版本的 flutter 中通过 `onGenerateRoute()` 传递 `arguments`

Question

我想做什么？

像往常一样，我正在尝试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 'Object?' can' t被赋值给参数类型'Map<String, dynamic>'.

我还尝试在接收器屏幕上更改参数类型 Object?，但现在我无法将数据从 Object 提取到 Map<String, dynamic>。我认为两者是相同的数据类型，但空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 flutter official documentation for navigate-with-arguments，当我在 交互示例 部分切换到 null-safety 时，它也显示错误。请查看此屏幕截图或自己尝试。

它在非空安全版本的 flutter 中正常工作

这是片段

路线生成器Class

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!

Answer 1

简答

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>;

回答说明

我也将这个问题提交为 an issue on GitHub and thanks to goderbauer (A Flutter Team Member) who identify correctly this issue and closed it by giving an appropriate solution。

goderbauer 说

In your example settings.arguments is typed as Object? and you're passing it to VerifyFirebaseOtpScreen.data which is typed as Map<String, dynamic>. Prior to null-safety this was legal and is called an implicit downcast. But with null safety Dart has removed implicit downcast altogether (you can read more about that here https://dart.dev/null-safety/understanding-null-safety, just search for "implicit downcast" on the page). So now, if you're sure that settings.arguments is in deed of type Map<String, dynamic> you need to do an explicit cast, something like: settings.arguments as Map<String, dynamic>.

他还说

(The example on the page will have to be updated as well once we migrate them to null safety)

dart.dev 解释和例子

参考文档页面 Understanding 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);
}