Cannot pass `arguments` through `onGenerateRoute()` in `null-safety` version of flutter

Question

What am I trying to do?

As usual, I am trying to sending data from one screen to another onGenerateRoute() but I screen does not accept arguments type and showing error The argument type 'Object?' can't be assigned to the parameter type 'Map<String, dynamic>'.

I also tried to change the argument type Object? on the receiver screen but now I am not able to extract data from Object to Map<String, dynamic>. I think both are the same data type but the null-safety version is treated differently. I think this is a bug.

I have seen flutter official documentation for navigate-with-arguments and when I switched to null-safety in Interactive example section then it also showing error. See this screenshot or try it yourself.

It's working properly in the non-null-safety version of flutter

Here's the snippet

RouteGenerator 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}'),
              ),
            ),
          ),
        );
    }
  }
}

VerifyFirebaseOtpScreen

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']}',
              ),
            ),
          ),
        ),
      ),
    );
  }
}

Logs

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

Answer in short

Use typecast operator as so above question's answer will be like final args = settings.arguments as Map<String, dynamic>;

Answer explanation

I also filed this question as 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 says

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>.

He also said

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

dart.dev explanation and example

Referred documentation page Understanding null-safety & using-nullable-types explanation also covering this.

Their below example is explain enough

// 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);
}