Flutter passing optional List argument through go router

Question

I want to pass an optional List argument through go_router.
Here's my class with an optional List argument:

class Loading extends StatefulWidget {
    List<int>? genres;
    Loading({Key? key, this.genres}) : super(key: key);

    @override
    _LoadingState createState() => _LoadingState();
}

And the router:

final GoRouter _router = GoRouter(routes: [
    GoRoute(name: 'loading', path: '/', builder: (context, state) {
            List<int> genres = state.extra as List;
            return Loading(genres: genres);
        },  
    ), 

  ...
 
]);

But it doesn't work. I get this error message:

How can I fix that ?

Answer 1

The error message suggests that you are trying to cast a non-nullable type to a nullable type, which is not allowed. Since genres is an optional list (List<int>?), you need to check whether it is null before casting it to a non-null List<int>.

You can modify your router code as follows:

final GoRouter _router = GoRouter(routes: [
  GoRoute(
    name: 'loading',
    path: '/',
    builder: (context, state) {
      List<int>? genres = state.extra as List<int>?;
      return Loading(genres: genres);
    },
  ),
  ...
]);

In this code, the genres variable is declared as a nullable list (List<int>?). The cast to List<int>? allows for null values, so if state.extra is null, genres will be set to null as well. If state.extra is not null, it will be cast to a non-null List<int> and assigned to genres.

Then, you can pass genres to the Loading widget, which is also an optional list, so you don't need to do any additional null checks inside the widget.

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