Don't use 'BuildContext's across async gaps. flutter警告

The `Don't use 'BuildContext's across async gaps` warning in Flutter is usually caused by using a `BuildContext` that was obtained in a different `async` context than the one in which it is being used. This can lead to unexpected behavior and errors.

To avoid this warning, you can use the `BuildContext` obtained in the current `async` context or pass it as a parameter to the function or method where it is needed. Here's an example of how to do this:

Future<void> showSnackBar(BuildContext context, String message) async {
  final snackBar = SnackBar(content: Text(message));
  ScaffoldMessenger.of(context).showSnackBar(snackBar);
}

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('SnackBar Demo')),
        body: Center(
          child: ElevatedButton(
            child: Text('Show SnackBar'),
            onPressed: () {
              showSnackBar(context, 'Hello, World!');
            },
          ),
        ),
      ),
    );
  }
}

In the example above, we define a function `showSnackBar` that takes a `BuildContext` and a `String` as parameters. We use the `ScaffoldMessenger.of(context)` method to obtain the `ScaffoldMessengerState` for the current `BuildContext` and show the `SnackBar`.

In the `onPressed` callback of the `ElevatedButton`, we pass the `BuildContext` obtained from the `build` method to the `showSnackBar` function. This ensures that we're using the `BuildContext` obtained in the current `async` context, and avoids the warning message.