use_build_context_synchronously is triggered when using context.mounted

[luccasclezar]

After upgrading to Flutter 3.7, using BuildContext.mounted in any way other than if (context.mounted) {} makes the linter complain about using context across async gaps.

@goderbauer told me to mention him.

Reproduction steps

1. Create a new project with flutter create
2. Add this Widget to main.dart

class AsyncContextTestWidget extends StatelessWidget {
  const AsyncContextTestWidget({super.key});

  @override
  Widget build(BuildContext context) {
    return ElevatedButton(
      onPressed: () async {
        await Future<void>.delayed(Duration.zero);

        // This line is marked as using context across async gaps
        if (!context.mounted) {
          return;
        }
      },
      child: const Text('Test'),
    );
  }
}

!context.mounted is just one example of this problem. This is reproducible when using ternary operators or initializing a variable to context.mounted too.

// This line triggers a warning too
final isMounted = context.mounted;

if (!isMounted) {
  return;
}

[andrewescutia]

If it helps here is an example failing test:

void methodWithBuildContextParameter4(BuildContext context) async { 
  await Future<void>.delayed(Duration());
  if (!context.mounted) {
    return;
  }
  await Navigator.of(context).pushNamed('routeName'); // OK
}

The added second if check below within lib/src/rules/use_build_context_synchronously.dart appears to correct it but I do not have an intimate knowledge of this repo.

        // if (mounted) { ... do ... }
        if (isMountedCheck(parent, positiveCheck: true)) {
          return;
        }

        // if (!context.mounted) { return; }
        if (isMountedCheck(parent)) {
          return;
        }

[LostInDarkMath]

This bug is pretty anoying. Hope you can fix this soon :)

[asashour]

For fixing this, should we search for specific statements, e.g. if (!context.mounted) return; or we should use FlowAnalysis, to check that context is not used before .mount, in the same function/method?

[pq]

It's unfortunate that we're approximating what flow analysis does more completely but I'm not aware of any way to leverage it in a lint implementation (though it has been discussed).

/fyi @stereotype441

[Levi-Lesches]

Just ran into this issue today, pretty unintuitive why

if (!context.mounted) return;

was failing but

if (context.mounted) doSomething(context)

wasn't

[luccasclezar]

@Levi-Lesches While this is not fixed, if you really want to return instead of placing more code inside an if, you can use:

if (context.mounted) {
} else {
  return;
}

It's not great, but sometimes it can be better than nesting ifs, plus when this is fixed the refactoring will be pretty quick.

[haslinger]

Cool tip, @luccasclezar! And to be able fo find these instances later, I modified it to

  if (context.mounted) {
    // FIXME
  } else {
    return;
  }

[JosephNK]

I hope it will be corrected. :)

[gnprice]

I'm continuing to see this issue as of a Dart SDK version from today (from the Flutter main/master channel):

$ dart --version
Dart SDK version: 3.1.0-75.0.dev (dev) (Thu May 4 03:59:43 2023 -0700) on "linux_x64"

In particular, if I take a test case from the PR dart-archive/linter#4312:

import 'package:flutter/widgets.dart';

void foo(BuildContext context) async {
  await f();
  if (!context.mounted) {
    return;
  }
  Navigator.of(context);
}

Future<void> f() async {}

and run dart analyze, I get the lint message:

$ dart analyze lib/tmp.dart
Analyzing tmp.dart...                  0.5s

   info • tmp.dart:5:8 • Don't use 'BuildContext's across async gaps. Try
          rewriting the code to not reference the 'BuildContext'. •
          use_build_context_synchronously

1 issue found.

Is that unexpected, or is there something I'm missing?

[pq]

@srawlins

[srawlins]

Dart SDK at head is at linter e36813b which does not include this fix.

[gnprice]

Good to know, thanks.

What's the typical cadence of when new linter versions enter the Dart SDK? The readme for this repo just says:

The linter is bundled with the Dart SDK; if you have an updated Dart SDK already, you're done!

It's perfectly reasonable that it doesn't propagate instantly; I'm just curious what to expect.

[gnprice]

Relatedly: is there a straightforward way to look up what linter version is in the SDK? It'd be great to document that in the readme file too, if possible.

[srawlins]

All good questions!

I'd say we're aspiring for continuous deployment; each commit here should land ASAP in the Dart SDK. But we're definitely not there yet, commits here typically make it over to the Dart SDK in under a month.

Relatedly: is there a straightforward way to look up what linter version is in the SDK? It'd be great to document that in the readme file too, if possible.

We use a homegrown vendoring system, so you'll see the commit SHA in this file: https://github.com/dart-lang/sdk/blob/main/DEPS#L147

[Wellcomezhangheng]

Take away without thanking