BuildContext across async gaps with mounted check false positive

[vendelin8]

Describe the issue
Lint error use_build_context_synchronously when it was already checked

To Reproduce

    TextButton(
      child: Text('Choose a file...'),
      onPressed: () async {
        FilePickerResult? result = await FilePicker.platform.pickFiles(
          dialogTitle: 'Choose a file',
          allowCompression: true,
          type: FileType.image,
        );
        if (!mounted) return;

        if (result == null) {
          Navigator.of(context).pop(); // LINT INFO HERE
          return;
        }
        Navigator.of(context).pop(await cp.getImage(result.files.single)); // LINT INFO HERE
      },
    ),

Expected behavior
No lint infos, because mounted was already checked previously.

Additional context

dart --version
Dart SDK version: 3.5.0 (stable) (Tue Jul 30 02:17:59 2024 -0700) on "linux_x64"

[srawlins]

Is TextButton the containing class a State subclass? If not, you need to do a mounted check with context.mounted, not mounted.

[vendelin8]

This is a function Future<void> newPhoto() async in class _ProfileImageState extends State<_ProfileImage> which is the state of class _ProfileImage extends StatefulWidget

[srawlins]

I see. Can you provide a full reproduction? I cannot reproduce this issue in unit tests.

[vendelin8]

https://github.com/vendelin8/dart-lintbug

[srawlins]

@goderbauer could you clarify for me, in this code:

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) => const Scaffold(
        body: Text(
          'You have pushed the button this many times:',
        ),
      );

  Future<int?> test() async {
    await Future.delayed(const Duration(seconds: 1));
    return 1;
  }

  Future<void> newPhoto() async {
    await showDialog(
        context: context,
        builder: (BuildContext context) => AlertDialog(
              actions: [
                TextButton(
                  onPressed: () async {
                    int? result = await test();
                    if (!mounted) return;
                    Navigator.of(context).pop(result);
                  },
                ),
              ],
            ));
  }
}

(simplified from https://github.com/vendelin8/dart-lintbug/blob/main/lib/main.dart)

Key points:

• We are in a subclass of State
• We are in a builder function that has a local context variable
• We want to call await foo, then call Navigator.of(context)

Do we need to guard that use of the local context variable via the surrounding State.mounted (this.mounted) or with context.mounted?

[goderbauer]

Do we need to guard that use of the local context variable via the surrounding State.mounted (this.mounted) or with context.mounted?

You are using the local context in Navigator.of(context) so you want to guard on that since that local context and the surrounding State can have different life cycles.

[srawlins]

OK good, the docs are accurate on this question. Thanks!

• When using a State's context property, the State's mounted property must be checked.
• For other BuildContext instances (like a local variable or function argument), the BuildContext's mounted property must be checked.

[vendelin8]

Ok, thanks!