Don't use 'BuildContext's across async gaps in Flutter warning doesn't go away

Question

Flutter gives me the warning "Don't use 'BuildContext's across async gaps. Try rewriting the code to not use the 'BuildContext', or guard the use with a 'mounted' check". The warning is in my app twice and I can't make any of them go away by adding a "mounted guard". Below is a full standalone app that demonstrates a simple version of my app. Can you please show me what is the ideal way of solving this problem, and if it's the "mounted guard", where should I put them?

import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';

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

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Search History Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(),
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Search History'),
      ),
      body: Center(
        child: ElevatedButton(
          onPressed: () {
            clearEnglishHistory(context);
          },
          child: const Text('Clear History'),
        ),
      ),
    );
  }

  Future<void> clearEnglishHistory(BuildContext context) async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    bool confirmClear = await showDialog(
      context: context,
      builder: (BuildContext context) {
        return AlertDialog(
          title: const Text('Confirmation'),
          content: const Text('Do you really want to clear search history?'),
          actions: <Widget>[
            TextButton(
              onPressed: () {
                Navigator.of(context).pop(false);
              },
              child: const Text('No'),
            ),
            TextButton(
              onPressed: () {
                Navigator.of(context).pop(true);
              },
              child: const Text('Yes'),
            ),
          ],
        );
      },
    );

    if (confirmClear == true) {
      await prefs.remove('english history');
      ScaffoldMessenger.of(context).showSnackBar(const SnackBar(
        content: Text('History cleared'),
      ));
    }
  }
}

Devon Ray · Accepted Answer · 2024-04-11 21:41:26Z

2

you need to either disable the message using lint message, or you can add a mounted check before you show your snackbar. Something like this:

if (confirmClear == true) {
      await prefs.remove('english history');
      if(!mounted) return;
      ScaffoldMessenger.of(context).showSnackBar(const SnackBar(
        content: Text('History cleared'),
      ));
    }

answered Apr 11, 2024 at 21:41

Devon Ray

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2 Comments

AvestaChang Over a year ago

In the code above when I did that it would say !mounted is undefined, but in my main app which complex and I have handled the snackbar in the exact way, there is no error, but doesn't also make the warnings go away. I also looked at all questions in the SO and none worked for me.

AvestaChang Over a year ago

I also tried if (confirmClear == true && mounted) {, but the warnings still persist, even after restarting VScode to make sure the warnings weren't cached.

FrankAi · Accepted Answer · 2024-07-25 07:07:15Z

2

Devon Ray was almost right but it is a StatelessWidget so you have to write if(!context.mounted) return; instead of if(!mounted) return;

answered Jul 25, 2024 at 7:07

FrankAi

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Comments

Randal Schwartz · Accepted Answer · 2024-04-11 21:51:02Z

0

Others have answered where to add the mounted guard, but I wanted to point out that

await SharedPreferences.getInstance()

should really appear only in your main, saving the result into a global prefs var. Once you get the instance, you have the instance, and you can use it totally synchronously to get values as well. The only async you'll need to worry about then is the writing of new values, and even that can be fire-and-forget for the most part.

answered Apr 11, 2024 at 21:51

Randal Schwartz

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2 Comments

AvestaChang Over a year ago

I really have no programming background (just a teacher making an educational app). I build the app by googling and courses. Would it be fine if I keep them? because when I tried to remove them, it made other errors I didn't know how to fix. Maybe until I get more experience, then I will in a few years fix them.

AvestaChang Over a year ago

By the way, the other answers didn't solve my problem. Could you share your own?

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