proposal: `use_set_state_synchronously`

[navaronbracke]

use_set_state_synchronously

I propose a new lint, akin to its counterpart, use_build_context_synchronously, but for setState() invocations.

Description

Use setState() synchronously and guard it with a mounted check if needed.

Details

Using setState() after an async gap is potentially unsafe. Consider guarding it with a mounted check.

We should consider both explicit awaits and sync/async versions of the then/catchError/whenComplete callbacks.

Kind

It guards against errors.

Bad Examples

await Future.value(42);

setState(() {});

await Future.value(42).then((int value) {
  setState(() {});
});

await Future.value(42).then((int value) async {
  await Future<void>.delayed(const Duration(seconds: 1));

  setState(() {});
});

await Future.value(42).catchError((Object error, StackTrace stackTrace) {
  setState(() {});
});

await Future.value(42).whenComplete(() {
  setState(() {});
});

Good Examples

await Future.value(42);

if (mounted) {
  setState(() {});
}

await Future.value(42);

if (!mounted) {
  return;
}

setState(() {});

await Future.value(42);

if (!context.mounted) {
  return;
}

setState(() {});

await Future.value(42).then((int value) {
  if (!mounted) {
    return;
  }
  
  setState(() {});
});

await Future.value(42).then((int value) async {
  await Future<void>.delayed(const Duration(seconds: 1));
  
  if (!mounted) return;

  setState(() {});
});

await Future.value(42).catchError((Object error, StackTrace stackTrace) {
  if (!mounted) {
    return;
  }
  
  setState(() {});
});

await Future.value(42).whenComplete(() {
  if (!mounted) {
    return;
  }

  setState(() {});
});

Discussion

I have seen bugs (both in the framework and in user code) due to bad usages of setState() after an async gap.

I.e. flutter/flutter#124597

Discussion checklist

• List any existing rules this proposal modifies, complements, overlaps or conflicts with.
• List any relevant issues (reported here, the SDK Tracker, or elsewhere).
• If there's any prior art (e.g., in other linters), please add references here.
• If this proposal corresponds to Effective Dart or Flutter Style Guide advice, please call it out. (If there isn't any corresponding advice, should there be?)
• If this proposal is motivated by real-world examples, please provide as many details as you can. Demonstrating potential impact is especially valuable.

[srawlins]

CC @goderbauer

[goderbauer]

Nice suggestion. I think this would be a good one to have. setState and using context after an async gap should follow the same rules. I wonder how many violations of this we have in the framework...

[pq]

@stereotype441 (re: flow analysis)