avoid_mounted_in_setstate

v0.4.0 Warning Fix State Management

Warns when mounted or context.mounted is checked inside a setState callback. If the widget has been disposed, setState itself throws an exception before the callback ever runs, making any mounted check inside it useless.

Why use this rule

A common misconception is that checking mounted inside setState protects against calling setState on a disposed widget. In reality, setState validates the state object immediately when called — if the widget is unmounted, it throws before executing the callback. The mounted check must happen before the setState call, not inside it.

See also: State.mounted | State.setState

Don’t

class _BadExampleState extends State<BadExample> {
  Future<void> _loadData() async {
    final data = await Future.delayed(const Duration(seconds: 1), () => 42);

    // mounted check inside setState is too late
    setState(() {
      if (mounted) {
        // If the widget was disposed, setState already threw
      }
    });

    // context.mounted inside setState is also wrong
    setState(() {
      if (context.mounted) {
        // Same problem
      }
    });
  }

  @override
  Widget build(BuildContext context) => const SizedBox();
}

Do

class _GoodExampleState extends State<GoodExample> {
  Future<void> _loadData() async {
    final data = await Future.delayed(const Duration(seconds: 1), () => 42);

    // Check mounted BEFORE calling setState
    if (!mounted) return;
    setState(() {
      // Safe — we already verified the widget is still mounted
    });

    // Or using context.mounted
    if (context.mounted) {
      setState(() {
        // Also safe
      });
    }
  }

  @override
  Widget build(BuildContext context) => const SizedBox();
}

Configuration

To disable this rule:

plugins:
  many_lints:
    diagnostics:
      avoid_mounted_in_setstate: false