avoid_unnecessary_setstate

v0.4.0 Warning Fix State Management

Warns when setState is called directly inside initState, didUpdateWidget, or build methods. In these lifecycle methods, mutating state directly is sufficient because the framework already schedules a build after they return. Calling setState in build triggers an unnecessary additional rebuild.

Why use this rule

In initState and didUpdateWidget, the framework will call build automatically after the method returns, so wrapping mutations in setState is redundant overhead. In build, calling setState triggers an infinite rebuild loop or at minimum a wasted frame. Event handler callbacks (like onTap) inside build are excluded from this rule since they run asynchronously and do need setState.

See also: setState

Don’t

class _BadInitStateState extends State<BadInitState> {
  String _data = '';

  @override
  void initState() {
    super.initState();
    // Unnecessary — framework rebuilds after initState anyway
    setState(() {
      _data = 'Hello';
    });
  }

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

class _BadBuildState extends State<BadBuild> {
  String _data = '';

  @override
  Widget build(BuildContext context) {
    // Triggers an unnecessary rebuild during build
    setState(() {
      _data = 'Hello';
    });
    return const SizedBox();
  }
}

Do

class _GoodInitStateState extends State<GoodInitState> {
  String _data = '';

  @override
  void initState() {
    super.initState();
    _data = 'Hello'; // Assign directly — no setState needed
  }

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

// setState in an async method is fine
class _GoodAsyncState extends State<GoodAsync> {
  String _data = '';

  Future<void> _loadData() async {
    final data = await Future.value('Hello');
    setState(() {
      _data = data; // Async method needs setState to trigger rebuild
    });
  }

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

// setState in event handler callbacks inside build is fine
class _GoodCallbackState extends State<GoodCallback> {
  String _data = '';

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: () {
        setState(() {
          _data = 'tapped'; // Event handler runs asynchronously
        });
      },
      child: const SizedBox(),
    );
  }
}

Configuration

To disable this rule:

plugins:
  many_lints:
    diagnostics:
      avoid_unnecessary_setstate: false