avoid-unnecessary-setstate

added in: 1.6.0 warning

Warns when setState is called inside initState, didUpdateWidget or build methods and when it's called from a sync method that is called inside those methods.

Calling setState in those cases will lead to an additional widget rerender which is bad for performance.

Consider changing state directly without calling setState.

Additional resources:

• https://stackoverflow.com/questions/53363774/importance-of-calling-setstate-inside-initstate/53373017#53373017

Example

Bad:

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  String myString = '';

  @override
  void initState() {
    super.initState();

    // LINT
    setState(() {
      myString = "Hello";
    });

    if (condition) {
      // LINT
      setState(() {
        myString = "Hello";
      });
    }

    myStateUpdateMethod(); // LINT
  }

  @override
  void didUpdateWidget(MyWidget oldWidget) {
    // LINT
    setState(() {
      myString = "Hello";
    });
  }

  void myStateUpdateMethod() {
    setState(() {
      myString = "Hello";
    });
  }

  @override
  Widget build(BuildContext context) {
    // LINT
    setState(() {
      myString = "Hello";
    });

    if (condition) {
      // LINT
      setState(() {
        myString = "Hello";
      });
    }

    myStateUpdateMethod(); // LINT

    return ElevatedButton(
      onPressed: () => myStateUpdateMethod(),
      onLongPress: () {
        setState(() {
          myString = data;
        });
      },
      child: Text('PRESS'),
    );
  }
}

Good:

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  String myString = '';

  final classWithMethod = SomeClassWithMethod();

  @override
  void initState() {
    super.initState();

    myString = "Hello";

    classWithMethod.myMethod();
    myAsyncMethod();
  }

  @override
  void didUpdateWidget(MyWidget oldWidget) {
    myString = "Hello";
  }

  void myStateUpdateMethod() {
    setState(() {
      myString = "Hello";
    });
  }

  Future<void> myAsyncMethod() async {
    final data = await service.fetchData();

    setState(() {
      myString = data;
    });
  }

  @override
  Widget build(BuildContext context) {
    myAsyncMethod();

    return ElevatedButton(
      onPressed: () => myStateUpdateMethod(),
      onLongPress: () {
        setState(() {
          myString = data;
        });
      },
      child: Text('PRESS'),
    );
  }
}