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Many Lints

prefer_widget_private_members

v1.0.0 Warning Widget Best Practices

Flags a public instance method or getter declared on a widget class.

A widget’s public surface is its constructor — the parameters a parent passes in. Everything else exists to serve build, and making it public invites a caller to reach in and drive part of the rendering out of band.

This matters more in Flutter than the general encapsulation argument suggests, because a widget instance is rebuilt constantly. A public method sits on an object the framework may discard on the next frame, so whatever a caller does with it cannot be relied upon.

This rule is in the pedantic preset: it imposes an architecture rather than catching a defect. No configuration.

See also: Flutter: widget classes over helper methods

Enabling this rule

Section titled “Enabling this rule”
many_lints.yaml
rules:
  prefer_widget_private_members: true

Don’t

Section titled “Don’t”
class SearchBar extends StatelessWidget {
  const SearchBar({super.key});


  // A caller can reach in and drive part of the rendering.
  void clear() {}


  String get query => '';


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

Do

Section titled “Do”

Make it private, and move anything a parent genuinely needs into the constructor:

class SearchBar extends StatelessWidget {
  const SearchBar({required this.query, required this.onClear, super.key});


  // Fields are the constructor's parameters — never reported.
  final String query;
  final VoidCallback onClear;


  void _handleSubmit() {}


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

Statics are exempt

Section titled “Statics are exempt”

A static is not reachable on a widget instance, so the rebuild argument does not apply to it. static Future<T> show(context) is the documented way to open a dialog or a sheet — on a real app this accounted for 14 of 16 reports:

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


  // Not reported
  static Future<bool?> show(BuildContext context) =>
      showDialog<bool>(context: context, builder: (_) => const ConfirmDialog());


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

Overrides and test seams are exempt

Section titled “Overrides and test seams are exempt”

@override names belong to the supertype, and @visibleForTesting is a deliberate widening — neither is reported:

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


  @override
  String toStringShort() => 'Chip';        // not reported: the name is the supertype's


  @visibleForTesting
  int computeWidth() => 0;                 // not reported: a deliberate widening


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

Known limitations

Section titled “Known limitations”

Fields are never reported. A widget’s fields are its constructor parameters, and public final fields are the idiom the framework itself uses.

State classes are not checked. Only classes that are themselves widgets are examined — a State subclass and its initState/dispose are out of scope.

The framework’s own hooks — build, createState, createElement, debugFillProperties, debugDescribeChildren, toStringShort, toString, noSuchMethod — cannot be private and are always exempt. Operators are skipped too.

Turning this rule off

Section titled “Turning this rule off”
many_lints.yaml
rules:
  prefer_widget_private_members: false

To keep the rule on but skip certain paths, use per-rule exclude.

Section titled “Related rules”