dumb_visitor.dart

/*
  sample usage:

  @override
  Widget build(BuildContext context) {
    // finds multiple instances of EditableTextState:
    return DumbVisitor(
      onFound: (EditableTextState ets) {
        print('=' * 60);
        print('1 $ets');
        print('2 ${ets.renderEditable}');
        final key = ets.context.findAncestorWidgetOfExactType<TextField>()?.key as ValueKey?;
        print('3 ${key?.value}');
      },
      child: const Column(
        children: [
          TextField(key: ValueKey('first key')),
          TextField(key: ValueKey('second key')),
        ],
      ),
    );
  }
*/

class DumbVisitor<T> extends StatelessWidget {
  const DumbVisitor({
    super.key,
    required this.onFound,
    required this.child,
  });

  final void Function(T object) onFound;
  final Widget child;

  @override
  Widget build(BuildContext context) => child;

  @override
  StatelessElement createElement() => _DumbVisitorElement<T>(this, onFound);
}

class _DumbVisitorElement<T> extends StatelessElement {
  _DumbVisitorElement(super.widget, this.onFound);

  final void Function(T object) onFound;
  Element? oldElement;

  @override
  Element? updateChild(Element? child, Widget? newWidget, Object? newSlot) {
    final element = super.updateChild(child, newWidget, newSlot);
    if (oldElement != element) {
      oldElement = element;
      element?.visitChildren(_visitor);
    }
    return element;
  }

  void _visitor(Element child) {
    if (child is StatefulElement && child.state is T) {
      onFound(child.state as T);
    } else
    if (child.renderObject is T) {
      onFound(child.renderObject as T);
    }
    child.visitChildren(_visitor);
  }
}