Reparenting FocusNode example.

main.dart

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

// Modified from the FocusNode example in the docs:
// https://main-api.flutter.dev/flutter/widgets/FocusNode-class.html

void main() => runApp(const FocusNodeExampleApp());

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: const Text('FocusNode Sample')),
        body: const FocusNodeExample(),
      ),
    );
  }
}

class ColorfulButton extends StatefulWidget {
  const ColorfulButton({super.key});

  @override
  State<ColorfulButton> createState() {
    print('justin createState.');
    return _ColorfulButtonState();
  }
}

class _ColorfulButtonState extends State<ColorfulButton> {
  late FocusNode _node;
  bool _focused = false;
  FocusAttachment? _nodeAttachment;
  Color _color = Colors.white;

  @override
  void initState() {
    print('justin ColorfulButtonState initState.');
    super.initState();
    _node = FocusNode(debugLabel: 'Button');
    _node.addListener(_handleFocusChange);
    if (_nodeAttachment != null) {
      _nodeAttachment!.detach();
    }
    _nodeAttachment = _node.attach(context, onKeyEvent: _handleKeyPress);
  }

  @override
  void didUpdateWidget(ColorfulButton oldWidget) {
    super.didUpdateWidget(oldWidget);
  }

  void _handleFocusChange() {
    if (_node.hasFocus != _focused) {
      setState(() {
        _focused = _node.hasFocus;
      });
    }
  }

  KeyEventResult _handleKeyPress(FocusNode node, KeyEvent event) {
    if (event is KeyDownEvent) {
      debugPrint(
          'Focus node ${node.debugLabel} got key event: ${event.logicalKey}');
      switch (event.logicalKey) {
        case LogicalKeyboardKey.keyR:
          debugPrint('Changing color to red.');
          setState(() {
            _color = Colors.red;
          });
          return KeyEventResult.handled;
        case LogicalKeyboardKey.keyG:
          debugPrint('Changing color to green.');
          setState(() {
            _color = Colors.green;
          });
          return KeyEventResult.handled;
        case LogicalKeyboardKey.keyB:
          debugPrint('Changing color to blue.');
          setState(() {
            _color = Colors.blue;
          });
          return KeyEventResult.handled;
      }
    }
    return KeyEventResult.ignored;
  }

  @override
  void dispose() {
    print('justin ColorfulButtonState initState.');
    _node.removeListener(_handleFocusChange);
    // The attachment will automatically be detached in dispose().
    _node.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    print('justin buildColorsfulbutton');
    _nodeAttachment!.reparent();
    return GestureDetector(
      onTap: () {
        if (_focused) {
          _node.unfocus();
        } else {
          _node.requestFocus();
        }
      },
      child: Center(
        child: Container(
          width: 400,
          height: 100,
          color: _focused ? _color : Colors.white,
          alignment: Alignment.center,
          child:
              Text(_focused ? "I'm in color! Press R,G,B!" : 'Press to focus'),
        ),
      ),
    );
  }
}

class FocusNodeExample extends StatefulWidget {
  const FocusNodeExample({super.key});

  @override
  State<FocusNodeExample> createState() => _FocusNodeExampleState();
}

class _FocusNodeExampleState extends State<FocusNodeExample> {
  final GlobalKey _buttonKey = GlobalKey();
  late final ColorfulButton _button;

  bool _inAlternativeParent = false;

  @override
  void initState() {
    super.initState();
    _button = ColorfulButton(
        key:
            _buttonKey); // Without this key, _button's state will be recreated.
    print('justin _FocusNodeExample initstate.');
  }

  @override
  void dispose() {
    print('justin _FocusNodeExample dispose.');
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    final TextTheme textTheme = Theme.of(context).textTheme;
    return Column(
      children: <Widget>[
        ColoredBox(
          color: Colors.cyan.withValues(alpha: 0.2),
          child: DefaultTextStyle(
            style: textTheme.headlineMedium!,
            child: _inAlternativeParent ? const Text('empty') : _button,
          ),
        ),
        ColoredBox(
          color: Colors.yellow.withValues(alpha: 0.2),
          child: DefaultTextStyle(
            style: textTheme.headlineMedium!,
            child: _inAlternativeParent ? _button : const Text('empty'),
          ),
        ),
        TextButton(
          onPressed: () {
            setState(() {
              _inAlternativeParent = !_inAlternativeParent;
            });
          },
          child: const Text('reparent'),
        ),
      ],
    );
  }
}