ChangeNotifier state synchronization

main.dart

import 'package:flutter/material.dart';

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

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(colorSchemeSeed: Colors.blue),
      home: const OverlayExample(title: 'Flutter Demo Home Page'),
    );
  }
}

class OverlayExample extends StatefulWidget {
  final String title;

  const OverlayExample({super.key, required this.title});

  @override
  State<OverlayExample> createState() => _OverlayExampleState();
}

class _OverlayExampleState extends State<OverlayExample>
    with SingleTickerProviderStateMixin {
  late final Controller controller = Controller();
  final GlobalKey key = GlobalKey();
  
  @override
  void initState() {
    super.initState();
    controller.addListener((){
      // Throws
      if (controller.isExpanded) {
        key.currentContext!.findRenderObject();
      }
    });
  }

  void open() {
    controller.expand();
  }

  void close() {
    controller.collapse();
  }

  @override
  void dispose() {
    controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text(widget.title)),
      body: SizedBox(
        width: 300,
        height: 400,
        child: Stack(
          children: [
            TextButton(
              onPressed: () {
                if (controller.isExpanded) {
                  close();
                } else {
                  open();
                }
              },
              child: Text("Toggle overlay"),
            ),
            AnimatedBuilder(
              animation: controller,
              builder: (context, child) {
                return Positioned(
                  top: 50,
                  left: 0,
                  child:
                      controller.isExpanded
                          ? Container(
                            key: key,
                            color: Colors.blue.shade100,
                            child: const Center(
                              child: FlutterLogo(size: 200.0),
                            ),
                          )
                          : SizedBox(),
                );
              },
            ),
          ],
        ),
      ),
    );
  }
}

class Controller extends ChangeNotifier {
  Controller();

  bool _isExpanded = false;

  void _setExpansionState(bool newValue) {
    if (newValue != _isExpanded) {
      _isExpanded = newValue;
      notifyListeners();
    }
  }

  bool get isExpanded => _isExpanded;

  void expand() {
    _setExpansionState(true);
  }

  void collapse() {
    _setExpansionState(false);
  }
}