FocusNode listener triggered on rebuild

main.dart

import 'package:flutter/material.dart';

const Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(
        scaffoldBackgroundColor: darkBlue,
      ),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: Container(child: MyWidget(), width: 300, height: 200)
        ),
      ),
    );
  }
}

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

class MyWidgetState extends State<MyWidget> {
  late final FocusNode markFocusNode;
  bool isLocked = false;

  @override
  void initState() {
    markFocusNode = FocusNode(debugLabel: "MarkFocusNode")
      ..addListener(_onChangeFocus);
    super.initState();
  }

  @override
  void dispose() {
    markFocusNode.dispose();
    super.dispose();
  }
  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        TextField(
          autofocus: false,
          enabled: !isLocked,
          focusNode: markFocusNode,
          onChanged: (val) => print('onChanged'),
          textInputAction: TextInputAction.next,
        ),
        ElevatedButton(child: Text('Toggle Lock'), onPressed: () => setState(() => isLocked = !isLocked))
      ]
    );
  }
  
    void _onChangeFocus() {
    if (!markFocusNode.hasFocus) {
      print('Lost focus');
    }
  }
}