verbosity_texteditingcontroller.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyExample(),
    );
  }
}

class MyExample extends StatefulWidget {
  MyExample({Key key, this.controller}) : super(key: key);

  final TextEditingController controller;

  @override
  _MyExampleState createState() => _MyExampleState();
}

class _MyExampleState extends State<MyExample> with SingleTickerProviderStateMixin {
  @override
  void initState() {
    super.initState();
    // Add initial listener.
    widget.controller.addListener(_textChanged);
  }

  @override
  void didUpdateWidget(MyExample oldWidget) {
    super.didUpdateWidget(oldWidget);
    // Move listener.
    if (widget.controller != oldWidget.controller) {
      oldWidget.controller.removeListener(_textChanged);
      widget.controller.addListener(_textChanged);
    }
  }

  @override
  void dispose() {
    super.dispose();
    // Remove listener.
    widget.controller.removeListener(_textChanged);
  }

  void _textChanged() {
    setState(() {
      // The controller state changed.
    });
  }

  @override
  Widget build(BuildContext context) {
    // Doing something useful with it.
    return Text(widget.controller.text);
  }
}