main.dart

    import 'package:flutter/material.dart';

    class MyHomePage extends StatefulWidget {
      MyHomePageState createState() => new MyHomePageState();
    }

    class MyHomePageState extends State<MyHomePage> {
      TextEditingController _controller = new TextEditingController();
      String _title = "";

      @override
      void initState() {
        _controller.addListener(() {
          setState(() {
            _title = _controller.value.text;
            print("Title value: $_title ${_title.length}");
          });
        });
      }

      @override
      Widget build(BuildContext context) {
        return new Scaffold(
          appBar: new AppBar(
            title: new Text(_title),
          ),
          body: new Container(
            alignment: FractionalOffset.center,
            padding: new EdgeInsets.all(20.0),
            child: new TextFormField(
              controller: _controller,
            ),
          ),
        );
      }
    }

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

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