homepage.dart

import 'package:flutter/material.dart';
import 'package:test_proj/nextpage.dart';

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  TextEditingController controller;

  @override
  void initState() {
    super.initState();
    controller = TextEditingController();
  }

  Widget _buildBody() {
    return Center(
      child: Column(
        mainAxisSize: MainAxisSize.min,
        children: [
          Container(
            width: 200,
            child: TextField(
              controller: controller,
            ),
          ),
          SizedBox(height: 15),
          RaisedButton(
            onPressed: () async {
              final res = await Navigator.of(context).push(
                MaterialPageRoute(
                  builder: (context) {
                    return NextPage(controller: controller);
                  },
                ),
              );
              setState(() {
                controller = res;
              });
            },
            child: Text('Next page'),
          )
        ],
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: _buildBody(),
    );
  }
}

main.dart

import 'package:flutter/material.dart';
import 'package:test_proj/homepage.dart';

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

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

nextpage.dart

import 'package:flutter/material.dart';

class NextPage extends StatefulWidget {
  final TextEditingController controller;

  NextPage({@required this.controller});

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

class _NextPageState extends State<NextPage> {
  Widget _buildBody() {
    return Center(
      child: RaisedButton(
        onPressed: () {
          widget.controller.text = 'This is the new text';
          Navigator.of(context).pop(widget.controller);
        },
        child: Text('Pop'),
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: _buildBody(),
    );
  }
}