import 'package:flutter/material.dart';

void main() {
  runApp(MaterialApp(home: HomePage()));
}

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

class _HomePageState extends State<HomePage> {
  TextEditingController _messageInputController;

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Demo"),
      ),
      body: SafeArea(
        child: Column(
          children: <Widget>[
            RaisedButton(
                child: Text("add text"),
                onPressed: () {
                  String oldText = _messageInputController.text;
                  String newText = oldText + "insertText";

                  var newValue = _messageInputController.value.copyWith(
                    text: newText,
                    selection: TextSelection.collapsed(
                      offset: newText.length,
                    ),
                    composing: TextRange.empty,
                  );

                  _messageInputController.value = newValue;
                }),
            TextField(
              controller: _messageInputController,
            ),
          ],
        ),
      ),
    );
  }
}