flutter_validation_bug.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: TextExample(),
    );
  }
}

class FormData {
  final String name;
  final double lat;

  FormData({ this.name, this.lat });

  FormData updateName(String newName) {
    return FormData(
      name: newName,
      lat: lat
    );
  }

  FormData updateLat(double newLat) {
    return FormData(
      name: name,
      lat: newLat
    );
  }
}

class TextExample extends StatefulWidget {
  @override
  _TextExampleState createState() => _TextExampleState();
}

class _TextExampleState extends State<TextExample> {
  FormData formData = FormData(
    name: ''
  );

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text('Sandbox'),
        ),
        body: Column(
          children: [
            TextFormField(
              autovalidateMode: AutovalidateMode.onUserInteraction,
              validator: (String value) {
                print('validate name $value');

                return null;
              },
              onChanged: (String value) {
                formData = formData.updateName(value);
              },
            ),
            TextFormField(
              autovalidateMode: AutovalidateMode.onUserInteraction,
              validator: (String value) {
                print('validate lat $value');

                return null;
              },
              onChanged: (String value) {
                // formData = formData.updateLat(double.parse(value));
              },
            ),
          ],
        )
    );
  }
}