example autovalidate

main.dart

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

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

/// This Widget is the main application widget.
class MyApp extends StatelessWidget {
  static const String _title = 'Flutter Code Sample';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: _title,
      home: MyStatefulWidget(),
    );
  }
}

class MyStatefulWidget extends StatefulWidget {
  MyStatefulWidget({Key key}) : super(key: key);

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

class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  TextEditingController _controller;

  void initState() {
    super.initState();
    _controller = TextEditingController();
  }

  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: TemperatureField(),
      ),
    );
  }
}

class TemperatureField extends StatelessWidget {
  @override
  Widget build(BuildContext context) => TextFormField(
        autovalidate: true,
        keyboardType: TextInputType.numberWithOptions(decimal: true),
        decoration: InputDecoration(
          border: OutlineInputBorder(),
          labelText: "Temperature",
        ),
        inputFormatters: <TextInputFormatter>[
          LengthLimitingTextInputFormatter(4),
          WhitelistingTextInputFormatter(RegExp(r"[\d.]")),
        ],
        validator: (value) {
          Pattern pattern = r'^\d{2}([.]\d)?$';
          RegExp regex = new RegExp(pattern);

          if (value.isEmpty) return "Can't be empty";
          if (!regex.hasMatch(value)) {
            return "Invalid number";
          }
          if (double.parse(value) > 50) {
            return "Temp too hight";
          }
          if (double.parse(value) < 20) {
            return "Temp too low";
          }

          return null;
        },
      );
}