main.dart

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

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

class MyApp extends StatefulWidget {
  
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  GlobalKey<FormState> _key = new GlobalKey();
  bool _validate = false;
  String name, email, mobile;
  final logger = Logger(
      printer: PrettyPrinter(
    methodCount: 0,
    errorMethodCount: 5,
    lineLength: 50,
    colors: true,
    printEmojis: true,
    printTime: false,
  ));
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: new Scaffold(
        appBar: new AppBar(
          title: new Text('Form Validation'),
        ),
        body: new SingleChildScrollView(
          child: new Container(
            margin: new EdgeInsets.all(15.0),
            child: new Form(
              key: _key,
              autovalidate: _validate,
              child: FormUI(),
            ),
          ),
        ),
      ),
    );
  }

  Widget FormUI() {
    return Column(
      children: <Widget>[
        new TextFormField(
          decoration: new InputDecoration(hintText: 'Full Name'),
          maxLength: 32,
          validator: validateName,
          onSaved: (String val) {
            name = val;
          },
        ),
        new TextFormField(
            decoration: new InputDecoration(hintText: 'Mobile Number'),
            keyboardType: TextInputType.phone,
            maxLength: 10,
            validator: validateMobile,
            onSaved: (String val) {
              mobile = val;
            }),
        new TextFormField(
            decoration: new InputDecoration(hintText: 'Email ID'),
            keyboardType: TextInputType.emailAddress,
            maxLength: 32,
            validator: validateEmail,
            onSaved: (String val) {
              email = val;
            }),
        new SizedBox(height: 15.0),
        new TextFormField(
            decoration: new InputDecoration(hintText: 'Email ID'),
            keyboardType: TextInputType.emailAddress,
            maxLength: 32,
            validator: validateEmail,
            onSaved: (String val) {
              email = val;
            }),
        RaisedButton(
          onPressed: _sendToServer,
          child: new Text('Send'),
        )
      ],
    );
  }

  String validateName(String value) {
    String patttern = r'(^[a-zA-Z ]*$)';
    RegExp regExp = new RegExp(patttern);
    if (value.length == 0) {
      return "Name is Required";
    } else if (!regExp.hasMatch(value)) {
      return "Name must be a-z and A-Z";
    }
    return null;
  }

  String validateMobile(String value) {
    String patttern = r'(^[0-9]*$)';
    RegExp regExp = new RegExp(patttern);
    if (value.length == 0) {
      return "Mobile is Required";
    } else if (value.length != 10) {
      return "Mobile number must 10 digits";
    } else if (!regExp.hasMatch(value)) {
      return "Mobile Number must be digits";
    }
    return null;
  }

  String validateEmail(String value) {
    String pattern =
        r'^(([^<>()[\]\\.,;:\s@\"]+(\.[^<>()[\]\\.,;:\s@\"]+)*)|(\".+\"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$';
    RegExp regExp = new RegExp(pattern);
    if (value.length == 0) {
      return "Email is Required";
    } else if (!regExp.hasMatch(value)) {
      return "Invalid Email";
    } else {
      return null;
    }
  }

  _sendToServer() {
    if (_key.currentState.validate()) {
      // No any error in validation
      _key.currentState.save();
      logger.i("Name $name");
      print("Mobile $mobile");
      print("Email $email");
    } else {
      // validation error
      setState(() {
        _validate = true;
      });
    }
  }
}