Form Validation in flutter

main.dart

import 'package:flutter/material.dart';
import 'package:medium/page1.dart';

void main() => runApp(MyApp());
var _formKey = GlobalKey<FormState>();

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

class MyPage extends StatefulWidget {
  @override
  _MyPageState createState() => _MyPageState();
}

class _MyPageState extends State<MyPage> {
  @override
  Widget build(BuildContext context) {
    return Form(
      key: _formKey,
      child: Scaffold(
          body: Padding(
              padding: EdgeInsets.symmetric(horizontal: 20),
              child: SingleChildScrollView(
                child: Column(
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: <Widget>[
                      RichText(
                        text: TextSpan(
                            text: "Hello!",
                            style: TextStyle(
                                color: Color(0xFF2508FF),
                                fontWeight: FontWeight.bold,
                                fontSize: 35),
                            children: <TextSpan>[
                              TextSpan(
                                  text: "Login",
                                  style: TextStyle(
                                    color: Color(0xFF14C115),
                                  ))
                            ]),
                      ),
                      SizedBox(height: 25),
                      TextFormField(
                        decoration: InputDecoration(
                          labelText: "Name",
                          hintStyle:
                              TextStyle(color: Colors.grey, fontSize: 16.0),
                        ),
                        validator: (String name) {
                          if (name.isEmpty) {
                            return "Please enter your name";
                          } else if (name.length > 30 || name.length < 2) {
                            return "Please enter your name correctly";
                          }
                        },
                      ),
                      SizedBox(height: 25),
                      TextFormField(
                        decoration: InputDecoration(
                          labelText: "Email",
                          hintStyle:
                              TextStyle(color: Colors.grey, fontSize: 16.0),
                        ),
                        validator: (String email) {
                          if (email.isEmpty) {
                            return "Please enter your email";
                          } else if (!email.contains("@") ||
                              !email.endsWith(".com")) {
                            return "Please enter your email correctly";
                          }
                        },
                      ),
                      SizedBox(height: 25),
                      buildButtonContainer(),
                      SizedBox(height: 25),
                      Row(
                        mainAxisAlignment: MainAxisAlignment.center,
                        children: <Widget>[
                          Text("Don't have an account?",
                              style: TextStyle(color: Color(0xFF2508FF))),
                          FlatButton(
                            child: Text("Signup",
                                style: TextStyle(color: Color(0xFF14C115))),
                            onPressed: () {},
                          ),
                        ],
                      )
                    ]),
              ))),
    );
  }

  Widget buildButtonContainer() {
    return Container(
      height: 56.0,
      width: MediaQuery.of(context).size.width,
      decoration: BoxDecoration(
        borderRadius: BorderRadius.circular(23.0),
        gradient: LinearGradient(colors: [
          Color(0xFF2508FF),
          Color(0xFF2585A9),
          Color(0xFF14C115),
          Color(0xFF14C115),
        ], begin: Alignment.centerRight, end: Alignment.centerLeft),
      ),
      child: Center(
        child: Container(
          height: 100.0,
          width: 500,
          child: FlatButton(
            shape: new RoundedRectangleBorder(
                borderRadius: new BorderRadius.circular(23.0)),
            child: Text(
              "Sign in",
              style: TextStyle(color: Colors.white, fontSize: 18.0),
            ),
            onPressed: () {
              if (_formKey.currentState.validate()) {
                Navigator.push(
                    context, MaterialPageRoute(builder: (context) => Page1()));
              }
            },
          ),
        ),
      ),
    );
  }
}

Page1.dart

import 'package:flutter/material.dart';

class Page1 extends StatefulWidget {
  @override
  _Page1State createState() => _Page1State();
}

class _Page1State extends State<Page1> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Container(
        child: Text("Welcome"),
      ),
    );
  }
}