login.dart

import 'package:flutter/material.dart';
import 'package:flutter_task/mainmenu.dart';

class Login extends StatefulWidget {
  @override
  _LoginPageState createState() => _LoginPageState();
}

class _LoginPageState extends State<Login> {
  String username, password;
  bool visible = false;
  _cekLogin() async {
    setState(() {
      visible = true;
    });
    try {
      if (username == "anandagalang" && password == "admin123") {
        Navigator.pushReplacement(context,
            MaterialPageRoute(builder: (context) {
              return MainMenu();
            }));
        AlertDialog alert = AlertDialog(
          title: Text("Sukses"),
          content: Text("Welcome to IT Catalog"),
        );
        showDialog(
          context: context,
          builder: (BuildContext context) {
            return alert;
          },
        );
      } else if(username == "" || password == "") {
        _popupAlert(context, "Oopss", "Silahkan Isi Username dan Password");
      } else {
        _popupAlert(context, "Gagal Login", "Username atau Password Salah");
      }
    } catch (e) {}
  }

  _popupAlert(BuildContext context, String ttl, String err) {
    Widget okButton = FlatButton(
      child: Text("OK"),
      onPressed: () => Navigator.pop(context),
    );
    AlertDialog alert = AlertDialog(
      title: Text(ttl),
      content: Text(err),
      actions: [
        okButton,
      ],
    );
    showDialog(
      context: context,
      builder: (BuildContext context) {
        return alert;
      },
    );
  }
  Widget _buildLogo(){
    return Row(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        Padding(
          padding: const EdgeInsets.symmetric(vertical: 70),
          child: Text(
            'Welcome',
            style: TextStyle(
              fontSize: MediaQuery.of(context).size.height / 25,
              fontWeight: FontWeight.bold,
              color: Colors.white,
            ),
          ),
        ),
      ],
    );
  }

  Widget _buildUsernameRow(){
    return Padding(
      padding: EdgeInsets.all(8),
      child: TextFormField(
        keyboardType: TextInputType.text,
        onChanged: (value){
          setState(() {
            username = value;
          });
        },
        decoration: InputDecoration(
            prefixIcon: Icon(
              Icons.person,
              color: Color(0xFFB8E994),
            ),
            labelText: 'Username'),
      ),
    );
  }

  Widget _buildPasswordRow(){
    return Padding(
      padding: EdgeInsets.all(8),
      child: TextFormField(
        keyboardType: TextInputType.text,
        obscureText: true,
        onChanged: (value){
          setState(() {
            password = value;
          });
        },
        decoration: InputDecoration(
            prefixIcon: Icon(
              Icons.lock,
              color: Color(0xFFB8E994),
            ),
            labelText: 'Password'),
      ),
    );
  }

  Widget _buildLoginButton(){
    return Row(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        Container(
          height: 1.4*(MediaQuery.of(context).size.height/20),
          width: 5*(MediaQuery.of(context).size.width/10),
          margin: EdgeInsets.only(bottom:20),
          child: RaisedButton(
            elevation: 5.0,
            shape: RoundedRectangleBorder(
              borderRadius: BorderRadius.circular(30.0),
            ),
            onPressed: _cekLogin,
            child: Text(
              "Login",
              style: TextStyle(
                color: Colors.white,
                letterSpacing: 1.5,
                fontSize: MediaQuery.of(context).size.height / 40,
              ),
            ),
          ),
        )
      ],
    );
  }

  Widget _buildContainer(){
    return Row(
      mainAxisAlignment: MainAxisAlignment.center,

      children: <Widget>[
        ClipRRect(
          borderRadius: BorderRadius.all(
              Radius.circular(20)
          ),
          child: Container(
            height: MediaQuery.of(context).size.height*0.6,
            width: MediaQuery.of(context).size.width*0.8,
            decoration: BoxDecoration(
              color: Colors.white,
            ),
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              crossAxisAlignment: CrossAxisAlignment.center,
              children: <Widget>[
                Row(
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: <Widget>[
                    Text(
                      "Login",
                      style: TextStyle(
                        fontSize: MediaQuery.of(context).size.height / 30,
                      ),
                    ),
                  ],
                ),
                _buildUsernameRow(),
                _buildPasswordRow(),
                _buildLoginButton(),
              ],
            ),
          ),
        ),
      ],
    );
  }

  @override
  Widget build(BuildContext context) {
    return SafeArea(
        child: Scaffold(
          resizeToAvoidBottomPadding: false,
          backgroundColor: Color(0xfff2f3f7),
          body: Stack(
            children: <Widget>[
              Container(
                height: MediaQuery.of(context).size.height*0.7,
                width: MediaQuery.of(context).size.width,
                child: Container(
                  decoration: BoxDecoration(
                      color: Color(0xFFB8E994) ,
                      borderRadius: BorderRadius.only(
                        bottomLeft: const Radius.circular(70),
                        bottomRight: const Radius.circular(70),
                      )
                  ),
                ),
              ),
              Column(
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget> [
                  _buildLogo(),
                  _buildContainer(),
                ],
              )
            ],
          ),
        ));
  }
}

main.dart

import 'package:flutter/material.dart';

import 'package:flutter/cupertino.dart';
import 'package:flutter_task/login.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home:Login(),
    );
  }
}