doctor_app_flutter/lib/screens/auth/login_screen.dart

import 'dart:async';

import 'package:doctor_app_flutter/widgets/auth/known_user_login.dart';
import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';

import '../../widgets/auth/auth_header.dart';
import '../../widgets/auth/login_form.dart';

class Loginsreen extends StatefulWidget {
  @override
  _LoginsreenState createState() => _LoginsreenState();
}

class _LoginsreenState extends State<Loginsreen> {
  Future<SharedPreferences> _prefs = SharedPreferences.getInstance();

  String platformImei;

  Future<String> platformImeiText;

  Future<void> getSharedPref() async {
    final SharedPreferences prefs = await _prefs;
    platformImei = prefs.getString('platformImei');
  }

  @override
  void initState() {
    super.initState();
    platformImeiText = _prefs.then((SharedPreferences prefs) {
      return (prefs.getString('platformImei'));
    });
  }

  @override
  Widget build(BuildContext context) {
    getSharedPref();
    return Scaffold(
      backgroundColor: Colors.white,
      body: SingleChildScrollView(
        child: FutureBuilder(
            future: platformImeiText,
            builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
              switch (snapshot.connectionState) {
                case ConnectionState.waiting:
                  return const CircularProgressIndicator();
                default:
                  if (snapshot.hasError) {
                    return Text('Error: ${snapshot.error}');
                  } else {
                    return Container(
                        margin: EdgeInsetsDirectional.fromSTEB(30, 0, 0, 0),
                        alignment: Alignment.topLeft,
                        child: Column(
                            crossAxisAlignment: CrossAxisAlignment.start,
                            children: <Widget>[
                              AuthHeader(),
                              (platformImei == null)
                                  ? LoginForm()
                                  : KnownUserLogin(),
                            ]));
                  }
              }
            }),
      ),
    );
  }
}