FLUTTER:- I need to return a widget from a method after Navigator.push() method

I have made a login app using Flutter and BLOC architecture. If login becomes success I need to return a widget to the scaffold and also want to navigate to the next screen using Navigator.push().

I have tried following ways:-

1. Method 1:-

Navigator.push(context,MaterialPageRoute(builder: (context) => DashboardPage()));
return ButtonWidget(); 

But it causes an error that you can not set a widget when shifted to the next screen.

2. Method 2:-

Future.delayed(
        Duration(days: 1200),
        () => {
              Navigator.push(context,
                  MaterialPageRoute(builder: (context) => DashboardPage())),
            }));
return ButtonWidget(); 

But it causes the code inside the future to run after every 1.2 seconds.

3.Method 3:-

var cancellableCompleter = CancelableCompleter(onCancel:(){print("onCancel");});
    cancellableCompleter.complete(
      //Future.value("future result"));
      Future.delayed(
        Duration(days: 1200),
        () => {
              print("i am here"),
              Navigator.push(context,
                  MaterialPageRoute(builder: (context) => DashboardPage())),
            }));
    cancellableCompleter.operation.value.then((value)=>print(value));
    cancellableCompleter.operation.value.whenComplete(()=>{print("Completed")});
    return ButtonWidget();

But it does not instantiate the Navigator.push() method. Even the "I am here" does not get printed.

I have also tried CancelableOperation but it gives the same error as Method 2.

Full Code

import 'dart:async';

import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:fluttertoast/fluttertoast.dart';
import 'package:sales_app/bloc/bloc.dart';
import 'package:shared_preferences/shared_preferences.dart';
import 'package:async/async.dart';

import 'dashboard_page.dart';

class LoginPage extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _LoginPageState();
  }
}

class _LoginPageState extends State<LoginPage> {
  String _email;
  String _password;
  LoginBloc bloc = LoginBloc();
  final blue_color = Color(0xff4490E8);
  final formKey = GlobalKey<FormState>();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: BlocProvider(
        builder: (context) => bloc,
        child: Container(
          decoration: BoxDecoration(color: blue_color),
          child: Padding(
            padding: const EdgeInsets.all(20.0),
            child: Form(
              key: formKey,
              child: Column(
                mainAxisAlignment: MainAxisAlignment.center,
                crossAxisAlignment: CrossAxisAlignment.center,
                children: <Widget>[
                  Text(
                    "Log In",
                    style: TextStyle(color: Colors.white, fontSize: 30),
                  ),
                  SizedBox(
                    height: 50,
                  ),
                  TextFormField(
                      autovalidate: true,
                      style: TextStyle(color: blue_color),
                      onSaved: (email) => {this._email = email},
                      keyboardType: TextInputType.emailAddress,
                      initialValue: "saurav.vidyarthi@connexrm.com",
                      validator: (val) =>
                          RegExp(r"^[a-zA-Z0-9.]+@[a-zA-Z0-9]+\.[a-zA-Z]+")
                                  .hasMatch(val)
                              ? null
                              : "Invalid Email",
                      decoration: InputDecoration(
                        filled: true,
                        helperStyle: TextStyle(color: blue_color),
                        labelStyle: TextStyle(color: blue_color),
                        labelText: "Email Id",
                        hintText: "e.g. joe@gmail.com",
                        hintStyle: TextStyle(color: blue_color),
                        prefixIcon: Icon(Icons.email, color: blue_color),
                        fillColor: Colors.white,
                        /*border: OutlineInputBorder(
                            borderSide:
                            BorderSide(color: const Color(0xffffffff)))),*/
                      )),
                  SizedBox(height: 15.0),
                  TextFormField(
                      style: TextStyle(color: blue_color),
                      obscureText: true,
                      autovalidate: true,
                      initialValue: "Saurav@1",
                      onSaved: (password) => {_password = password},
                      validator: (val) => val.length < 6
                          ? "Password must be of at least 6 char."
                          : null,
                      decoration: InputDecoration(
                          filled: true,
                          labelStyle: TextStyle(color: blue_color),
                          hintStyle: TextStyle(color: blue_color),
                          prefixIcon: Icon(Icons.lock, color: blue_color),
                          fillColor: Colors.white,
                          /* border: OutlineInputBorder(
                              borderSide:
                              BorderSide(color: const Color(0xffffffff))),*/
                          labelText: "Password",
                          hintText: "*******")),
                  SizedBox(height: 25.0),
                  BlocBuilder(
                    bloc: bloc,
                    builder:
                        (BuildContext buildContext, LoginState loginState) {
                      print(loginState.toString());
                      if (loginState is InitialLoginState)
                        return _buildInitialState();
                      else if (loginState is LoggingInState)
                        return _buildLoadingState();
                      else if (loginState is LoginSuccessState) {
                        LoginSuccessState state = loginState;
                        return _buildSuccessState(state.user.StatusText);
                      } else {
                        LoginFailedState state = loginState;
                        return _buildFailureState(state.user.StatusText);
                      }
                    },
                  )
                ],
              ),
            ),
          ),
        ),
      ),
    );
  }

  void _onButtonPressed() {
    if (formKey.currentState.validate()) {
      formKey.currentState.save();
      bloc.dispatch(GetUser(_email, _password));
    } else {
      Fluttertoast.showToast(msg: "Please enter a valid email and password");
    }
  }

  Widget _buildInitialState() {
    return RaisedButton(
        onPressed: _onButtonPressed,
        color: Colors.white,
        child: Text(
          "Log In",
          style: TextStyle(color: blue_color),
        ));
  }

  Widget _buildLoadingState() {
    return CircularProgressIndicator(backgroundColor: Colors.white);
  }

  Widget _buildFailureState(message) {
    Fluttertoast.showToast(msg: message);
    return RaisedButton(
        onPressed: _onButtonPressed,
        color: Colors.white,
        child: Text(
          "Log In",
          style: TextStyle(color: blue_color),
        ));
  }

  Widget _buildSuccessState(message) {
    Fluttertoast.showToast(msg: message);
    _setAuthState();
    var cancellableCompleter = CancelableCompleter(onCancel:(){print("onCancel");});
    cancellableCompleter.complete(
      //Future.value("future result"));
      Future.delayed(
        Duration(days: 1200),
        () => {
              print("i am here"),
              Navigator.push(context,
                  MaterialPageRoute(builder: (context) => DashboardPage())),
            }));
    cancellableCompleter.operation.value.then((value)=>print(value));
    cancellableCompleter.operation.value.whenComplete(()=>{print("Completed")});
    return RaisedButton(
      onPressed: _onButtonPressed,
      color: Colors.white,
      child: Text(
        "Log In",
        style: TextStyle(color: blue_color),
      ),
    );
  }

  _setAuthState() async {
    SharedPreferences sharedPreferences = await SharedPreferences.getInstance();
    await sharedPreferences.setBool("isLoggedIn", true);
  }

  @override
  void dispose() {
    super.dispose();
    bloc.dispose();
  }
}

Use BlocListener

return BlocListener<LoginBloc, LoginState>(
  listener: (context, state) {
    if (state is loginSuccess) {
      Navigator.push(context,MaterialPageRoute(builder: (context) => DashboardPage()));
    }
  },
  child: ButtonWidget(),
)