setState is not working inside the floating action button and not able to render the ui in flutter?

I am trying to use setState class inside the floating action button but, setState class is not able to rerender the ui.Here is my code: Here i use isLogin bool type variable so it able to toggle the two different ui.In fact the is not abe to working when ever the setState function call it must change the value of that variable is rerender the ui but it is not working .

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

class AuthForm extends StatefulWidget {
  @override
  _AuthFormState createState() => _AuthFormState();
}

class _AuthFormState extends State<AuthForm> {
  @override
  Widget build(BuildContext context) {
    final _formKey = GlobalKey<FormState>();
    var _isLogin = true;
    var _userEmail = '';
    var _userName = '';
    var _userPassword = '';

    void _trySubmit() {
      final isValid = _formKey.currentState.validate();
      FocusScope.of(context).unfocus();
      if (isValid) {
        _formKey.currentState.save();
        print(_userEmail);
        print(_userPassword);
        print(_userName);
      }
    }

    return Center(
      child: Card(
        margin: EdgeInsets.all(20),
        child: SingleChildScrollView(
          child: Padding(
            padding: EdgeInsets.all(16),
            child: Form(
              key: _formKey,
              child: Column(
                mainAxisSize: MainAxisSize.min,
                children: <Widget>[
                  TextFormField(
                    validator: (value) {
                      if (value.isEmpty || !value.contains('@')) {
                        return 'Please enter the valid email address.';
                      }
                      return null;
                    },
                    keyboardType: TextInputType.emailAddress,
                    decoration: InputDecoration(
                      labelText: 'Email Address',
                    ),
                    onSaved: (newValue) {
                      _userEmail = newValue;
                    },
                  ),
                  TextFormField(
                    validator: (value) {
                      if (value.isEmpty || value.length < 4) {
                        return 'Please enter at least 4 character.';
                      }
                      return null;
                    },
                    decoration: InputDecoration(
                      labelText: 'Username',
                    ),
                    onSaved: (newValue) {
                      _userName = newValue;
                    },
                  ),
                  TextFormField(
                    validator: (value) {
                      if (value.isEmpty || value.length < 7) {
                        return 'Password must be at least 7 character long.';
                      }
                      return null;
                    },
                    decoration: InputDecoration(
                      labelText: 'Password',
                    ),
                    obscureText: true,
                    onSaved: (newValue) {
                      _userPassword = newValue;
                    },
                  ),
                  SizedBox(
                    height: 20,
                  ),
                  RaisedButton(
                    onPressed: () {
                      _trySubmit();
                    },
                    child: Text(
                      _isLogin ? 'Login' : 'Signup',
                    ),
                  ),
                  FlatButton(
                    textColor: Theme.of(context).primaryColor,
                    child: Text(
                      _isLogin
                          ? 'Create New Account.'
                          : 'I Already Have Account.',
                    ),
                    onPressed: () {
                      setState(() {
                        _isLogin = !_isLogin;
                      });
                    },
                  ),
                ],
              ),
            ),
          ),
        ),
      ),
    );
  }
}

Solution 1: Sagar Acharya

The problem is with the var _isLogin = true; you have defined it inside the build method that's why whenever the setState is called it becomes true and does not change the ui.

Try placing it outside the build method. Example below:

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return AuthForm();
  }
}

class AuthForm extends StatefulWidget {
  @override
  _AuthFormState createState() => _AuthFormState();
}

class _AuthFormState extends State<AuthForm> {
   var _isLogin = true;
  @override
  Widget build(BuildContext context) {
    final _formKey = GlobalKey<FormState>();

    var _userEmail = '';
    var _userName = '';
    var _userPassword = '';

    void _trySubmit() {
      final isValid = _formKey.currentState.validate();
      FocusScope.of(context).unfocus();
      if (isValid) {
        _formKey.currentState.save();
        print(_userEmail);
        print(_userPassword);
        print(_userName);
      }
    }

    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: Card(
            margin: EdgeInsets.all(20),
            child: SingleChildScrollView(
              child: Padding(
                padding: EdgeInsets.all(16),
                child: Form(
                  key: _formKey,
                  child: Column(
                    mainAxisSize: MainAxisSize.min,
                    children: <Widget>[
                      TextFormField(
                        validator: (value) {
                          if (value.isEmpty || !value.contains('@')) {
                            return 'Please enter the valid email address.';
                          }
                          return null;
                        },
                        keyboardType: TextInputType.emailAddress,
                        decoration: InputDecoration(
                          labelText: 'Email Address',
                        ),
                        onSaved: (newValue) {
                          _userEmail = newValue;
                        },
                      ),
                      TextFormField(
                        validator: (value) {
                          if (value.isEmpty || value.length < 4) {
                            return 'Please enter at least 4 character.';
                          }
                          return null;
                        },
                        decoration: InputDecoration(
                          labelText: 'Username',
                        ),
                        onSaved: (newValue) {
                          _userName = newValue;
                        },
                      ),
                      TextFormField(
                        validator: (value) {
                          if (value.isEmpty || value.length < 7) {
                            return 'Password must be at least 7 character long.';
                          }
                          return null;
                        },
                        decoration: InputDecoration(
                          labelText: 'Password',
                        ),
                        obscureText: true,
                        onSaved: (newValue) {
                          _userPassword = newValue;
                        },
                      ),
                      SizedBox(
                        height: 20,
                      ),
                      RaisedButton(
                        onPressed: () {
                          _trySubmit();
                        },
                        child: Text(
                          _isLogin ? 'Login' : 'Signup',
                        ),
                      ),
                      FlatButton(
                        textColor: Theme.of(context).primaryColor,
                        child: Text(
                          _isLogin
                              ? 'Create New Account.'
                              : 'I Already Have Account.',
                        ),
                        onPressed: () {

                          setState(() {
                            _isLogin = !_isLogin;
                          });
                        },
                      ),
                    ],
                  ),
                ),
              ),
            ),
          ),
        ),
      ),
    );
  }
}

Let me know if it works.