I can't change my widget's visibility with using mobx - flutter

Question

I'm trying to change my second TextField's visibility in my other Auth class but I'm using mobx. I tried this version. But I can't solve my problem anyway.
If I declare my otpvisibility variable in SignInView class, I need to use setstate when i change the value of this variable in the Auth class. But i can't use setstate because I am using mobx. On the other hang if I declare otpvisibility variable in my mobx class, changes won't effect.
class SignInView extends StatelessWidget {
final BuildContext context;
SignInView({required this.context, Key? key}) : super(key: key);
TextEditingController phonecontroller = TextEditingController();
TextEditingController otpcontroller = TextEditingController();
Auth auth = Auth();
SignInViewModel svm = SignInViewModel();
#override
Widget build(BuildContext context) {
return Scaffold(
body: buildbody,
);
}
Widget get buildbody {
double screenWidth = MediaQuery.of(context).size.width;
return Observer(builder: (_) {
return SizedBox(
width: screenWidth,
child: Padding(
padding: const EdgeInsets.all(8.0),
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
const Text(
"Welcome to Whatsapp Clone, Let's begin!",
style: TextStyle(color: Colors.green, fontSize: 20),
),
const SizedBox(height: 30),
Container(
padding: const EdgeInsets.all(8),
height: 80,
child: TextField(
controller: phonecontroller,
decoration: const InputDecoration(
border: OutlineInputBorder(borderSide: BorderSide()),
),
keyboardType: TextInputType.phone,
textInputAction: TextInputAction.done,
onSubmitted: (String value) {
if (phonecontroller.text != '') {
if (svm.otpVisibility) {
svm.otpvisiblty(context);
auth.verifyotp(context, otpcontroller.text);
} else {
print('otpvisible false');
auth.loginWithPhone(
context: context, phone: phonecontroller.text);
}
} else {
ScaffoldMessenger.of(context).showSnackBar(
const SnackBar(
content:
Text('Please enter your phone number')));
}
},
)),
Visibility(
visible: svm.otpVisibility,
child: Container(
padding: const EdgeInsets.all(8),
height: 80,
child: TextField(
controller: otpcontroller,
decoration: const InputDecoration(
border: OutlineInputBorder(
borderSide: BorderSide(),
),
),
keyboardType: TextInputType.number,
maxLength: 6,
),
),
),
TextButton(
onPressed: () {
print(phonecontroller.text);
if (phonecontroller.text != '') {
if (svm.otpVisibility) {
svm.otpvisiblty(context);
auth.verifyotp(context, otpcontroller.text);
} else {
print('otpvisible false');
auth.loginWithPhone(
context: context, phone: phonecontroller.text);
}
} else {
ScaffoldMessenger.of(context).showSnackBar(const SnackBar(
content: Text('Please enter your phone number')));
}
},
child: const Text(
'Verify',
style: TextStyle(color: Colors.green),
),
)
],
),
),
);
});
}
}