flutter : value not passed to the previous screen and is shown null while using constructor

Issue

Basically, I have two classes Register and AddUser. I want to navigate value from the AddUser page to the RegisterPage but I am not getting any values despite using the constructor and getting null value while debugging. User First lands on the Register page where there is floatingAction button and it navigates to the AddUser Page. After providing the input , on clicking save button, it navigates back to the Register page where user will get the list of input.

**Register**

class Register extends StatefulWidget {
  late  String? names;
  Register({required this.names});


  @override
  _RegisterState createState() => _RegisterState(names);
}

class _RegisterState extends State<Register> {
  late String? names;

  _RegisterState(this.names);

 List<UserModel> getUserModel() {
  return [
    UserModel(

      name: widget.names??'',
    )
  ];
}

 // final user = UserSimplePreferences.getUser();

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
     }

  @override
  Widget build(BuildContext context) {
    return   Column(
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: [
        Padding(
        padding: const EdgeInsets.only(top: 10, bottom: 10),
    child: Text('Seleccione una categoría:',
    textAlign: TextAlign.center,
    style: TextStyle(color: Colors.black)
    ),
    ),
    Expanded(
    child: ListView.builder(
    itemCount: getUserModel().length,
    itemBuilder: (BuildContext ctx, int index) {
    return Container(
      margin: EdgeInsets.all(20),
      height: 150,
      child: Stack(
        children: [
          Text(getUserModel()[index].name)
        ],

     )


    );
    },
    ),
    ),
     FloatingActionButton(
        backgroundColor: Colors.indigo[900],
        onPressed: () {
          print(names);
          Navigator.push(
            context,
            MaterialPageRoute(builder: (_) {

              return AddUser(idUser: '',);

            }),
          );
        },
        child: Icon(Icons.add, color: Colors.white),
      ),
    ]
    );


  }
}



**AddUser**
class AddUser extends StatefulWidget {
  final String? idUser;

  const AddUser({required this.idUser});

  @override
  _AddUserState createState() => _AddUserState();
}

class _AddUserState extends State<AddUser> {
  final formKey = GlobalKey<FormState>();

  TextEditingController saaaaa = new TextEditingController();
  @override
  void initState() {
    super.initState();
  }

  @override
  Widget build(BuildContext context) => Scaffold(
        appBar: AppBar(
            automaticallyImplyLeading: false,
            title: Text(
              'Please Enter Your Details',
              textAlign: TextAlign.center,
            )),
        body: SafeArea(
          child: ListView(
            padding: EdgeInsets.all(16),
            children: [
              buildName(),
              const SizedBox(height: 12),
                          ],
          ),
        ),
      );

  Widget buildName() => buildTitle(
        title: 'Name',
        child: TextFormField(
          controller: saaaaa,
          //initialValue: name,
          decoration: InputDecoration(
            border: OutlineInputBorder(),
            hintText: 'Your Name',
          ),
          onChanged: (namer) => setState(() => namer = saaaaa.text),
        ),
      );

  

  Widget buildButton() => ButtonWidget(
      text: 'Save',

      onClicked: () async {
        
        setState(() async {
             Register(names : saaaaa.text );
             Navigator.pop(context);


        });
      });

  Widget buildTitle({
    required String title,
    required Widget child,
  }) =>
      Column(
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: [
          Text(
            title,
            style: TextStyle(fontWeight: FontWeight.bold, fontSize: 18),
          ),
          const SizedBox(height: 8),
          child,
        ],
      );
}

Solution

You can achive this things by then callback in navigator and pass your value when you pop add screen. Please replace your code with below code

Register

class Register extends StatefulWidget {
  @override
  _RegisterState createState() => _RegisterState();
}


class _RegisterState extends State<Register> {
  List<UserModel> userList = [];

  // final user = UserSimplePreferences.getUser();

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Column(crossAxisAlignment: CrossAxisAlignment.stretch, children: [
      Padding(
        padding: const EdgeInsets.only(top: 10, bottom: 10),
        child: Text('Seleccione una categoría:',
            textAlign: TextAlign.center, style: TextStyle(color: Colors.black)),
      ),
      Expanded(
        child: ListView.builder(
          itemCount: userList.length,
          itemBuilder: (BuildContext ctx, int index) {
            return Container(
                margin: EdgeInsets.all(20),
                height: 150,
                child: Stack(
                  children: [Text(userList[index].name)],
                ));
          },
        ),
      ),
      FloatingActionButton(
        backgroundColor: Colors.indigo[900],
        onPressed: () {
          Navigator.push(
            context,
            MaterialPageRoute(builder: (_) {
              return AddUser(
                idUser: '',
              );
            }),
          ).then((value) {
            if (value != null) {
              userList.add(UserModel(name: value));
              setState(() {});
            }
          });
        },
        child: Icon(Icons.add, color: Colors.white),
      ),
    ]);
  }
}



**AddUser**
class AddUser extends StatefulWidget {
  final String? idUser;

  const AddUser({required this.idUser});

  @override
  _AddUserState createState() => _AddUserState();
}

class _AddUserState extends State<AddUser> {
  final formKey = GlobalKey<FormState>();

  TextEditingController saaaaa = new TextEditingController();

  @override
  void initState() {
    super.initState();
  }

  @override
  Widget build(BuildContext context) => Scaffold(
        appBar: AppBar(
            automaticallyImplyLeading: false,
            title: Text(
              'Please Enter Your Details',
              textAlign: TextAlign.center,
            )),
        body: SafeArea(
          child: ListView(
            padding: EdgeInsets.all(16),
            children: [
              buildName(),
              buildButton(),
              const SizedBox(height: 12),
            ],
          ),
        ),
      );

  Widget buildName() => buildTitle(
        title: 'Name',
        child: TextFormField(
          controller: saaaaa,
          //initialValue: name,
          decoration: InputDecoration(
            border: OutlineInputBorder(),
            hintText: 'Your Name',
          ),
          onChanged: (namer) => setState(() => namer = saaaaa.text),
        ),
      );

  Widget buildButton() => ButtonWidget(
      text: 'Save',
      onClicked: () async {
        setState(() async {
//          Register(names : saaaaa.text );
          Navigator.pop(context, saaaaa.text);
        });
      });

  Widget buildTitle({
    required String title,
    required Widget child,
  }) =>
      Column(
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: [
          Text(
            title,
            style: TextStyle(fontWeight: FontWeight.bold, fontSize: 18),
          ),
          const SizedBox(height: 8),
          child,
        ],
      );
}

Answered By - Sagar Patel