Passing Data to a Stateful Widget

Don't pass parameters to State using it's constructor. You should only access these using this.widget.myField.

Not only editing the constructor requires a lot of manual work ; it doesn't bring anything. There's no reason to duplicate all the fields of Widget.

EDIT :

Here's an example:

class ServerIpText extends StatefulWidget {
  final String serverIP;

  const ServerIpText ({ Key key, this.serverIP }): super(key: key);

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

class _ServerIpTextState extends State<ServerIpText> {
  @override
  Widget build(BuildContext context) {
    return Text(widget.serverIP);
  }
}

class AnotherClass extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Center(
      child: ServerIpText(serverIP: "127.0.0.1")
    );
  }
}

Best way is don't pass parameters to State class using it's constructor. You can easily access in State class using widget.myField.

For Example

class UserData extends StatefulWidget {
  final String clientName;
  final int clientID;
  const UserData(this.clientName,this.clientID);

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

class UserDataState extends State<UserData> {
  @override
  Widget build(BuildContext context) {
    // Here you direct access using widget
    return Text(widget.clientName); 
  }
}

Pass your data when you Navigate screen :

 Navigator.of(context).push(MaterialPageRoute(builder: (context) => UserData("WonderClientName",132)));

Another answer, building on @RémiRousselet's anwser and for @user6638204's question, if you want to pass initial values and still be able to update them in the state later:

class MyStateful extends StatefulWidget {
  final String foo;

  const MyStateful({Key key, this.foo}): super(key: key);

  @override
  _MyStatefulState createState() => _MyStatefulState(foo: this.foo);
}

class _MyStatefulState extends State<MyStateful> {
  String foo;

  _MyStatefulState({this.foo});

  @override
  Widget build(BuildContext context) {
    return Text(foo);
  }
}