Passing Data to a Stateful Widget in Flutter

Don't pass parameters to State using it's constructor. You should only access the parameters 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);
  }
}

For passing initial values (without passing anything to the constructor)

class MyStateful extends StatefulWidget {
  final String foo;

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

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

class _MyStatefulState extends State<MyStateful> {
  @override
  void initState(){
    super.initState();
    // you can use this.widget.foo here
  }

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

Flutter's stateful widgets API is kinda awkward: storing data in Widget in order to access it in build() method which resides in State object 🤦 If you don't want to use some of bigger state management options (Provider, BLoC), use flutter_hooks (https://pub.dev/packages/flutter_hooks) - it is a nicer and cleaner substitute for SatefullWidgets:

class Counter extends HookWidget {
  final int _initialCount;

  Counter(this._initialCount = 0);
  
  @override
  Widget build(BuildContext context) {
    final counter = useState(_initialCount);

    return GestureDetector(
      // automatically triggers a rebuild of Counter widget
      onTap: () => counter.value++,
      child: Text(counter.value.toString()),
    );
  }
}

@Rémi Rousselet, @Sanjayrajsinh, @Daksh Shah is also better. but I am also defined this is in from starting point.that which parameter is which value

   import 'package:flutter/material.dart';
    
    
    void main() => runApp(MyApp());
    
    class MyApp extends StatelessWidget {
      String name = "Flutter Demo";
      String description = "This is Demo Application";
    
      @override
      Widget build(BuildContext context) {
        return MaterialApp(
          home: MainActivity(
            appName: name,
            appDescription: description,
          ),
        );
      }
    }
    
    class MainActivity extends StatefulWidget {
      MainActivity({Key key, this.appName, this.appDescription}) : super(key: key);
      var appName;
      var appDescription;
    
      @override
      _MainActivityState createState() => _MainActivityState();
    }
    
    class _MainActivityState extends State<MainActivity> {
      @override
      Widget build(BuildContext context) {
        return Scaffold(
          appBar: AppBar(
            title: Text(widget.appName),
          ),
          body: Scaffold(
            body: Center(
              child: Text(widget.appDescription),
            ),
          ),
        );
      }
    }

To pass data to stateful widget, first of all, create two pages. Now from the first page open the second page and pass the data.

class PageTwo extends StatefulWidget {
  final String title;
  final String name;

  PageTwo ({ this.title, this.name });

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

class PageTwoStateState extends State<PageTwo> {
  @override
  Widget build(BuildContext context) {
      return Text(
         widget.title,
         style: TextStyle(
               fontSize: 18, fontWeight: FontWeight.w700),
               ),
  }
}

class PageOne extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialButton(
          text: "Open PageTwo",
          onPressed: () {
                var destination = ServicePage(
                   title: '<Page Title>',
                   provider: '<Page Name>',
                );
                Navigator.push(context,
                    MaterialPageRoute(builder: (context) => destination));
                        },);
  }
}