Check out example codes for "send params to stateful widget". It will help you in understanding the concepts better.

Code Example 1

class Screen1 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Center(
      child: Screen2(userId: "89d6db7e-7469-4888-9fac-38c300628ff1") // sends the value
    );
  }
}

class Screen2 extends StatefulWidget {
  final String userId; // receives the value

  Screen2({ Key key, this.userId }): super(key: key);

  @override
  _Screen2State createState() => _Screen2State();
}

class _Screen2State extends State<Screen2> {
  @override
  Widget build(BuildContext context) {
    return Text(widget.userId); // uses the value
  }
}

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