Passing Data to a Stateful Widget

Question

I'm wondering what the recommended way of passing data to a stateful widget, while creating it, is.

The two styles I've seen are:

class ServerInfo extends StatefulWidget {

  Server _server;

  ServerInfo(Server server) {
    this._server = server;
  }

  @override
    State<StatefulWidget> createState() => new _ServerInfoState(_server);
}

class _ServerInfoState extends State<ServerInfo> {
  Server _server;

  _ServerInfoState(Server server) {
    this._server = server;
  }
}

This method keeps a value both in ServerInfo and _ServerInfoState, which seems a bit wasteful.

The other method is to use widget._server:

class ServerInfo extends StatefulWidget {

  Server _server;

  ServerInfo(Server server) {
    this._server = server;
  }

  @override
    State<StatefulWidget> createState() => new _ServerInfoState();
}

class _ServerInfoState extends State<ServerInfo> {
  @override
    Widget build(BuildContext context) {
      widget._server = "10"; // Do something we the server value
      return null;
    }
}

This seems a bit backwards as the state is no longer stored in _ServerInfoSate but instead in the widget.

Is there a best practice for this?

Answer 1

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")
    );
  }
}

Answer 2

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)));