Passing data to StatefulWidget and accessing it in its state in Flutter

Question

I have 2 screens in my Flutter app: a list of records and a screen for creating and editing records.

If I pass an object to the second screen that means I am going to edit this and if I pass null it means that I am creating a new item. The editing screen is a Stateful widget and I am not sure how to use this approach https://flutter.io/cookbook/navigation/passing-data/ for my case.

class RecordPage extends StatefulWidget {
  final Record recordObject;

  RecordPage({Key key, @required this.recordObject}) : super(key: key);

  @override
  _RecordPageState createState() => new _RecordPageState();
}

class _RecordPageState extends State<RecordPage> {
  @override
  Widget build(BuildContext context) {
   //.....
  }
}

How can I access recordObject inside _RecordPageState?

Answer 1

To use recordObject in _RecordPageState, you have to just write widget.objectname like below

class _RecordPageState extends State<RecordPage> {
  @override
  Widget build(BuildContext context) {
   .....
   widget.recordObject
   .....
  }
}

Answer 2

Full Example

You don't need to pass parameters to State using it's constructor. You can easily access these using widget.myField.

class MyRecord extends StatefulWidget {
  final String recordName;
  const MyRecord(this.recordName);

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

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

Pass your data when you Navigate screen :

 Navigator.of(context).push(MaterialPageRoute(builder: (context) => MyRecord("WonderWorld")));

Answer 3

class RecordPage extends StatefulWidget {
  final Record recordObject;

  RecordPage({Key key, @required this.recordObject}) : super(key: key);

  @override
  _RecordPageState createState() => new _RecordPageState(recordObject);
}

class _RecordPageState extends State<RecordPage> {
  Record  recordObject
 _RecordPageState(this. recordObject);  //constructor
  @override
  Widget build(BuildContext context) {.    //closure has access
   //.....
  }
}

Answer 4

example as below:

class nhaphangle extends StatefulWidget {
  final String username;
  final List<String> dshangle;// = ["1","2"];
  const nhaphangle({ Key key, @required this.username,@required this.dshangle }) : super(key: key);


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

class _nhaphangleState extends State<nhaphangle> {
  TextEditingController mspController = TextEditingController();
  TextEditingController soluongController = TextEditingController();
  final scrollDirection = Axis.vertical;
  DateTime Ngaysx  = DateTime.now();
  ScrollController _scrollController = new ScrollController();

  ApiService _apiService;
  List<String> titles = [];

  @override
  void initState() {
    super.initState();
    _apiService = ApiService();
    titles = widget.dshangle;  //here var is call and set to 
  }

    

Answer 5

You don't have to create any new object or variable for this reason in the state class, you need to define just a final and nullable variable for your object in the stateful class. you can access this variable by widget. like widget.recordObject

class RecordPage extends StatefulWidget {
   final Record? recordObject;
   RecordPage({required this.recordObject});
   @override
   _RecordPageState createState() => new _RecordPageState();
}

class _RecordPageState extends State<RecordPage> {
   @override
   Widget build(BuildContext context) {
      return Text(
         widget.recordObject == null? "the object is null" : "the object isn't null",
   );
 }
}

Answer 6

I have to Navigate back to any one of the screens in the list pages but when I did that my onTap function stops working and navigation stops.

class MyBar extends StatefulWidget {
  MyBar({this.pageNumber});
  final pageNumber;
  static const String id = 'mybar_screen';
  @override
  _MyBarState createState() => _MyBarState();
}

class _MyBarState extends State<MyBar> {
  final List pages = [
    NotificationScreen(),
    AppointmentScreen(),
    RequestBloodScreen(),
    ProfileScreen(),
  ];
  @override
  Widget build(BuildContext context) {
    var _selectedItemIndex = widget.pageNumber;
    return Scaffold(
        bottomNavigationBar: BottomNavigationBar(
          elevation: 0,
          backgroundColor: Colors.white,
          unselectedItemColor: Colors.grey.shade700,
          selectedItemColor: Color(kAppColor),
          selectedIconTheme: IconThemeData(color: Color(kAppColor)),
          currentIndex: _selectedItemIndex,
          type: BottomNavigationBarType.fixed,
          onTap: (int index) {
            setState(() {
              _selectedItemIndex = index;
            });
          },

Answer 7

You should use a Pub/Sub mechanism. I prefer to use Rx in many situations and languages. For Dart/Flutter this is the package: https://pub.dev/packages/rxdart

For example, you can use a BehaviorSubject to emit data from widget A, pass the stream to widget B which listens for changes and applies them inside the setState.

Widget A:

// initialize subject and put it into the Widget B
BehaviorSubject<LiveOutput> subject = BehaviorSubject();
late WidgetB widgetB = WidgetB(deviceOutput: subject);

// when you have to emit new data
subject.add(deviceOutput);

Widget B:

// add stream at class level
class WidgetB extends StatefulWidget {
   final ValueStream<LiveOutput> deviceOutput;
   const WidgetB({Key? key, required this.deviceOutput}) : super(key: key);

   @override
   State<WidgetB> createState() => _WidgetBState();
}

// listen for changes
@override
void initState() {
   super.initState();

   widget.deviceOutput.listen((event) {
      print("new live output");
      setState(() {
         // do whatever you want
      });
   });
}

Answer 8

In my app, often instead of using stateful widgets, I use mainly ChangeNotifierProvider<T> in main.dart, some model class

class FooModel extends ChangeNotifier {

var _foo = false;

void changeFooState() {
   _foo = true;
   notifyListeners();
}

bool getFoo () => _foo;

}

and

var foo = context.read<FooModel>();
# or
var foo = context.watch<FooModel>();

in my stateless widgets. IMO this gives me more precise control over the rebuilding upon runtime state change, compared to stateful widgets.

The recipe can be found in the official docs, the concept is called "lifting state up".