How to pass a variable inside the build method to its parent state in Flutter?-CodePudding

I have a state that looks like this where I call the method _getData inside the initState

class _DataPageState extends State<DataPage> {
  @override
  void initState() {
    super.initState();
    _selectedEvents = ValueNotifier(_getData(value!));
  }

  @override
  void dispose() {
    _selectedEvents.dispose();
    super.dispose();
  }

  List<dynamic> _getData(DateTime value) {
    //trying to access 'events' variable here
    return events[day] ?? [];
  }

And my build method looks likes this

@override
  Widget build(BuildContext context) {
    final _source = toResults(data);
    //'events' variable I'm trying to pass
    final events = LinkedHashMap<DateTime, List<dynamic>>(
      equals: isSameDay,
      hashCode: getHashCode,
    )..addAll(_source);
  }

I'm trying to pass the events variable to my _getData method inside the state to be able to run it inside the initState, is there an appropriate way of doing that?

And if not, is there a way of moving the final variable up without having the issue of "Instance member can't be accessed in an initializer" ?

CodePudding user response：

Your build method should be "pure", meaning it should not have side effects, since Flutter can call it whenever it needs to rebuild the widget tree (e.g. if a keyboard appears, if changing to landscape mode, because a parent rebuilds, etc).

In Dart, if you have a variable that will be initialized before it is first used, but cannot be immediately initialized, you can use the late keyword. Perhaps something like this would work:

class MyState extends State<MyWidget> {
  late LinkedHashMap<DateTime, List<dynamic>> _events;  // late means no initializer needed

  @override
  void initState() {
    super.initState();
    final _source = toResults(data);
    _events = LinkedHashMap<DateTime, List<dynamic>>(
      equals: isSameDay,
      hashCode: getHashCode,
    )..addAll(_source);
  }

  // other stuff
  
}

It's not entirely clear where data is coming from, but if it can be passed in from higher up in the tree, you could make it a property on the Widget class and use widget.data to read it safely inside initState()