如何在Flutter中的单例内部使用Provider.of（...）？

Question

I've this widget somewhere deep inside my widget tree:

Widget build(BuildContext context) {
return ChangeNotifierProvider(
  builder: (context) => TimersModel(context: context),
  child: Scaffold(...

TimersModel get the context:

class TimersModel extends ChangeNotifier {
  final BuildContext context;
  NotificationsService _notificationsService;

  TimersModel({@required this.context}) {
    _notificationsService = NotificationsService(context: context);
  }

And instantiates this NotificationsService singleton for the first and only time:

class NotificationsService {
  static FlutterLocalNotificationsPlugin _flutterLocalNotificationsPlugin;

  final BuildContext context;

  static NotificationsService _instance;

  factory NotificationsService({@required BuildContext context}) {
    _instance ??= NotificationsService._internalConstructor(context: context);
    return _instance;
  }

  NotificationsService._internalConstructor({@required this.context}) {

As you can see, this is a singleton for the FlutterLocalNotificationsPlugin

Problem is that if I call Provider.of<TimersModel>(context)... from this singleton, although it's getting the right context, it's always throwing ProviderNotFoundError .

If I place a breakpoint on this code from Provider:

static T of<T>(BuildContext context, {bool listen = true}) {
    // this is required to get generic Type
    final type = _typeOf<InheritedProvider<T>>();
    final provider = listen
        ? context.inheritFromWidgetOfExactType(type) as InheritedProvider<T>
        : context.ancestorInheritedElementForWidgetOfExactType(type)?.widget
            as InheritedProvider<T>;

    if (provider == null) {
      throw ProviderNotFoundError(T, context.widget.runtimeType);
    }

    return provider._value;
  }

The context ChangeNotifierProvider and type TimersModel are right. But provider is always null.

I know the singleton is not a widget and, of course, it's not in the widget tree.

But shouldn't I be able to call Provider.of<TimersModel>(context)... from anywhere as long as I provide the right context and type?

Or should this work and I'm doing something wrong?

Answer 1

As Provider does the lookup by type, try giving your ChangeNotifierProvider a type when you return it in your build method:

return ChangeNotifierProvider<TimersModel>(...);

I can imagine that Provider simply doesn't find an instance for TimersModel because you didn't declare a provider of that type.