wait_until_provider.dart

class WaitUntilFutureProvider<T> extends SingleChildStatefulWidget {
  const WaitUntilFutureProvider({
    Key key,
    this.waitUntil,
    this.create,
    Widget child,
  }) : super(key: key, child: child);

  final bool Function(BuildContext) waitUntil;
  final Create<Future<T>> create;

  @override
  _WaitUntilFutureProviderState<T> createState() =>
      _WaitUntilFutureProviderState<T>();
}

class _WaitUntilFutureProviderState<T>
    extends SingleChildState<WaitUntilFutureProvider<T>> {
  final createCompleter = Completer<T>();

  var running = false;

  void start() {
    running = true;
    tryCreate();
  }

  void tryCreate() {
    if (widget.waitUntil(context)) {
      running = false;
      createCompleter.complete(widget.create(context));
    }
  }

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    if (running) {
      tryCreate();
    }
  }

  @override
  Widget buildWithChild(BuildContext context, Widget child) {
    return FutureProvider<T>(
      create: (_) async {
        // not in initState to preserve lazy-loading
        start();
        return createCompleter.future;
      },
      child: child,
    );
  }
}