grabAt<S> method

S grabAt<S>(BuildContext context, GrabSelector<R, S> selector)

Returns an object chosen with the selector, and starts listening for changes in the ValueListenable to rebuild the widget associated with the provided BuildContext when there is a change in the selected object.

The callback of the selector receives the value of the ValueListenable that this method was called on.

A Grab is necessary as an ancestor of the widget this method is used for. GrabMissingError is thrown otherwise.

void main() {
  runApp(
    const Grab(child: ...),
  );
}

class Item extends ChangeNotifier {
  Item({required this.name, required this.quantity});

  final String name;
  final int quantity;
}

final notifier = ValueNotifier(
  Item(name: 'Milk', quantity: 3),
};

...

class InventoryItem extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final name = notifier.grabAt(context, (item) => item.name);
    return Text(name);
  }
}

The object to select can be anything as long as it is possible to evaluate its equality with the previous object using the == operator.

final hasEnough = notifier.grabAt(context, (item) => item.quantity > 5);

Supposing that the quantity was 3 in the previous build and has changed to 2 now, the widget is not rebuilt because the value returned by the selector has remained false.

Implementation

S grabAt<S>(BuildContext context, GrabSelector<R, S> selector) {
  if (Grab.stateOf(context) case final grabState?) {
    return grabState.listen(
      context: context,
      listenable: this,
      selector: selector,
    );
  }
  throw GrabMissingError();
}