grab 0.0.1

A flutter package to control rebuilds of a widget based on updates of Listenables.

A flutter package to control rebuilds of a widget based on updates of Listenables.

Although this package is extremely simple, it becomes a powerful state management tool if combined with some DI package like GetIt.

Usage

This package contains mixins and extensions of BuildContext.

Mixins

Add StatelessGrabMixin or StatefulGrabMixin to a widget in which you want to use extensions.

class MyWidget extends StatelessWidget with StatelessGrabMixin

class MyWidget extends StatefulWidget with StatefulGrabMixin

Each mixin has an alias.
Use Grab for StatelessGrabMixin or Grabful for StatefulMixin if you like shorter names.

class MyWidget extends StatelessWidget with Grab

class MyWidget extends StatefulWidget with Grabful

Extensions

You can use grab() and grabAt() on a BuildContext.
They are like watch() and select() of package:provider.

Make sure to add a mixin to the StatelessWidget/StatefulMixin where these extensions are used. An error occurs otherwise.

grab()

grab() listens to the listenable passed to it, and the widget that the BuildContext belongs to is rebuilt every time the listenable is updated.

final notifier = ValueNotifier(0);

@override
Widget build(BuildContext context) {
  final count = context.grab<int>(counterNotifier);
  return Text('$count');
}

The return value is the listenable itself, or its value if the listenable is ValueListenable.

In the above example, the listenable is a ValueNotifier extending ValueListenable, so the count returned by grab() is not the listenable itself but the value of ValueNotifier.

grabAt()

The widget is rebuilt only when there is a change in the value returned by the selector, which is a callback function passed as the second argument.

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

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

The selector receives the listenable itself, or its value if the listenable is ValueListenable.

In this example, the value of the notifier is Item that has name and quantity. The widget is not rebuilt if quantity has a new value but name is not updated.

Tip

The value returned by the selector can be anything as long as it is possible to evaluate its equality with the previous value with the == operator.

final bool isEnough = context.grabAt(
  notifier,
  (Item item) => item.quantity > 5,
);

Supposing that the quantity was 3 in the previous build, if it's changed to 2, the widget is not going to be rebuilt because isEnough remains false.