Proposal: Make ScrollPhysics reactive #80051

Hello there!
Right now a ScrollPhysics is only updated if you change the class type.

I think it would be great to follow some other patterns inside the flutter framework to allow this class to be reactive. This would be to add a new method inside the ScrollPhysics like

bool shouldReload (covariant ScrollPhysics old) => false;

Notice by default would return false, so the behaviour for all the current ScrollPhysics would be the same as now.

Then when the Scrollable widget updates we would add

  if (newPhysics?.runtimeType != oldPhysics?.runtimeType || 
      newPhysics?.shouldUpdate(oldPhysics!))
        return true;

This pattern can be seen in LocalizationsDelegate, SliverPersistentHeaderDelegate, SliverChildDelegate, ScrollBehavior, and many more. (In the search box from the api website write should to see the rest)

Use case

I am building a ScrollPhysics that allows snapping to different positions if the scrollOffset is too close (Similiar to PageScrollPhysics). This would be very useful for SliverAppBars to accomplish this behaviour, where it only snaps to a expanded/collapsed state, but the scroll can not stop in the middle.

It works, but right now it is not reactive. People need to do a hot-restart to see the changes made to it.

CustomScrollView(
   physics: SnapScrollView(offset: 64, range: 20),
   slivers: [
      ....
   ]
);

It is important to consider this comment from the Scrollable doc though.

I don't understand why it would be needed to create this object for each frame to see if has changed its configuration. Right now the type runtime is compared only when the widget or its dependencies are updated. So I think it would be the same to also check it through a shouldUpdate method if the configuration has changed.

Notice the ScrollPhysics class is immutable so it should not change its configuration after created. And when it changes didDependenciesUpdate or didWidgetUpdate should be triggered if is used in that scrollable.

cc: @Piinks