previousItem method
Animate to previous item in viewport.
Implementation
Future<void> previousItem(
{Duration duration = _kDefaultDuration,
Curve curve = _kDefaultCurve}) async {
if (!hasClients) return;
await Future.wait<void>([
for (final position in positions.cast<_InfiniteScrollPosition>())
position.animateTo(offset - position.itemExtent,
duration: duration, curve: curve),
]);
}