backward method
backward will move the scrollbar with a distance of one item in
the top direction.
If value of align is true
, the first visible item will be aligned.
Implementation
void backward({bool align = true, bool? animate}) {
if (align) {
final firstIdx = (offset / scrollPerItem - 1).ceil();
if (firstIdx < 0) {
return;
}
scrollToItem(firstIdx, animate: animate);
} else {
_scrollToPos(offset - scrollPerItem, animate: animate);
}
}