openSection method
Implementation
void openSection(int index) {
if (limitExpandedToOne) {
_openSections.value = [index];
} else {
final newOpenSections = List<int>.from(_openSections.value);
if (!newOpenSections.contains(index)) {
newOpenSections.add(index);
}
_openSections.value = newOpenSections;
}
notifyListeners();
}