toggleExpansion method
Implementation
void toggleExpansion(int index) {
final newExpanded = Set<int>.from(expanded.value);
if (widget.singleExpand) {
if (newExpanded.contains(index)) {
newExpanded.clear();
} else {
newExpanded
..clear()
..add(index);
}
} else {
if (newExpanded.contains(index)) {
newExpanded.remove(index);
} else {
newExpanded.add(index);
}
}
expanded.value = newExpanded;
}