moveNext method
move to the next element, return false if no more elements there
Implementation
@override
bool moveNext() {
if (_element == null) {
return false;
}
if (_element == _scope && _element!.children.isEmpty) {
_element = null;
return false;
}
if (_reverse) {
_navigateBackward();
} else {
_navigateForward();
}
if (_element == _startingElement) {
_element = null;
}
return (_element != null);
}