SliderIterableChildDelegate

main.dart

class SliverIterableChildDelegate extends SliverChildDelegate {
  SliverIterableChildDelegate(this.children);

  final Iterable<Widget> children;
  int _lastAccessedIndex;
  Iterator<Widget> _lastAccessedIterator;

  @override
  Widget build(BuildContext context, int index) {
    if (_lastAccessedIndex == null || _lastAccessedIndex > index) {
      _lastAccessedIndex = -1;
      _lastAccessedIterator = children.iterator;
    }

    while (_lastAccessedIndex < index) {
      _lastAccessedIterator.moveNext();
      _lastAccessedIndex++;
    }

    return _lastAccessedIterator.current;
  }

  @override
  bool shouldRebuild(SliverIterableChildDelegate oldDelegate) {
    return children != oldDelegate.children
      || _lastAccessedIndex != oldDelegate._lastAccessedIndex
      || _lastAccessedIterator != oldDelegate._lastAccessedIterator;
  }
}