scroll_notification_to_percent.dart

  double _calculateScrollProgress(ScrollNotification notification) {
    var containerExtent = notification.metrics.viewportDimension;

    if (notification is ScrollUpdateNotification) {
      _dragOffset -= notification.scrollDelta;
    }

    if (notification is OverscrollNotification) {
      _dragOffset -= notification.overscroll;
    }

    var percent = _dragOffset / (containerExtent * widget.dragExtentPercentage);
    return percent.clamp(0.0, 1.0);
  }