LazyListViewport({ExtentsChangedCallback onExtentsChanged, double scrollOffset: 0.0, Axis mainAxis: Axis.vertical, ViewportAnchor anchor: ViewportAnchor.start, @required double itemExtent, EdgeInsets padding, int itemCount, @required ItemListBuilder itemBuilder })

Creates a virtual viewport onto an extremely large or infinite list of equally sized children.

The mainAxis, anchor, itemExtent, and itemBuilder arguments must not be null.

Source 

LazyListViewport({
  ExtentsChangedCallback onExtentsChanged,
  double scrollOffset: 0.0,
  Axis mainAxis: Axis.vertical,
  ViewportAnchor anchor: ViewportAnchor.start,
  @required double itemExtent,
  EdgeInsets padding,
  this.itemCount,
  @required this.itemBuilder
}) : super(
  onExtentsChanged,
  scrollOffset,
  mainAxis,
  anchor,
  itemExtent,
  false, // Don't support wrapping yet.
  padding
) {
  assert(itemBuilder != null);
}