RenderVirtualViewport({int virtualChildCount, LayoutCallback callback, Offset paintOffset: Offset.zero, Axis mainAxis: Axis.vertical, ViewportAnchor anchor: ViewportAnchor.start })

Initializes fields for subclasses.

The paintOffset and mainAxis arguments must not be null.

Source 

RenderVirtualViewport({
  int virtualChildCount,
  LayoutCallback callback,
  Offset paintOffset: Offset.zero,
  Axis mainAxis: Axis.vertical,
  ViewportAnchor anchor: ViewportAnchor.start
}) : _virtualChildCount = virtualChildCount,
     _callback = callback,
     super(paintOffset, mainAxis, anchor);