LazyBlockViewport({Key key, @required LazyBlockDelegate delegate, double startOffset: 0.0, Axis mainAxis: Axis.vertical, EdgeInsets padding, LazyBlockExtentsChangedCallback onExtentsChanged })

Creates a viewport on an infinite list of variable height children.

The delegate argument must not be null.

Source 

LazyBlockViewport({
  Key key,
  @required this.delegate,
  this.startOffset: 0.0,
  this.mainAxis: Axis.vertical,
  this.padding,
  this.onExtentsChanged
}) : super(key: key) {
  assert(delegate != null);
}