Viewport({Key key, Offset paintOffset: Offset.zero, Axis mainAxis: Axis.vertical, ViewportAnchor anchor: ViewportAnchor.start, ViewportDimensionsChangeCallback onPaintOffsetUpdateNeeded, Widget child })

Creates a widget that's bigger on the inside.

The mainAxis and paintOffset arguments must not be null.

Source 

Viewport({
  Key key,
  this.paintOffset: Offset.zero,
  this.mainAxis: Axis.vertical,
  this.anchor: ViewportAnchor.start,
  this.onPaintOffsetUpdateNeeded,
  Widget child
}) : super(key: key, child: child) {
  assert(mainAxis != null);
  assert(paintOffset != null);
}