ScrollableViewport({Key key, double initialScrollOffset, Axis scrollDirection: Axis.vertical, ViewportAnchor scrollAnchor: ViewportAnchor.start, ScrollListener onScrollStart, ScrollListener onScroll, ScrollListener onScrollEnd, SnapOffsetCallback snapOffsetCallback, Key scrollableKey, Widget child })

Creates a simple scrolling widget that has a single child.

The scrollDirection and scrollAnchor arguments must not be null.

Source 

ScrollableViewport({
  Key key,
  this.initialScrollOffset,
  this.scrollDirection: Axis.vertical,
  this.scrollAnchor: ViewportAnchor.start,
  this.onScrollStart,
  this.onScroll,
  this.onScrollEnd,
  this.snapOffsetCallback,
  this.scrollableKey,
  this.child
}) : super(key: key) {
  assert(scrollDirection != null);
  assert(scrollAnchor != null);
}