GestureDragUpdateCallback onVerticalDragUpdate

Called when the user scrolls up or down.

Source 

GestureDragUpdateCallback get onVerticalDragUpdate => _onVerticalDragUpdate;
void onVerticalDragUpdate=(GestureDragUpdateCallback value)

Source 

set onVerticalDragUpdate(GestureDragUpdateCallback value) {
  if (_onVerticalDragUpdate == value)
    return;
  bool wasSemanticBoundary = isSemanticBoundary;
  bool hadHandler = _onVerticalDragUpdate != null;
  _onVerticalDragUpdate = value;
  if ((value != null) != hadHandler)
    markNeedsSemanticsUpdate(onlyChanges: isSemanticBoundary == wasSemanticBoundary);
}