update method

void update (ScrollMetrics metrics, AxisDirection axisDirection)

Update with new ScrollMetrics. The scrollbar will show and redraw itself based on these new metrics.

The scrollbar will remain on screen.

Implementation 

void update(
  ScrollMetrics metrics,
  AxisDirection axisDirection,
) {
  _lastMetrics = metrics;
  _lastAxisDirection = axisDirection;
  notifyListeners();
}