update method

Implementation

@override
void update(DragUpdateDetails details) {
  assert(details.primaryDelta != null);
  _lastDetails = details;
  double offset = details.primaryDelta!;
  if (offset != 0.0) {
    _lastNonStationaryTimestamp = details.sourceTimeStamp;
  }
  // By default, iOS platforms carries momentum and has a start threshold
  // (configured in [BouncingScrollPhysics]). The 2 operations below are
  // no-ops on Android.
  _maybeLoseMomentum(offset, details.sourceTimeStamp);
  offset = _adjustForScrollStartThreshold(offset, details.sourceTimeStamp);
  if (offset == 0.0) {
    return;
  }
  if (_reversed) {
    offset = -offset;
  }
  delegate.applyUserOffset(offset);
}