sample method

PointerSample? sample(Offset globalPointer)

Converts globalPointer into every coordinate the drag pipeline needs, or null when the scrollable is unmounted (or its viewport box is detached mid-teardown).

Every path that resolves a target samples first, so this is also the correctness floor for the scroll subscription: a swapped position is re-bound here even with no widget layer above the controller (the widget's didChangeDependencies edge trigger only removes the latency until the next event).

Implementation

PointerSample? sample(Offset globalPointer) {
  syncScrollSubscription();
  if (!_scrollable.mounted) {
    return null;
  }
  final box = _scrollable.context.findRenderObject();
  if (box is! RenderBox || !box.attached) {
    return null;
  }
  final local = box.globalToLocal(globalPointer);
  return (
    sliverX: globalPointer.dx - _renderPort.crossAxisGlobalOrigin,
    sliverY:
        _scrollable.position.pixels +
        local.dy -
        _renderPort.precedingScrollExtent,
    viewportDy: local.dy,
    viewportHeight: box.size.height,
  );
}