planScrollables function

ScrollPlan planScrollables(UiSnapshot snap)

Pure planner — see ScrollPlan.

Implementation

ScrollPlan planScrollables(UiSnapshot snap) {
  final all = snap.nodes
      .where((n) => n.scrollable && n.visible)
      .toList(growable: false);
  if (all.isEmpty) return const ScrollPlan(vertical: [], horizontal: []);
  var topLayer = 0;
  for (final n in all) {
    if (n.layer > topLayer) topLayer = n.layer;
  }
  final onTop = all.where((n) => n.layer == topLayer);
  final vertical = <UiNode>[];
  final horizontal = <UiNode>[];
  final vpH = snap.viewportSize.height;
  for (final n in onTop) {
    if (!scrollsHorizontally(n)) {
      vertical.add(n);
      continue;
    }
    // Degenerate viewport ⇒ the strip test is unanswerable; skipping it beats
    // filtering every candidate to nothing.
    if (vpH > 0 && n.bounds.height > vpH * kMaxCarouselHeightFraction) {
      continue; // a pager
    }
    horizontal.add(n);
  }
  double area(UiNode n) => n.bounds.width * n.bounds.height;
  vertical.sort((a, b) => area(b).compareTo(area(a)));
  horizontal.sort((a, b) {
    final dy = a.bounds.top.compareTo(b.bounds.top);
    return dy != 0 ? dy : a.bounds.left.compareTo(b.bounds.left);
  });
  return ScrollPlan(vertical: vertical, horizontal: horizontal);
}