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perf(ios): request layout once per turn and append subviews without copying - #11507
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farfromrefug
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…opying - requestLayout returns early when the view already climbed in this turn and is still flagged. CSS calls it once per layout-affecting property, so each call climbed to the root with a native setNeedsLayout at every level. A layout pass in between clears the flag, so the next request climbs again; the set is cleared by a microtask. - insertNativeSubview calls addSubview without reading `subviews` when no index is given. Reading it copies the array, so appending k children was O(k^2).
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triniwiz
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A synchronous layout pass (e.g. layoutIfNeeded) can lay out an ancestor and clear its flag while skipping a collapsed child, which stays flagged. The child's next requestLayout in the same turn was skipped, so the ancestor never re-laid out and the child stayed hidden after becoming visible. The shortcut now applies only while every ancestor is still flagged.
NathanWalker
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PR Checklist
What is the current behavior?
requestLayouton iOS climbs to the root on every call and callssetNeedsLayoutat each level. CSS calls it once per layout-affecting property, so styling one view costs depth times properties native calls, all reaching ancestors that were already flagged by the first call.insertNativeSubviewreadssubviewsbefore checking whether an index was given. Reading it copies the array, so appending k children to one parent is O(k^2).What is the new behavior?
requestLayoutreturns early when the view already climbed in this turn and is still flagged. A layout pass in between clears the flag, so the next request climbs again. The set of views is cleared by a microtask at the end of the turn.addSubviewwithout readingsubviews.New specs: a repeat request in the same turn does not climb (fails on
main), and a request climbs again in the next turn and after a layout pass.Numbers
How this was measured
xplat-benchmarks
ns-coreapp (plain core, no plugins), iPhone 17 Pro Max simulator, iOS 26.4. Each scenario mounts about 1,000 views at size M. Base and change were installed alternately for 3 rounds of 10 measured iterations, median ms, lower is better. "Attach" is assigning the built tree to a loaded parent (native view creation, CSS, native setters); "painted" ends one frame after layout. A GC is forced before each mutation, as it already is after each unmount, so a collection pause lands in the same phase for both builds.Part of a series: #11505, #11506, #11507 (this), #11510.