Fix _flushViewController crash by using viewController index map#301
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When UIPageViewController asks for the view controller before/after a given view controller, the current implementation uses `currentIndex` to determine which page to return. However, `currentIndex` can become stale after a cancelled or interrupted transition, causing the data source to return an incorrect view controller. This leads to an internal state inconsistency in UIPageViewController, which eventually triggers a `_flushViewController` crash. This fix resolves the issue by looking up the actual index of the passed `viewController` from `viewControllerIndexMap` first, falling back to `currentIndex` only when the view controller is not found in the map. This ensures the correct adjacent page is always returned, regardless of whether `currentIndex` has been updated yet. Refs: uias#286 Refs: uias/Tabman#556
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Summary
Fixes a crash in
UIPageViewController's internal_flushViewControllermethod caused by stalecurrentIndexvalues in theUIPageViewControllerDataSourcemethods.@msaps
Root cause:
pageViewController(_:viewControllerBefore:)andpageViewController(_:viewControllerAfter:)usecurrentIndexto determine which adjacent page to return. After a cancelled or interrupted page transition,currentIndexcan become stale (not yet updated to reflect the actual displayed page). This causes the data source to return an incorrect view controller, leading to an internal state inconsistency inUIPageViewControllerthat triggers the_flushViewControllercrash.Fix: Use
viewControllerIndexMap.index(for: viewController)to look up the actual index of theviewControllerparameter passed byUIPageViewController, falling back tocurrentIndexonly when the view controller is not found in the map. This ensures the correct adjacent page is always returned regardless ofcurrentIndexstate.Safety: This change is backward-compatible.
viewControllerIndexMapalready tracks all managed view controllers and their indices. The fallback tocurrentIndexpreserves existing behavior for any edge case where the view controller is not in the map.Related Issues
Test Plan
swift buildpasses successfully