Open Radar

 
Area:
UIKit

Summary:
See title. A subview in the view of a child view controller updated via `setOverrideTraitCollection` is given an incorrect `previousTraitCollection` when `preferredContentSizeCategory` is involved. Since many implementations (including those backing `UILabel.adjustsFontForContentSizeCategory` et. al.) check for duplicate content size categories in order to not do duplicate work, these subviews never get the effect of the override trait collection.

Steps to Reproduce:
See attached sample code.

1. Set up a view controller containing another view controller using "Container View" in the Interface Builder.
2. In the child view controller, set up a `UILabel` with `adjustsFontForContentSizeCategory = true`.
2. Call `setOverrideTraitCollection` from the parent view controller.

Expected Results:
I can drag the slider in the sample project to resize the label.

Observed Results:
The label does not automatically update its font size from a custom event.

Version:
iOS 11.0 Beta 1 (15A5278f) / Xcode Version 9.0 beta (9M136h)

Also occurs on iOS 10.3 (14E8301)

Notes:
A workaround was not forthcoming. The root view of a UIViewController propagates trait notifications down asynchronously, after the view controller has re-cached the new trait collection as its "previously notified" trait collection.

Configuration:
N/A