Some properties changed from
UIKit from iOS 9 to iOS 10. Because there are no
@available guards for optionality changes, how should these be handled? Are they safe to call for apps that target iOS 9, or does that implicitly unwrap an Optional?
One such property that changed its nullability attributes is
UIViewControllerContextTransitioning, which changed the return value of
public func containerView() from
UIView. AnimatedTransitioning.swift and OptionalityChangeWrapper.swift explore one way to workaround these changes in a safer way.