CAN LAYOUTIFNEEDED BE USED IN VIEWDIDLOAD TO DETERMINE A CORNER RADIUS SIZE? IN IOS

You can call it in viewDidLayoutSubviews() if you have issue.

From Apple iOS 10 Release notes is:

Sending layoutIfNeeded to a view is not expected to move the view, but in earlier releases, if the view had translatesAutoresizingMaskIntoConstraints set to NO, and if it was being positioned by constraints, layoutIfNeeded would move the view to match the layout engine before sending layout to the subtree. These changes correct this behavior, and the receiver’s position and usually its size won’t be affected by layoutIfNeeded.

Some existing code may be relying on this incorrect behavior that is now corrected. There is no behavior change for binaries linked before iOS 10, but when building on iOS 10 you may need to correct some situations by sending -layoutIfNeeded to a superview of the translatesAutoresizingMaskIntoConstraints view that was the previous receiver, or else positioning and sizing it before (or after, depending on your desired behavior) layoutIfNeeded.

Third party apps with custom UIView subclasses using Auto Layout that override layoutSubviews and dirty layout on self before calling super are at risk of triggering a layout feedback loop when they rebuild on iOS 10. When they are correctly sent subsequent layoutSubviews calls they must be sure to stop dirtying layout on self at some point (note that this call was skipped in release prior to iOS 10)."

Essentially you cannot call layoutIfNeeded on a child object of the View - now calling layoutIfNeeded has to be on the superView, and you can still call this in viewDidLayoutSubviews.

I think a better approach would be to do this either in viewWillAppear or viewDidLayoutSubviews

Those are times when the frame is actually finalized. viewDidLayoutSubviews is good because it will react to changes in the views frame.

If you have a requirement of different cornerRadius values for different size classes, I would use traitCollectionDidChange instead of viewWillLayoutSubviews or viewDidLayoutSubviews.

override func traitCollectionDidChange(previousTraitCollection: UITraitCollection?) is called only when the iOS interface environment changes. If the view is being loaded for the first time, previousTraitCollection will be nil.

That way, if your application supports multitasking, as soon as the view size changes to show two apps on your device screen at once, your views and will resize properly to fit the new size class. Using the layoutSubviews methods will also work, but might put additional processing strain on your application.