iOS 13 Dark mode: traitCollectionDidChange only called the first time

Question

My app uses custom color themes but iOS13 users can opt into following dark mode.

I thought I could simply update my colors in the ViewController's traitCollectionDidChange() but for some reason, this function is only called the first time the user changes the interface style in the iOS settings. While this may be sufficient for most users, ideally traitCollectionDidChange() should be called every time the user changes their iOS settings.

Just very basic:

override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
  super.traitCollectionDidChange(previousTraitCollection)
  print(#function)

  guard traitCollection.userInterfaceStyle != previousTraitCollection?.userInterfaceStyle else { return }

  NotificationCenter.default.post(name: NSNotification.Name(Keys.Notifications.updateColorTheme), object: nil)
}

I launch my app, leave it, change appearance in the iOS settings: the next time I open the app (open, not launch), the above function is called and the app updates colors. Now, when I repeat the same process, the function is not called anymore.

Answer 1

I found that if I set UIViewController's overrideUserInterfaceStyle property, then the traitCollectionDidChange method not called, however when I don't set this overrideUserInterfaceStyle property, the traitCollectionDidChange is called.

maybe there have some internal method call judge in UIKit based on overrideUserInterfaceStyle property.

Hope this help.