a.swift

func iterateOverViewAndSubviews(view: UIView, block: (UIView) -> Void) {
    block(view)
    for subview in view.subviews {
        iterateOverViewAndSubviews(view: subview, block: block)
    }
}

func runCheckers() {
    if let keyWindow = UIApplication.shared.keyWindow {
        iterateOverViewAndSubviews(view: keyWindow) { (view) in
            if checkersFail() {
                fatalError()
            }
        }
    }
}