swizzling.swift

extension UIView {
    static let swizzle: Void = {
        let originalSelector = #selector(layoutSubviews)
        let swizzledSelector = #selector(swizzled_layoutSubviews)

        guard let originalMethod = class_getInstanceMethod(UIView.self, originalSelector) else { return }
        guard let swizzledMethod = class_getInstanceMethod(UIView.self, swizzledSelector) else { return }
        method_exchangeImplementations(originalMethod, swizzledMethod)
    }()

    @objc func swizzled_layoutSubviews() {
        swizzled_layoutSubviews()

        // do stuff
    }
}

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    override init() {
        super.init()
        UIView.swizzle
    }

}