MethodSwizzling.swift

private let swizzling: (AnyClass, Selector, Selector) -> () = { forClass, originalSelector, swizzledSelector in
    let originalMethod = class_getInstanceMethod(forClass, originalSelector)
    let swizzledMethod = class_getInstanceMethod(forClass, swizzledSelector)
    method_exchangeImplementations(originalMethod!, swizzledMethod!)
}

extension UIViewController {
    static let swizzled: Void = {
        let originalSelector = #selector(viewDidLoad)
        let swizzledSelector = #selector(swizzled_viewDidload)
        swizzling(UIViewController.self, originalSelector, swizzledSelector)
    }()

    @objc func swizzled_viewDidload() {
        swizzled_viewDidload()
        print("view did load at" + String(self.description))
    }
}