关于Method Swizzling 原理什么的有很多帖子讲述的已经很清楚这里不再赘述，

这里仅仅处理Method Swizzling 在swift4.0中的使用方法

因为Swift本身对Runtime的支持并不是很到位，尤其是Method-Swizzling在OC中非常常用，但是到Swift后发现load方法不见了进而需要用initialize代替，甚至到了Swift4中直接取消了initialize方法。因此需要自己初始化

解决方案需要在appdelegate 添加这一行代码
 UIViewController.initializeMethod()

/**需要在appdelegate 添加这一行代码UIViewController.initializeMethod()*/

private let onceToken = "Method Swizzling"extension UIViewController {public class func initializeMethod() {// Make sure This isn't a subclass of UIViewController, So that It applies to all UIViewController childsif self != UIViewController.self {return}//DispatchQueue函数保证代码只被执行一次，防止又被交换回去导致得不到想要的效果DispatchQueue.once(token: onceToken) {let originalSelector = #selector(UIViewController.viewWillAppear(_:))let swizzledSelector = #selector(UIViewController.swizzled_viewWillAppear(animated:))let originalMethod = class_getInstanceMethod(self, originalSelector)let swizzledMethod = class_getInstanceMethod(self, swizzledSelector)//在进行 Swizzling 的时候,需要用 class_addMethod 先进行判断一下原有类中是否有要替换方法的实现let didAddMethod: Bool = class_addMethod(self, originalSelector, method_getImplementation(swizzledMethod!), method_getTypeEncoding(swizzledMethod!))//如果 class_addMethod 返回 yes,说明当前类中没有要替换方法的实现,所以需要在父类中查找,这时候就用到 method_getImplemetation 去获取 class_getInstanceMethod 里面的方法实现,然后再进行 class_replaceMethod 来实现 Swizzingif didAddMethod {class_replaceMethod(self, swizzledSelector, method_getImplementation(originalMethod!), method_getTypeEncoding(originalMethod!))} else {method_exchangeImplementations(originalMethod!, swizzledMethod!)}let originalSelector1 = #selector(UIViewController.viewWillDisappear(_:))let swizzledSelector1 = #selector(UIViewController.swizzled_viewWillDisappear(animated:))let originalMethod1 = class_getInstanceMethod(self, originalSelector1)let swizzledMethod1 = class_getInstanceMethod(self, swizzledSelector1)//在进行 Swizzling 的时候,需要用 class_addMethod 先进行判断一下原有类中是否有要替换方法的实现let didAddMethod1: Bool = class_addMethod(self, originalSelector1, method_getImplementation(swizzledMethod1!), method_getTypeEncoding(swizzledMethod1!))if didAddMethod1 {class_replaceMethod(self, swizzledSelector1, method_getImplementation(originalMethod1!), method_getTypeEncoding(originalMethod1!))} else {method_exchangeImplementations(originalMethod1!, swizzledMethod1!)}}}@objc func swizzled_viewWillAppear(animated: Bool) {//需要注入的代码写在此处self.swizzled_viewWillAppear(animated: animated)DDLOG(message: "\(NSStringFromClass(classForCoder))--Appear")}@objc func swizzled_viewWillDisappear(animated: Bool) {//需要注入的代码写在此处self.swizzled_viewWillDisappear(animated: animated)DDLOG(message: "\(NSStringFromClass(classForCoder))--Disappear")}
}

由于swift 没有DispatchQueue.once 方法 所以手动扩展了一个 方便使用

extension DispatchQueue {private static var _onceTracker = [String]()public class func once(token: String, block: () -> ()) {objc_sync_enter(self)defer {objc_sync_exit(self)}if _onceTracker.contains(token) {return}_onceTracker.append(token)block()}func async(block: @escaping ()->()) {self.async(execute: block)}func after(time: DispatchTime, block: @escaping ()->()) {self.asyncAfter(deadline: time, execute: block)}
}