扩展 UIViewController 和 Swizzling viewDidLoad

在 Objective-C 中，方法调配是改变现有选择器的实现的过程。这可能是由于选择器映射到调度表的方式，或指向函数或方法的指针表。

Objective-C 运行时不会动态调度 Pure Swift 方法，但我们仍然可以在从 NSObject 继承的任何类上利用这些技巧。

在这里，我们将扩展 UIViewController 和 swizzle viewDidLoad 以添加一些自定义日志记录：

extension UIViewController {
    
    // We cannot override load like we could in Objective-C, so override initialize instead
    public override static func initialize() {
        
        // Make a static struct for our dispatch token so only one exists in memory
        struct Static {
            static var token: dispatch_once_t = 0
        }
        
        // Wrap this in a dispatch_once block so it is only run once
        dispatch_once(&Static.token) {
            // Get the original selectors and method implementations, and swap them with our new method
            let originalSelector = #selector(UIViewController.viewDidLoad)
            let swizzledSelector = #selector(UIViewController.myViewDidLoad)
            
            let originalMethod = class_getInstanceMethod(self, originalSelector)
            let swizzledMethod = class_getInstanceMethod(self, swizzledSelector)
            
            let didAddMethod = class_addMethod(self, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod))
            
            // class_addMethod can fail if used incorrectly or with invalid pointers, so check to make sure we were able to add the method to the lookup table successfully
            if didAddMethod {
                class_replaceMethod(self, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod))
            } else {
                method_exchangeImplementations(originalMethod, swizzledMethod);
            }
        }
    }
    
    // Our new viewDidLoad function
    // In this example, we are just logging the name of the function, but this can be used to run any custom code
    func myViewDidLoad() {
        // This is not recursive since we swapped the Selectors in initialize().
        // We cannot call super in an extension.
        self.myViewDidLoad()
        print(#function) // logs myViewDidLoad()
    }
}