Swift Language Method Swizzling Extending UIViewController and Swizzling viewDidLoad
Example
In Objective-C, method swizzling is the process of changing the implementation of an existing selector. This is possible due to the way Selectors are mapped on a dispatch table, or a table of pointers to functions or methods.
Pure Swift methods are not dynamically dispatched by the Objective-C runtime, but we can still take advantage of these tricks on any class that inherits from NSObject.
Here, we will extend UIViewController and swizzle viewDidLoad to add some custom logging:
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()
}
}
