How to implement method swizzling swift 3.0?

How can I implement method swizzling in Swift 3.0 ?

I've read nshipster article about it, but in this code's chunk

struct Static {
    static var token: dispatch_once_t = 0
}

the compiler gives me an error

dispatch_once_t is unavailable in Swift: Use lazily initialized globals instead

First of all dispatch_once_t is unavailable in Swift 3.0. You can choose from two alternatives:

1. Global variable

2. Static property of struct, enum or class

For more details, see that Whither dispatch_once in Swift 3

For different purposes you must use different implementation of swizzling

• Swizzling CocoaTouch class, for example UIViewController;
• Swizzling custom Swift class;

Swizzling CocoaTouch class

example swizzling viewWillAppear(_:) of UIViewController using global variable

private let swizzling: (UIViewController.Type) -> () = { viewController in

    let originalSelector = #selector(viewController.viewWillAppear(_:))
    let swizzledSelector = #selector(viewController.proj_viewWillAppear(animated:))

    let originalMethod = class_getInstanceMethod(viewController, originalSelector)
    let swizzledMethod = class_getInstanceMethod(viewController, swizzledSelector)

    method_exchangeImplementations(originalMethod, swizzledMethod) }

extension UIViewController {

    open override class func initialize() {
        // make sure this isn't a subclass
        guard self === UIViewController.self else { return }
        swizzling(self)
    }

    // MARK: - Method Swizzling

    func proj_viewWillAppear(animated: Bool) {
        self.proj_viewWillAppear(animated: animated)

        let viewControllerName = NSStringFromClass(type(of: self))
        print("viewWillAppear: \(viewControllerName)")
    } 
 }

Swizzling custom Swift class

To use method swizzling with your Swift classes there are two requirements that you must comply with (for more details):

• The class containing the methods to be swizzled must extend NSObject
• The methods you want to swizzle must have the dynamic attribute

And example swizzling method of custom Swift base class Person

class Person: NSObject {
    var name = "Person"
    dynamic func foo(_ bar: Bool) {
        print("Person.foo")
    }
}

class Programmer: Person {
    override func foo(_ bar: Bool) {
        super.foo(bar)
        print("Programmer.foo")
    }
}

private let swizzling: (Person.Type) -> () = { person in

    let originalSelector = #selector(person.foo(_:))
    let swizzledSelector = #selector(person.proj_foo(_:))

    let originalMethod = class_getInstanceMethod(person, originalSelector)
    let swizzledMethod = class_getInstanceMethod(person, swizzledSelector)

    method_exchangeImplementations(originalMethod, swizzledMethod)
}

extension Person {

    open override class func initialize() {
        // make sure this isn't a subclass
        guard self === Person.self else { return }
        swizzling(self)
    }

    // MARK: - Method Swizzling

    func proj_foo(_ bar: Bool) {
        self.proj_foo(bar)

        let className = NSStringFromClass(type(of: self))
        print("class: \(className)")
    }
}

@TikhonovAlexander: Great answer

I modified the swizzler to take both selectors and made it more generic.

Swift 4 / Swift 5

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

extension UIView {
    
    static let classInit: Void = {            
        let originalSelector = #selector(layoutSubviews)
        let swizzledSelector = #selector(swizzled_layoutSubviews)
        swizzling(UIView.self, originalSelector, swizzledSelector)
    }()
    
    @objc func swizzled_layoutSubviews() {
        swizzled_layoutSubviews()
        print("swizzled_layoutSubviews")
    }
    
}

// perform swizzling in AppDelegate.init()

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

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

}

Swift 3

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)
}

// perform swizzling in initialize()

extension UIView {
    
    open override class func initialize() {
        // make sure this isn't a subclass
        guard self === UIView.self else { return }
        
        let originalSelector = #selector(layoutSubviews)
        let swizzledSelector = #selector(swizzled_layoutSubviews)
        swizzling(self, originalSelector, swizzledSelector)
    }
    
    func swizzled_layoutSubviews() {
        swizzled_layoutSubviews()
        print("swizzled_layoutSubviews")
    }
    
}