How to implement method swizzling swift 3.0?

Question

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

Answer 1

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)")
    }
}

Answer 2

@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")
    }
    
}

Answer 3

Swizzling in Playground
Swift 5

import Foundation
class TestSwizzling : NSObject {
    @objc dynamic func methodOne()->Int{
        return 1
    }
}

extension TestSwizzling {
    @objc func methodTwo()->Int{
        // It will not be a recursive call anymore after the swizzling
        return 2
    }
    //In Objective-C you'd perform the swizzling in load(),
    //but this method is not permitted in Swift
    func swizzle(){
        let i: () -> () = {
            let originalSelector = #selector(TestSwizzling.methodOne)
            let swizzledSelector = #selector(TestSwizzling.methodTwo)
            let originalMethod = class_getInstanceMethod(TestSwizzling.self, originalSelector);
            let swizzledMethod = class_getInstanceMethod(TestSwizzling.self, swizzledSelector)
            method_exchangeImplementations(originalMethod!, swizzledMethod!)
            print("swizzled!")
        }
        i()
    }
}

var c = TestSwizzling()
print([c.methodOne(), c.methodTwo()])  // [1, 2]
c.swizzle()                            // swizzled!
print([c.methodOne(), c.methodTwo()])  // [2, 1]

Answer 4

Swift swizzling

@objcMembers
class sA {
    dynamic
    class func sClassFooA() -> String {
        return "class fooA"
    }
    
    dynamic
    func sFooA() -> String {
        return "fooA"
    }
}

@objcMembers
class sB {
    dynamic
    class func sClassFooB() -> String {
        return "class fooB"
    }
    
    dynamic
    func sFooB() -> String {
        return "fooB"
    }
}

Swizzling.swift

import Foundation

@objcMembers
public class Swizzling: NSObject {
    
    public class func sExchangeClass(cls1: AnyClass, sel1: Selector, cls2: AnyClass, sel2: Selector) {
        
        let originalMethod = class_getClassMethod(cls1, sel1)
        let swizzledMethod = class_getClassMethod(cls2, sel2)
        
        method_exchangeImplementations(originalMethod!, swizzledMethod!)
    }

    public class func sExchangeInstance(cls1: AnyClass, sel1: Selector, cls2: AnyClass, sel2: Selector) {
        
        let originalMethod = class_getInstanceMethod(cls1, sel1)
        let swizzledMethod = class_getInstanceMethod(cls2, sel2)
        
        method_exchangeImplementations(originalMethod!, swizzledMethod!)
    }
}

Using via Swift

func testSExchangeClass() {
    Swizzling.sExchangeClass(cls1: sA.self, sel1: #selector(sA.sClassFooA), cls2: sB.self, sel2: #selector(sB.sClassFooB))
    
    XCTAssertEqual("class fooB", sA.sClassFooA())
}

func testSExchangeInstance() {
    Swizzling.sExchangeInstance(cls1: sA.self, sel1: #selector(sA.sFooA), cls2: sB.self, sel2: #selector(sB.sFooB))
    
    XCTAssertEqual("fooB", sA().sFooA())
}

[Add Objective-C as an consumer]

using via Objective-C

- (void)testSExchangeClass {
    [Swizzling sExchangeClassWithCls1:[cA class] sel1:@selector(cClassFooA) cls2:[cB class] sel2:@selector(cClassFooB)];
    
    XCTAssertEqual(@"class fooB", [cA cClassFooA]);
}

- (void)testSExchangeInstance {
    [Swizzling sExchangeInstanceWithCls1:[cA class] sel1:@selector(cFooA) cls2:[cB class] sel2:@selector(cFooB)];
    
    XCTAssertEqual(@"fooB", [[[cA alloc] init] cFooA]);
}

[Objective-C swizzling]