MethodSwizzling.swift

import Foundation

public struct Swizzle {
    private init() {}

    public static func classMethod(_ cls: AnyClass, to: Selector, from: Selector) {
        let cls: AnyClass = object_getClass(cls)!
        let toMethod = class_getInstanceMethod(cls, to)
        let fromMethod = class_getInstanceMethod(cls, from)

        if class_addMethod(cls, from, method_getImplementation(toMethod!), method_getTypeEncoding(toMethod!)) {
            class_replaceMethod(cls, to, method_getImplementation(fromMethod!), method_getTypeEncoding(fromMethod!))
        } else {
            method_exchangeImplementations(fromMethod!, toMethod!)
        }
    }

    public static func instanceMethod(_ cls: AnyClass, to: Selector, from: Selector) {
        let toMethod = class_getInstanceMethod(cls, to)
        let fromMethod = class_getInstanceMethod(cls, from)

        if class_addMethod(cls, from, method_getImplementation(toMethod!), method_getTypeEncoding(toMethod!)) {
            class_replaceMethod(cls, to, method_getImplementation(fromMethod!), method_getTypeEncoding(fromMethod!))
        } else {
            method_exchangeImplementations(fromMethod!, toMethod!)
        }
    }
}

class Foo: NSObject {
    @objc static var staticVar: String {
        return "original"
    }

    @objc static func staticFunc() -> String {
        return "original"
    }

    @objc var instanceVar: String {
        return "original"
    }

    @objc func instanceFunc() -> String {
        return "original"
    }
//}
//
//extension Foo {
    @objc static var newStaticVar: String {
        return "new"
    }

    @objc static func newStaticFunc() -> String {
        return "new"
    }

    @objc var newInstanceVar: String {
        return "new"
    }

    @objc func newInstanceFunc() -> String {
        return "new"
    }
}

Swizzle.classMethod(
    Foo.self,
    to: #selector(getter: Foo.newStaticVar),
    from: #selector(getter: Foo.staticVar)
)
Swizzle.classMethod(
    Foo.self,
    to: #selector(Foo.newStaticFunc),
    from: #selector(Foo.staticFunc)
)

Swizzle.instanceMethod(
    Foo.self,
    to: #selector(getter: Foo.newInstanceVar),
    from: #selector(getter: Foo.instanceVar)
)
Swizzle.instanceMethod(
    Foo.self,
    to: #selector(Foo.newInstanceFunc),
    from: #selector(Foo.instanceFunc)
)

print(Foo.staticVar)
print(Foo.staticFunc())
print(Foo().instanceVar)
print(Foo().instanceFunc())

print(Foo.newStaticVar)
print(Foo.newStaticFunc())
print(Foo().newInstanceVar)
print(Foo().newInstanceFunc())