An attempt at creating a subclass of an unknown/hidden NSProxy type.

runtime-class.m

func test() {
    // Allocate a new class-pair from an unknown superclass type.
    let selector = Selector(("doStuff"))
    let oldClass_: AnyClass = Something.self
    let newClass_: AnyClass = objc_allocateClassPair(NSClassFromString("_NSObjectAnimator"), "MyNewClass", 0)!
    
    // Copy instance and class methods over.
    let m = class_getInstanceMethod(oldClass_, selector)!
    class_addMethod(newClass_, selector, method_getImplementation(m), method_getTypeEncoding(m))
    objc_registerClassPair(newClass_);
    
    // Required to avoid +isSubclassOfClass: failure due to NSProxy types.
    let oldClass = unsafeBitCast(oldClass_, to: NSObject.Type.self)
    let newClass = unsafeBitCast(newClass_, to: NSObject.Type.self)
    
    // Test the newly cloned subclass!
    print("old!")
    _ = oldClass.perform("alloc").takeUnretainedValue().perform(Selector(("initWithTarget:"))).takeUnretainedValue().perform(selector)
    print("new!")
    _ = newClass.perform("alloc").takeUnretainedValue().perform(Selector(("initWithTarget:"))).takeUnretainedValue().perform(selector)
}

@objc public class Something: NSObject {
    @objc(initWithTarget:)
    public init(_ target: Any) {
        super.init()
        print("**** INIT \(target) ****")
    }
    @objc public func doStuff() {
        print("\(type(of: self)) stuff done!")
    }
}

@_silgen_name("objc_alloc")
private func objc_alloc(_ cls: AnyClass!) -> AnyObject!

/*
old!
**** INIT 2001-01-01 00:00:00 +0000 ****
Something stuff done!
new!
MyNewClass stuff done!
*/