Generate dynamic subclasses of Objective-C objects in Swift

DynamicSubclassGenerator.swift

import Foundation
import ObjectiveC

private var subclassCounts: [String: Int] = [:]

func GenerateDynamicSubclass<T: NSObject>(of class: T.Type) -> T.Type! {
    let oldClassName = NSStringFromClass(`class`) as String
    var subclassCount = subclassCounts[oldClassName] ?? 0
    let className = "\(oldClassName)_DynamicSubclass_\(subclassCount)"
    
    subclassCounts[oldClassName] = subclassCount + 1
    
    guard let newClass = objc_allocateClassPair(T.self, (className as NSString).utf8String!, 0) else { return nil }
    objc_registerClassPair(newClass)
    
    return newClass as! T.Type
}

GenerateDynamicSubclass(of: NSObject.self).init().debugDescription

// <NSObject_DynamicSubclass_0: 0x101066c30>