Stored Properties In Swift Extensions - https://marcosantadev.com/stored-properties-swift-extensions/

objc_setAssociatedObject.playground

import UIKit

protocol PropertyStoring {
    associatedtype T
    func getAssociatedObject(_ key: UnsafeRawPointer, defaultValue: T) -> T
}

extension PropertyStoring {
    func getAssociatedObject(_ key: UnsafeRawPointer, defaultValue: T) -> T {
        guard let value = objc_getAssociatedObject(self, key) as? T else {
            return defaultValue
        }
        return value
    }
}

protocol ToggleProtocol {
    func toggle()
}

enum ToggleState {
    case on
    case off
}

extension UIButton: ToggleProtocol, PropertyStoring {
    typealias T = ToggleState
    
    private struct CustomProperties {
        static var toggleState = ToggleState.off
    }
    
    private(set) var toggleState: ToggleState {
        get {
            return getAssociatedObject(&CustomProperties.toggleState, defaultValue: CustomProperties.toggleState)
        }
        set {
            objc_setAssociatedObject(self, &CustomProperties.toggleState, newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
    }
    
    func toggle() {
        toggleState = toggleState == .on ? .off : .on
        
        if toggleState == .on {
            self.backgroundColor = .green
        } else {
            self.backgroundColor = .red
        }
    }
}

let button = UIButton()
print(button.toggleState)
button.toggle()
print(button.toggleState)