Button Spring Behavior

ButtonSpringBehavior.swift

import UIKit

var SpringKey :Int = 0

class ButtonSpringBehavior: NSObject {
    var scaleFactor :CGFloat = 1.3
    init(button: UIButton) {
        super.init()
        
        button.addTarget(self, action: #selector(springOut(sender:)), for: UIControlEvents.touchDown)
        button.addTarget(self, action: #selector(restore(sender:)), for: UIControlEvents.touchUpInside)
        button.addTarget(self, action: #selector(restore(sender:)), for: UIControlEvents.touchDragExit)
        objc_setAssociatedObject(button, &SpringKey, self, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
    }
    
    class func addToButton(button: UIButton) {
        _ = ButtonSpringBehavior(button: button)
    }
    
    @objc private func springOut(sender: UIButton!) {
        UIView.animate(withDuration: 0.3, delay: 0,
                       usingSpringWithDamping: 0.5, initialSpringVelocity: 0,
                       options: UIViewAnimationOptions.allowUserInteraction,
                       animations: { () -> Void in
                        sender.layer.transform = CATransform3DMakeScale(self.scaleFactor, self.scaleFactor, 1.0)
        }, completion: nil)
    }
    
    @objc private func restore(sender: UIButton!) {
        UIView.animate(withDuration: 0.3, delay: 0,
                       usingSpringWithDamping: 0.5, initialSpringVelocity: 0,
                       options: UIViewAnimationOptions.allowUserInteraction,
                       animations: { () -> Void in
                        sender.layer.transform = CATransform3DMakeScale(1.0, 1.0, 1.0)
        }, completion: nil)
    }
}