UIControlEventTouchDragExit是
“将手指从控件内拖到其边界外的事件”
UIControlEventTouchDragEnter是
“将手指拖入控件边界的事件”
如果我模拟一个恒定的向下拖动，本质上是退出控件边界一次，为什么touchDragExit和touchDragEnter会被调用多次？

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        let btn = CustomButton(frame: CGRect(x: 100, y: 100, width: 100, height: 100), image:UIImage())
        btn.setTitle("", for: .normal)
        btn.backgroundColor = UIColor.green
        self.view.addSubview(btn)
    }
}

class CustomButton: UIButton {

    init(frame: CGRect, image:UIImage?) {
        super.init(frame: frame)
        self.addTargets()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }

    private func addTargets() {
        self.addTarget(self, action: #selector(self.touchDown), for: UIControlEvents.touchDown)
        self.addTarget(self, action: #selector(self.touchUpInside), for: UIControlEvents.touchUpInside)
        self.addTarget(self, action: #selector(self.touchDragExit), for: UIControlEvents.touchDragExit)
        self.addTarget(self, action: #selector(self.touchDragEnter), for: UIControlEvents.touchDragEnter)
        self.addTarget(self, action: #selector(self.touchCancel), for: UIControlEvents.touchCancel)
    }

    func touchDown() {
        print("touched down")
        UIView.animate(withDuration: 0.05, animations: {
            self.transform = CGAffineTransform(scaleX: 0.9, y: 0.9)
        },completion: nil)
    }

    func touchUpInside() {
        print("touch up inside")
        UIView.animate(withDuration: 0.7, delay: 0.0, usingSpringWithDamping: 0.2, initialSpringVelocity: 9.0, options: [.curveEaseInOut, .allowUserInteraction], animations: {
            self.transform = CGAffineTransform.identity

        }, completion: nil)
    }

    func touchDragExit() {
        print("touch drag exit")
        UIView.animate(withDuration: 0.7, delay: 0.0, usingSpringWithDamping: 1.0, initialSpringVelocity: 0.0, options: [.curveEaseInOut], animations: {
            self.transform = CGAffineTransform.identity

        }, completion: nil)

    }

    func touchDragEnter() {
        print("touch drag enter")
        UIView.animate(withDuration: 0.05, animations: {
            self.transform = CGAffineTransform(scaleX: 0.9, y: 0.9)
        },completion: nil)
    }

    func touchCancel() {
        print("touch canceled")
        UIView.animate(withDuration: 0.05) {
            self.transform = CGAffineTransform.identity
        }
    }

}