Why are touchDragExit and touchDragEnter being called repetitively multiple times? - ios

Question

UIControlEventTouchDragExit is
"an event where a finger is dragged from within a control to outside
its bounds"
UIControlEventTouchDragEnter is
"an event where a finger is dragged into the bounds of the control"
If I simulate a constant drag down, essentially exiting the bounds of the control once, why are touchDragExit and touchDragEnter being called multiple times?
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
}
}
}

Answer 1

Consider the size of the end of your finger compared to the size of a pixel (particularly on a Retina display). There's a lot of room for error in that relative difference. The OS has to make some estimations to figure out exactly where your finger is "pointing" on the screen and as you wiggle your finger that estimation might change slightly. As a result figuring out whether your finger is inside or outside of a one pixel boundary can be a bit tough and some fluctuation is reasonable.