Swift PanGesture : Problems Managing Constraints: Flash on New Pan - ios

Question

This is the code used to control a view with a pan-capable View.
import UIKit
class ViewController: UIViewController {
#IBOutlet weak var redRect: PaletteView!
#IBOutlet var paletteTop : NSLayoutConstraint?
#IBOutlet var paletteLeading : NSLayoutConstraint?
override func viewDidLoad() {
super.viewDidLoad()
addRedViewUI(tgt: redRect!)
redRect.addGestureRecognizer(UIPanGestureRecognizer(target: self, action: #selector(palettePan(_:))))
redRect?.translatesAutoresizingMaskIntoConstraints = false
NSLayoutConstraint.activate(paletteConstraints())
}
func addRedViewUI(tgt : PaletteView) {
var pos = CGPoint(x: 10, y: 10)
var size = CGSize(width: 200, height: 240)
var v = UIView(frame: CGRect(origin: pos, size: size))
v.backgroundColor = .black
tgt.addSubview(v)
}
func paletteConstraints() -> Array<NSLayoutConstraint> {
var constraints : Array<NSLayoutConstraint> = []
constraints.append((redRect?.leadingAnchor.constraint( equalTo: view.leadingAnchor, constant: 100))!)
paletteLeading = constraints.last
constraints.append((redRect?.topAnchor.constraint( equalTo: view.topAnchor, constant: 100))!)
paletteTop = constraints.last
constraints.append((redRect?.widthAnchor.constraint(equalToConstant: 250))! )
constraints.append((redRect?.heightAnchor.constraint(equalToConstant: 350))! )
return constraints
}
func paletteConstraints(_ tgtView : PaletteView ) -> Array<NSLayoutConstraint> {
var constraints : Array<NSLayoutConstraint> = []
constraints.append((tgtView.leadingAnchor.constraint( equalTo: redRect!.leadingAnchor)))
constraints.append((tgtView.topAnchor.constraint( equalTo: redRect!.topAnchor)))
return constraints
}
#objc func palettePan(_ sender: UIPanGestureRecognizer) {
let translation = sender.translation(in: view)
switch sender.state {
case .began:
print("start \(redRect!.frame)")
paletteLeading?.constant = redRect!.frame.origin.x
paletteTop?.constant = redRect!.frame.origin.y
case .changed:
redRect!.transform = CGAffineTransform(translationX: translation.x, y: translation.y)
case .ended:
print("done \(redRect!.frame)")
default:
_ = true
}
}
}
class PaletteView : UIView {
var lastPos: CGPoint = CGPoint(x: 20, y: 20)
convenience init() {
self.init(frame: CGRect(x: 10, y: 10, width: 200, height: 200))
translatesAutoresizingMaskIntoConstraints = false
}
}
It works fine, except that when beginning a new pan, it jumps from its position from the end of the last pan, before instantaneously jumping back and the pan can be done. This jump is in the same direction as the last pan, so the second pan the view won't jump, and subsequent pans it jumps in the direction that the last pan went, in roughly the same amount.
This makes me think that it's something associated with the sender state, but I can't figure out what. If the amount of the last pan is small, or the pan is begun slowly, there's no flash, but I haven't been able to understand where that comes from.
The view being panned has no constraints in the storyBoard, but it does get defined and connected to its outlet in the storyboard.

Answer 1

The solution is to change the function so both start and changed perform the CGAffineTransform, after the constraints are updated in the case of changed:
#objc func palettePan(_ sender: UIPanGestureRecognizer) {
let translation = sender.translation(in: view)
switch sender.state {
case .began:
print("start \(redRect!.frame)")
paletteLeading?.constant = redRect!.frame.origin.x
paletteTop?.constant = redRect!.frame.origin.y
redRect!.transform = CGAffineTransform(translationX: translation.x, y: translation.y)
case .changed:
redRect!.transform = CGAffineTransform(translationX: translation.x, y: translation.y)
case .ended:
print("done \(redRect!.frame)")
default:
_ = true
}
}