UIViewPropertyAnimator not stopping properly with pan gesture between views - swift

Question

I have a view controller that setups 3 children view controllers, puts the three views side by side, and centers the middle view.. like snapchat. The sliding between view controllers is controlled by using a pangesture that adjusts the left anchor constraint of the far left child view controller.
Right now... when I pan and then let go it will go to the proper constraint.
THE ISSUE: when I pan, let go, pan again the constraints snap back and the animation isn't looking proper. Any insight would be appreciated....
this link shows what I mean by the constraint and animations snapping and being funky!! (the first pan is proper, but when i try to interrupt the second pan you see my issue)
https://giphy.com/gifs/mEDld5sbR81gJBHcl1
import UIKit
class ViewController: UIViewController {
private let viewControllerOne = UIViewController()
private let viewControllerTwo = UIViewController()
private let viewControllerThree = UIViewController()
private var viewControllerOneLeftAnchorConstraint: NSLayoutConstraint?
override func loadView() {
view = UIView()
view.backgroundColor = .white
//removed code for stackoverflow readability, here i setup view controllers and pangestures
guard let viewControllerOneLeftAnchorConstraint = viewControllerOneLeftAnchorConstraint else { return }
NSLayoutConstraint.activate([
viewControllerOne.view.topAnchor.constraint(equalTo: view.topAnchor),
viewControllerTwo.view.topAnchor.constraint(equalTo: view.topAnchor),
viewControllerThree.view.topAnchor.constraint(equalTo: view.topAnchor),
viewControllerOne.view.bottomAnchor.constraint(equalTo: view.bottomAnchor),
viewControllerTwo.view.bottomAnchor.constraint(equalTo: view.bottomAnchor),
viewControllerThree.view.bottomAnchor.constraint(equalTo: view.bottomAnchor),
viewControllerOne.view.widthAnchor.constraint(equalTo: view.widthAnchor),
viewControllerTwo.view.widthAnchor.constraint(equalTo: view.widthAnchor),
viewControllerThree.view.widthAnchor.constraint(equalTo: view.widthAnchor),
viewControllerOneLeftAnchorConstraint,
viewControllerTwo.view.leftAnchor.constraint(equalTo: viewControllerOne.view.rightAnchor),
viewControllerThree.view.leftAnchor.constraint(equalTo: viewControllerTwo.view.rightAnchor),
])
}
private var animator: UIViewPropertyAnimator?
private var previous: CGPoint?
private var current: CGPoint?
#objc private func handlePan(gesture: UIPanGestureRecognizer) {
guard let viewControllerOneLeftAnchorConstraint = viewControllerOneLeftAnchorConstraint else { return }
if gesture.state == .began {
previous = gesture.location(in: view)
animator?.stopAnimation(true)
} else if gesture.state == .changed {
let current = gesture.location(in: view)
guard let previous = previous else { return }
let xPositionDifference = current.x - previous.x
if viewControllerOneLeftAnchorConstraint.constant + xPositionDifference < 0 && viewControllerOneLeftAnchorConstraint.constant + xPositionDifference > -(UIScreen.main.bounds.width * 2) {
self.viewControllerOneLeftAnchorConstraint?.constant += xPositionDifference
self.previous = current
}
} else if gesture.state == .ended {
viewControllerOneLeftAnchorConstraint.constant = calculateLeftAnchorConstant(viewControllerOneLeftAnchorConstraint: viewControllerOneLeftAnchorConstraint)
animator = UIViewPropertyAnimator(duration: 2.0, curve: .easeOut) {
self.view.layoutIfNeeded()
}
animator?.startAnimation()
}
}
func calculateLeftAnchorConstant(viewControllerOneLeftAnchorConstraint: NSLayoutConstraint) -> CGFloat {
if viewControllerOneLeftAnchorConstraint.constant <= 0
&& viewControllerOneLeftAnchorConstraint.constant > -(UIScreen.main.bounds.width/2.0) {
return 0.0
} else if viewControllerOneLeftAnchorConstraint.constant > -(UIScreen.main.bounds.width/2.0 * 3)
&& viewControllerOneLeftAnchorConstraint.constant < -(UIScreen.main.bounds.width/2.0) {
return -(UIScreen.main.bounds.width)
} else {
return -(UIScreen.main.bounds.width * 2)
}
}
}

Answer 1

I'd advise you to abandon the entirety of your code. Your words and your animated gif are describing a UIPageViewController, so why not use a UIPageViewController?