Animation bug when swiping UIView

Question

I'm developing an app which has a 3 view and which is a card view like in Tinder. I'm creating views in a for loop. When I have more than 4 views, everything works fine. When It has only 3 cards, everything looks okey at first ,when the app opens, but after swiping one card, It gets broken. Last card moves with some bug. I'm trying to edit the code to work with 3 card but can't figure out. By the way, ImageCard is just a UIView class.

EDIT: My problem is that when It has 3 cards, App opens with 3 cards shown on screen but after a swipe, last card doesn't show on the screen, only 2 cards shown in screen. After swipe card on the front should goes to backmost and 3 cards should be seen again. When It has more than 5 cards, everything works fine like I explained and 3 cards shown on screen (What It needs to be)

I'm sure showNextCard() function occurs the problem but to be sure here is the full code :

class WelcomeViewController: UIViewController {

/// Data structure for custom cards
var cards = [ImageCard]()

override func viewDidLoad() {
    super.viewDidLoad()
    dynamicAnimator = UIDynamicAnimator(referenceView: self.view)
    print(self.view.frame.height)
    print(self.view.frame.width)
    let screenWidth = self.view.frame.width
    let screenHeight = self.view.frame.height
    //When add new cards to self.cards and call layoutCards() again
    for i in 1...5 {
        let card = ImageCard(frame: CGRect(x: 0, y: 0, width: screenWidth - screenWidth / 5, height: screenWidth))
        card.tag = i
        card.label.text = "Card Number: \(i)"
        cards.append(card)
    }
    lastIndex = cards.count

    // 2. layout the first cards for the user
    layoutCards()

}

/// Scale and alpha of successive cards visible to the user
let cardAttributes: [(downscale: CGFloat, alpha: CGFloat)] = [(1, 1), (0.92, 0.8), (0.84, 0.6), (0.76, 0.4)]
let cardInteritemSpacing: CGFloat = 12


/// Set up the frames, alphas, and transforms of the first 4 cards on the screen
func layoutCards() {
    // frontmost card (first card of the deck)
    let firstCard = cards[0]
    self.view.addSubview(firstCard)
    firstCard.layer.zPosition = CGFloat(cards.count)
    firstCard.center = self.view.center
    firstCard.frame.origin.y += 23
    firstCard.addGestureRecognizer(UIPanGestureRecognizer(target: self, action: #selector(handleCardPan)))

    // the next 3 cards in the deck
    for i in 1...3 {
        if i > (cards.count - 1) { continue }

        let card = cards[i]

        card.layer.zPosition = CGFloat(cards.count - i)

        // here we're just getting some hand-picked vales from cardAttributes (an array of tuples)
        // which will tell us the attributes of each card in the 4 cards visible to the user
        let downscale = cardAttributes[i].downscale
        let alpha = cardAttributes[i].alpha
        card.transform = CGAffineTransform(scaleX: downscale, y: downscale)
        card.alpha = alpha
        // position each card so there's a set space (cardInteritemSpacing) between each card, to give it a fanned out look
        card.center.y = self.view.center.y + 23
        card.frame.origin.x = cards[0].frame.origin.x + (CGFloat(i) * cardInteritemSpacing * 3)
        // workaround: scale causes heights to skew so compensate for it with some tweaking
        if i == 3 {
            card.frame.origin.x += 1.5
        }

        self.view.addSubview(card)
    }

    // make sure that the first card in the deck is at the front
    self.view.bringSubview(toFront: cards[0])
}

/// This is called whenever the front card is swiped off the screen or is animating away from its initial position.
/// showNextCard() just adds the next card to the 4 visible cards and animates each card to move forward.
func showNextCard() {
    let animationDuration: TimeInterval = 0.2
    // 1. animate each card to move forward one by one
    for i in 1...3{
        if i > (cards.count - 1) { continue }
        let card = cards[i]
        let newDownscale = cardAttributes[i - 1].downscale
        let newAlpha = cardAttributes[i - 1].alpha
        UIView.animate(withDuration: animationDuration, delay: (TimeInterval(i - 1) * (animationDuration / 2)), usingSpringWithDamping: 0.8, initialSpringVelocity: 0.0, options: [], animations: {
            card.transform = CGAffineTransform(scaleX: newDownscale, y: newDownscale)
            card.alpha = newAlpha
            if i == 1 {
                card.center = self.view.center
                card.frame.origin.y += 23
            } else {
                card.center.y = self.view.center.y + 23
                card.frame.origin.x = self.cards[1].frame.origin.x + (CGFloat(i - 1) * self.cardInteritemSpacing * 3)
            }
        }, completion: { (_) in
            if i == 1 {
                card.addGestureRecognizer(UIPanGestureRecognizer(target: self, action: #selector(self.handleCardPan)))
            }
        })

    }

    // 2. add a new card (now the 4th card in the deck) to the very back
    if 4 > (cards.count - 1) {
        if cards.count != 1 {
            self.view.bringSubview(toFront: cards[1])
        }else{
            //self.view.bringSubview(toFront: cards.last!)
        }
        return
    }
    let newCard = cards[4]
    newCard.layer.zPosition = CGFloat(cards.count - 4)
    let downscale = cardAttributes[3].downscale
    let alpha = cardAttributes[3].alpha

    // initial state of new card
    newCard.transform = CGAffineTransform(scaleX: 0.5, y: 0.5)
    newCard.alpha = 0
    newCard.center.y = self.view.center.y + 23
    newCard.frame.origin.x = cards[1].frame.origin.x + (4 * cardInteritemSpacing * 3)
    self.view.addSubview(newCard)

    // animate to end state of new card
    UIView.animate(withDuration: animationDuration, delay: (3 * (animationDuration / 2)), usingSpringWithDamping: 0.8, initialSpringVelocity: 0.0, options: [], animations: {
        newCard.transform = CGAffineTransform(scaleX: downscale, y: downscale)
        newCard.alpha = alpha
        newCard.center.y = self.view.center.y + 23
        newCard.frame.origin.x = self.cards[1].frame.origin.x + (3 * self.cardInteritemSpacing) + 1.5
    }, completion: { (_) in

    })
    // first card needs to be in the front for proper interactivity
    self.view.bringSubview(toFront: self.cards[1])

}

/// Whenever the front card is off the screen, this method is called in order to remove the card from our data structure and from the view.
func removeOldFrontCard() {
    cards.append(cards[0])
    cards[0].removeFromSuperview()
    cards.remove(at: 0)
    layoutCards()
}

private func isVerticalGesture(_ recognizer: UIPanGestureRecognizer) -> Bool {
    let translation = recognizer.translation(in: self.view!)
    if fabs(translation.y) > fabs(translation.x) {
        return true
    }
    return false
}

/// UIKit dynamics variables that we need references to.
var dynamicAnimator: UIDynamicAnimator!
var cardAttachmentBehavior: UIAttachmentBehavior!

/// This method handles the swiping gesture on each card and shows the appropriate emoji based on the card's center.
@objc func handleCardPan(sender: UIPanGestureRecognizer) {

    // Ensure it's a horizontal drag
    let velocity = sender.velocity(in: self.view)
    if abs(velocity.y) > abs(velocity.x) {
        return
    }

    // if we're in the process of hiding a card, don't let the user interace with the cards yet
    if cardIsHiding { return }
    // change this to your discretion - it represents how far the user must pan up or down to change the option
    // distance user must pan right or left to trigger an option
    let requiredOffsetFromCenter: CGFloat = 80

    let panLocationInView = sender.location(in: view)
    let panLocationInCard = sender.location(in: cards[0])

    switch sender.state {
    case .began:
        dynamicAnimator.removeAllBehaviors()
        let offset = UIOffsetMake(cards[0].bounds.midX, panLocationInCard.y)
        // card is attached to center
        cardAttachmentBehavior = UIAttachmentBehavior(item: cards[0], offsetFromCenter: offset, attachedToAnchor: panLocationInView)
        //dynamicAnimator.addBehavior(cardAttachmentBehavior)
        let translation = sender.translation(in: self.view)
        print(sender.view!.center.x)

        if(sender.view!.center.x < 555) {

            sender.view!.center = CGPoint(x: sender.view!.center.x + translation.x, y: sender.view!.center.y)

        }else {
            sender.view!.center = CGPoint(x:sender.view!.center.x, y:554)
        }
        sender.setTranslation(CGPoint(x: 0, y: 0), in: self.view)
    case .changed:
        //cardAttachmentBehavior.anchorPoint = panLocationInView
        let translation = sender.translation(in: self.view)
        print(sender.view!.center.y)

        if(sender.view!.center.x < 555) {

            sender.view!.center = CGPoint(x: sender.view!.center.x + translation.x, y: sender.view!.center.y)

        }else {
            sender.view!.center = CGPoint(x:sender.view!.center.x, y:554)
        }
        sender.setTranslation(CGPoint(x: 0, y: 0), in: self.view)

    case .ended:

        dynamicAnimator.removeAllBehaviors()

        if !(cards[0].center.x > (self.view.center.x + requiredOffsetFromCenter) || cards[0].center.x < (self.view.center.x - requiredOffsetFromCenter)) {
            // snap to center
            let snapBehavior = UISnapBehavior(item: cards[0], snapTo: CGPoint(x: self.view.frame.midX, y: self.view.frame.midY + 23))
            dynamicAnimator.addBehavior(snapBehavior)
        } else {
            let velocity = sender.velocity(in: self.view)
            let pushBehavior = UIPushBehavior(items: [cards[0]], mode: .instantaneous)
            pushBehavior.pushDirection = CGVector(dx: velocity.x/10, dy: velocity.y/10)
            pushBehavior.magnitude = 175
            dynamicAnimator.addBehavior(pushBehavior)
            // spin after throwing
            var angular = CGFloat.pi / 2 // angular velocity of spin

            let currentAngle: Double = atan2(Double(cards[0].transform.b), Double(cards[0].transform.a))

            if currentAngle > 0 {
                angular = angular * 1
            } else {
                angular = angular * -1
            }
            let itemBehavior = UIDynamicItemBehavior(items: [cards[0]])
            itemBehavior.friction = 0.2
            itemBehavior.allowsRotation = true
            itemBehavior.addAngularVelocity(CGFloat(angular), for: cards[0])
            dynamicAnimator.addBehavior(itemBehavior)

            showNextCard()
            hideFrontCard()

        }
    default:
        break
    }
}

/// This function continuously checks to see if the card's center is on the screen anymore. If it finds that the card's center is not on screen, then it triggers removeOldFrontCard() which removes the front card from the data structure and from the view.
var cardIsHiding = false
func hideFrontCard() {
    if #available(iOS 10.0, *) {
        var cardRemoveTimer: Timer? = nil
        cardRemoveTimer = Timer.scheduledTimer(withTimeInterval: 0.1, repeats: true, block: { [weak self] (_) in
            guard self != nil else { return }
            if !(self!.view.bounds.contains(self!.cards[0].center)) {
                cardRemoveTimer!.invalidate()
                self?.cardIsHiding = true
                UIView.animate(withDuration: 0.2, delay: 0, options: [.curveEaseIn], animations: {
                    self?.cards[0].alpha = 0.0
                }, completion: { (_) in
                    self?.removeOldFrontCard()
                    self?.cardIsHiding = false
                })
            }
        })
    } else {
        // fallback for earlier versions
        UIView.animate(withDuration: 0.2, delay: 1.5, options: [.curveEaseIn], animations: {
            self.cards[0].alpha = 0.0
        }, completion: { (_) in
            self.removeOldFrontCard()
        })
    }
}
}

ImageCard Class:

class ImageCard: UIView {
let label = UILabel()
override init(frame: CGRect) {
    super.init(frame: frame)

    // card style
    self.backgroundColor = UIColor.blue

    self.layer.cornerRadius = 26

    label.font = Font.gothamBold?.withSize(30)
    label.textColor = UIColor.white
    self.addSubview(label)
    label.anchor(self.topAnchor, left: self.leftAnchor, bottom: nil, right: nil, topConstant: 0, leftConstant: 0, bottomConstant: 0, rightConstant: 0, widthConstant: 0, heightConstant: 0)
}

required init?(coder aDecoder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
}

}

Answer 1

I found you forget to turn off your dynamicAnimator after animations. At least, you need to turn off animator about cards[0]. Otherwise, it becomes unpredictable. You can use your removeOldFrontCard() like this. Hope this is the answer.

  func removeOldFrontCard() {
    dynamicAnimator.removeAllBehaviors()
    cards.append( cards.remove(at: 0))
    layoutCards()
}

Answer 2

You start at index 1 but index of an Array starts with 0

// the next 3 cards in the deck
for i in 1...3 {
    if i > (cards.count - 1) { continue }
    let card = cards[i]
...
}

Change that to:

// the next 3 cards in the deck
for i in 0...2 {
    if i > (cards.count - 1) { break }
    let card = cards[i]
...
}