Change alpha of UIView inside UIPanGestureRecognizer change state

Question

I have a card view which looks like in below gif. I want to fade out first card UIView) and fadein second card when user swiping it. UIViews are swappable to both left and right. I only found below post about this question but can't doesn't help me.

Change View Alpha on Pan Gesture or Dragging

/// This method handles the swiping gesture on each card.
@objc func handleCardPan(sender: UIPanGestureRecognizer) {

    // 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])
    print(panLocationInView.x)
    print(panLocationInCard)

    guard let view = sender.view as? ImageCard else { return }
    if(view.layer.zPosition != CGFloat(cards.count)){ return }
    switch sender.state {
    case .began:
        dynamicAnimator.removeAllBehaviors()
        // card is attached to center
        //cardAttachmentBehavior = UIAttachmentBehavior(item: cards[0], offsetFromCenter: offset, attachedToAnchor: panLocationInView)
        let translation = sender.translation(in: self.view)
        sender.view!.center = CGPoint(x: sender.view!.center.x + translation.x, y: sender.view!.center.y)
        sender.setTranslation(CGPoint(x: 0, y: 0), in: self.view)
        break
    case .changed:
        let translation = sender.translation(in: self.view)
        sender.view!.center = CGPoint(x: sender.view!.center.x + translation.x, y: sender.view!.center.y)
        sender.setTranslation(CGPoint(x: 0, y: 0), in: self.view)

        //            view.alpha = panLocationInView.x / self.view.frame.width
        //cards[1].imageView.alpha = 0.0 - alphaPercentage
        break
    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.difference + cards[0].frame.height / 2))
            dynamicAnimator.addBehavior(snapBehavior)
        } else {
            let velocity = sender.velocity(in: self.view)
            let pushBehavior = UIPushBehavior(items: [cards[0]], mode: .instantaneous)
            //velocity.y add this for y position vector
            pushBehavior.pushDirection = CGVector(dx: velocity.x/10, dy: 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))
            let currentAngle: Double = 0.0
            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
    }
}

Answer 1

Here is solution what I successfully execute for this condition:

 case .changed:
    let translation = sender.translation(in: self.view)
    sender.view!.center = CGPoint(x: sender.view!.center.x + translation.x, y: sender.view!.center.y)
    sender.setTranslation(CGPoint(x: 0, y: 0), in: self.view)

    let percentage = (sender.view!.center.x*100.0 / self.view.center.x)/100.0

        let alpha = percentage >= 1.0 ? (1.0 - (percentage-1.0)) : percentage

        sender.view!.alpha = alpha
        cards[1].imageView.alpha = previousCardAlpha + (1.0 - alpha) // previousCardAlpha == cards[1].alpha eg: 0.5

    break

This code is working fine in XCode 10 with swift 4.2. Hope this will work for you.