Converting SpringyCollectionView to Swift

Question

I'm trying to convert the following FLowLayout to Swift 3 from Objective C. Unfortunately I get a null error. Anyone has already implemented similar or converted this code to Swift 3? Springy Collection View Library

Here is what I got so far:

override func prepare() {
    super.prepare()


    // Need to overflow our actual visible rect slightly to avoid flickering.
    let visibleRect = self.collectionView!.bounds.insetBy(dx: -100, dy: -100)
    let itemsInVisibleRectArray: NSArray = super.layoutAttributesForElements(in: visibleRect)! as NSArray
    let itemsIndexPathsInVisibleRectSet: NSSet = NSSet(array: itemsInVisibleRectArray.value(forKey: "indexPath") as! [AnyObject])

    // Step 1: Remove any behaviours that are no longer visible.
    let noLongerVisibleBehaviours = (self.dynamicAnimator.behaviors as NSArray).filtered(using: NSPredicate(block: {behaviour, bindings in
        let currentlyVisible: Bool = itemsIndexPathsInVisibleRectSet.member((behaviour as! UIAttachmentBehavior).items.first!) != nil
        return !currentlyVisible
    }))


    for (_, obj) in noLongerVisibleBehaviours.enumerated() {
        self.dynamicAnimator.removeBehavior(obj as! UIDynamicBehavior)
        self.visibleIndexPathsSet.remove((obj as! UIAttachmentBehavior).items.first!)
    }

    // Step 2: Add any newly visible behaviours.
    // A "newly visible" item is one that is in the itemsInVisibleRect(Set|Array) but not in the visibleIndexPathsSet
    let newlyVisibleItems = itemsInVisibleRectArray.filtered(using: NSPredicate(block: {item, bindings in
        let currentlyVisible: Bool = self.visibleIndexPathsSet.member(item!) != nil
        return !currentlyVisible
    }))

    let touchLocation: CGPoint = self.collectionView!.panGestureRecognizer.location(in: self.collectionView)

    for (_, item) in newlyVisibleItems.enumerated() {
        let springBehaviour: UIAttachmentBehavior = UIAttachmentBehavior(item: item as! UIDynamicItem, attachedToAnchor: (item as AnyObject).center)

        springBehaviour.length = CGFloat(kLength)
        springBehaviour.damping = CGFloat(kDamping)
        springBehaviour.frequency = CGFloat(kFrequence)

        let zeropoint = CGPoint(x: 0,y :0)

        // If our touchLocation is not (0,0), we'll need to adjust our item's center "in flight"
        if (!zeropoint.equalTo(touchLocation)) {
            let yDistanceFromTouch = fabsf(Float(touchLocation.y - springBehaviour.anchorPoint.y))
            let xDistanceFromTouch = fabsf(Float(touchLocation.x - springBehaviour.anchorPoint.x))
            let scrollResistance = (yDistanceFromTouch + xDistanceFromTouch) / Float(kResistence)

            let item = springBehaviour.items.first as! UICollectionViewLayoutAttributes
            var center = item.center

            if self.latestDelta < 0 {
                center.x += max(self.latestDelta, self.latestDelta * CGFloat(scrollResistance))
            } else {
                center.x += min(self.latestDelta, self.latestDelta * CGFloat(scrollResistance))
            }

            item.center = center
        }

        self.dynamicAnimator.addBehavior(springBehaviour)
        self.visibleIndexPathsSet.add(item)
    }
}

I get the first error here:

Answer 1

It would be better to use native swift types. Something like:

    override func prepare()
    {
        super.prepare()
        let visibleRect = CGRect(origin: self.collectionView?.bounds.origin ?? CGPoint.zero, size: self.collectionView?.frame.size ?? CGSize.zero).insetBy(dx: -100, dy: -100)

        let itemsInVisibleRectArray = super.layoutAttributesForElements(in: visibleRect) ?? []
        let itemsIndexPathsInVisibleRectSet = Set(itemsInVisibleRectArray.map{ $0.indexPath })

        let noLongerVisibleBehaviors = self.dynamicAnimator.behaviors.filter
        { behavior in
            guard let behavior = behavior as? UIAttachmentBehavior else { return false }
            guard let attribute = behavior.items.first as? UICollectionViewLayoutAttributes else { return false }
            let currentlyVisible = itemsIndexPathsInVisibleRectSet.contains(attribute.indexPath)
            return !currentlyVisible
        }

        noLongerVisibleBehaviors.forEach
        { behavior in
            self.dynamicAnimator.removeBehavior(behavior)
            guard let behavior = behavior as? UIAttachmentBehavior else { return }
            guard let attribute = behavior.items.first as? UICollectionViewLayoutAttributes else { return }
            self.visibleIndexPathsSet.remove(attribute.indexPath)
        }

        let newlyVisibleItems = itemsInVisibleRectArray.filter
        { item in
            let currentlyVisible = self.visibleIndexPathsSet.contains(item.indexPath)
            return !currentlyVisible
        }
        let touchLocation = self.collectionView?.panGestureRecognizer.location(in: self.collectionView)

        newlyVisibleItems.forEach
        { item in
            var center = item.center
            let springBehavior = UIAttachmentBehavior(item: item, attachedToAnchor: center)

            springBehavior.length = 0.0
            springBehavior.damping = 0.8
            springBehavior.frequency = 1.0

            if CGPoint.zero != touchLocation
            {
                let yDistanceFromTouch = fabs(touchLocation?.y ?? 0 - springBehavior.anchorPoint.y)
                let xDistanceFromTouch = fabs(touchLocation?.x ?? 0 - springBehavior.anchorPoint.x)
                let scrollResistance = (yDistanceFromTouch + xDistanceFromTouch) / 1500.0

                if self.latestDelta < 0.0
                {
                    center.y += max(self.latestDelta, self.latestDelta * scrollResistance)
                }
                else
                {
                    center.y += min(self.latestDelta, self.latestDelta * scrollResistance)
                }
                item.center = center
            }
            self.dynamicAnimator.addBehavior(springBehavior)
            self.visibleIndexPathsSet.insert(item.indexPath)
        }
    }

Using NSArray and NSPredicate are not statically typed and are prone to runtime errors and all-around not very swifty.

The spring effect works for me.

You can see all of the code here. Perhaps it's more subtle than you expect.