SpringyCollectionViewFlowLayout

import UIKit

class SpringyCollectionViewFlowLayout: UICollectionViewFlowLayout
{
    lazy var dynamicAnimator: UIDynamicAnimator =
    {
        return UIDynamicAnimator(collectionViewLayout: self)
    }()
    
    var visibleIndexPathsSet = Set<IndexPath>()
    
    var latestDelta: CGFloat = 0
    
    override init()
    {
        super.init()
        self.minimumInteritemSpacing = 10
        self.minimumLineSpacing = 10
        self.itemSize = CGSize(width: 44, height: 44)
        self.sectionInset = UIEdgeInsets(top: 10, left: 10, bottom: 10, right: 10)
    }
    
    required init?(coder aDecoder: NSCoder)
    {
        fatalError("Don't use a coder")
    }
    
    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)
        
        for item in newlyVisibleItems
        {
            var center = item.center
            let springBehavior = UIAttachmentBehavior(item: item, attachedToAnchor: center)
            
            springBehavior.length = 0.0
            springBehavior.damping = 0.8
            springBehavior.frequency = 1.0
            
            if let touchLocation = touchLocation, CGPoint.zero != touchLocation
            {
                let yDistanceFromTouch = fabs(touchLocation.y - springBehavior.anchorPoint.y)
                let xDistanceFromTouch = fabs(touchLocation.x - 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)
        }
    }
    
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]?
    {
        guard let attributes = self.dynamicAnimator.items(in: rect) as? [UICollectionViewLayoutAttributes] else { return nil }
        return attributes
    }
    
    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes?
    {
        return self.dynamicAnimator.layoutAttributesForCell(at: indexPath)
    }
    
    override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool
    {
        let scrollView = self.collectionView
        let delta = newBounds.origin.y - (scrollView?.bounds.origin.y ?? 0)
        self.latestDelta = delta
        let touchLocation = self.collectionView?.panGestureRecognizer.location(in: self.collectionView)
        
        for springBehavior in self.dynamicAnimator.behaviors
        {
            guard let springBehavior = springBehavior as? UIAttachmentBehavior, let touchLocation = touchLocation else { continue }
            let yDistanceFromTouch = fabs(touchLocation.y - springBehavior.anchorPoint.y)
            let xDistanceFromTouch = fabs(touchLocation.x - springBehavior.anchorPoint.x)
            let scrollResistance: CGFloat = (yDistanceFromTouch + xDistanceFromTouch) / 1500.0
            
            guard let item = springBehavior.items.first as? UICollectionViewLayoutAttributes else { continue }
            var center = item.center
            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
            print("\(item.center) \(item.indexPath)")
            self.dynamicAnimator.updateItem(usingCurrentState: item)
        }
        
        return false
    }
}

class ViewController: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource
{
    let cellIdentifier = "Cell"
    
    var collectionView: UICollectionView!
    
    override func viewDidLoad()
    {
        super.viewDidLoad()
        
        let flowLayout = SpringyCollectionViewFlowLayout()
        self.collectionView = UICollectionView(frame: self.view.bounds, collectionViewLayout: flowLayout)
        self.view.addSubview(collectionView)
        collectionView.delegate = self
        collectionView.dataSource = self
        collectionView.register(UICollectionViewCell.self, forCellWithReuseIdentifier: "Cell")
        
        
        self.collectionView.backgroundColor = .white
    }
    
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        self.collectionView.reloadData()
    }
    
    override func viewDidLayoutSubviews()
    {
        super.viewDidLayoutSubviews()
        self.collectionView.frame = self.view.bounds
    }
    
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int
    {
        return 10000
    }
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell
    {
        
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: cellIdentifier, for: indexPath)
        cell.contentView.backgroundColor = .orange
        return cell
    }
}