//
//  ViewController.swift
//  StackCollectionView
//
//  Created by Krzysztof Babis on 20/03/2020.
//  Copyright © 2020 Krzysztof Babis. All rights reserved.
//

import UIKit
import SnapKit

class ViewController: UIViewController {
    private var collectionView: UICollectionView!

    override func viewDidLoad() {
        super.viewDidLoad()
        initializeCollectionView()
    }

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(true)
        //collectionView.scrollToItem(at: IndexPath(item: 10, section: 0), at: .centeredVertically, animated: true)
    }

    private func initializeCollectionView() {
        collectionView = UICollectionView(frame: .zero, collectionViewLayout: StickyCollectionViewFlowLayout())
        collectionView.backgroundColor = .yellow
        collectionView.register(UICollectionViewCell.self, forCellWithReuseIdentifier: "cell")
        collectionView.dataSource = self
        collectionView.delegate = self
        view.addSubview(collectionView)

        collectionView.snp.makeConstraints { (make) in
            make.left.right.bottom.equalTo(view.safeAreaLayoutGuide)
            make.height.equalToSuperview().multipliedBy(0.3)
        }
    }
}

extension ViewController: UICollectionViewDelegateFlowLayout {
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        return CGSize(width: UIScreen.main.bounds.width * 0.8, height: UIScreen.main.bounds.height * 0.25)
    }
}

extension ViewController: UICollectionViewDataSource {
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 10
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath)
        let label = UILabel(frame: cell.bounds)
        cell.layer.borderWidth = 2.0
        cell.layer.borderColor = UIColor.black.cgColor
        cell.backgroundColor = .red
        label.text = "\(indexPath.row)"
        label.textAlignment = .center
        cell.addSubview(label)
        return cell
    }
}

final class StickyCollectionViewFlowLayout: UICollectionViewFlowLayout {
    private let overlap: CGFloat
    private let scaleMultiplier: CGFloat?

    init(overlap: CGFloat = 10.0, scaleMultiplier: CGFloat = 0.06) {
        self.overlap = overlap
        self.scaleMultiplier = scaleMultiplier
        super.init()
    }

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

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        guard let superLayoutAttributes = super.layoutAttributesForElements(in: rect) else { return nil }

        let items = NSArray(array: superLayoutAttributes, copyItems: true)
        var headerAttributes: UICollectionViewLayoutAttributes?

        for item in items {
            guard let attributes = item as? UICollectionViewLayoutAttributes else { continue }

            if attributes.representedElementKind == UICollectionView.elementKindSectionHeader {
                headerAttributes = attributes
            } else {
                self.updateCellAttributes(attributes, headerAttributes: headerAttributes)
            }
        }

        return items as? [UICollectionViewLayoutAttributes]
    }

    func updateCellAttributes(_ attributes: UICollectionViewLayoutAttributes, headerAttributes: UICollectionViewLayoutAttributes?) {
        guard let collectionView = collectionView else { return }

        let minY = collectionView.bounds.minY + collectionView.contentInset.top
        var maxY = attributes.frame.origin.y

        if let headerAttributes = headerAttributes {
            maxY -= headerAttributes.bounds.height
        }

        let finalY = max(minY, maxY)
        var origin = attributes.frame.origin

        if let scaleMultiplier = scaleMultiplier {
            let deltaY = (finalY - origin.y) / attributes.frame.height
            let scale = 1 - deltaY * scaleMultiplier
            attributes.transform = CGAffineTransform(scaleX: scale, y: scale)
        }

        origin.y = finalY + CGFloat(attributes.indexPath.row) * overlap
        attributes.frame = CGRect(origin: origin, size: attributes.frame.size)
        attributes.zIndex = attributes.indexPath.row
    }

    override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
        return true
    }
}