UIView extension to maintain z-ordering in a superview

ZIndexedView.swift

//
//  ZIndexedView.swift
//
//  Created by Victor Pavlychko on 7/30/17.
//  Copyright © 2017. All rights reserved.
//

import UIKit

public protocol ZIndexed {
    
    var zIndex: CGFloat { get }
}

public extension ZIndexed where Self: UIView {
    
    public func didUpdateZIndex(_ zIndex: CGFloat, from oldValue: CGFloat) {
        if zIndex != oldValue {
            didUpdateZIndex()
        }
    }
    
    public func didUpdateZIndex() {
        guard let superview = superview else {
            return
        }
        let siblings = superview.subviews.filter({ $0 !== self })
        if let insertionPoint = findInsertionPoint(in: siblings) {
            superview.insertSubview(self, belowSubview: insertionPoint)
        } else {
            superview.addSubview(self)
        }
    }

    private func findInsertionPoint(in input: [UIView]) -> UIView? {
        var lowerIndex = -1
        var upperIndex = input.count
        while upperIndex - lowerIndex > 1 {
            let currentIndex = (lowerIndex + upperIndex) / 2
            if (input[currentIndex] as? ZIndexed)?.zIndex ?? 0 > zIndex {
                upperIndex = currentIndex
            } else {
                lowerIndex = currentIndex
            }
        }
        return upperIndex < input.count ? input[upperIndex] : nil
    }
}

open class ZIndexedCollectionViewCell: UICollectionViewCell, ZIndexed {
    
    open var zIndex: CGFloat = 0 {
        didSet { didUpdateZIndex(zIndex, from: oldValue) }
    }
    
    open override func didMoveToSuperview() {
        super.didMoveToSuperview()
        didUpdateZIndex()
    }

    open override func apply(_ layoutAttributes: UICollectionViewLayoutAttributes) {
        super.apply(layoutAttributes)
        zIndex = CGFloat(layoutAttributes.zIndex)
    }
}