Playground for http://stackoverflow.com/questions/41091177/swift-3-how-to-center-horizontally-the-last-row-of-cells-in-a-uicollectionview

CenteringLastRow

//: Playground - noun: a place where people can play

import UIKit
import PlaygroundSupport

class LastRowCenteredLayout: UICollectionViewFlowLayout {
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        guard let originalAttributes = super.layoutAttributesForElements(in: rect) else { return nil }
        guard originalAttributes.isEmpty == false else { return originalAttributes }

        let finalAttributes = originalAttributes.map { $0.copy() as! UICollectionViewLayoutAttributes }

        // Find maxY among all element attributes, then all elements line up with that maxY. Those elements are the ones from the last row
        let maxY = finalAttributes.max { $0.frame.maxY < $1.frame.maxY }?.frame.maxY
        let lastRowAttrs = finalAttributes.filter { $0.frame.maxY == maxY }

        // If the elements in the last row don't sit flush against the right edge, then they need to be centered by shifting them a little to the right
        if lastRowAttrs.frame.maxX < insettedFrame.maxX {
            let shift = (insettedFrame.maxX - lastRowAttrs.frame.maxX) / 2.0
            
            for attrs in lastRowAttrs {
                attrs.frame = attrs.frame.offsetBy(dx: shift, dy: 0)
            }
        }
        
        return finalAttributes
    }
}

public class DataSource: NSObject, UICollectionViewDataSource {
    public func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 32;
    }
    
    public func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath)
        cell.backgroundColor = UIColor.green
        return cell
    }
}

extension CGRect {
    func insettedBy(insets: UIEdgeInsets) -> CGRect {
        return UIEdgeInsetsInsetRect(self, insets)
    }
}

extension Array where Element: UICollectionViewLayoutAttributes {
    var frame: CGRect {
        return map { $0.frame }.reduce(self[0].frame, { $0.union($1) })
    }
}

extension UICollectionViewFlowLayout {
    var insettedFrame: CGRect {
        guard let frame = collectionView?.frame else { return .zero }
        
        return frame.insettedBy(insets: sectionInset)
    }
}

let screen = UIView(frame: CGRect(x: 0, y: 0, width: 320, height: 500))
screen.backgroundColor = UIColor.white

let l = LastRowCenteredLayout()
l.sectionInset = UIEdgeInsets(top: 10, left: 10, bottom: 10, right: 10)
l.itemSize = CGSize(width: 30, height: 30)
let c = UICollectionView(frame: screen.bounds, collectionViewLayout: l)
c.backgroundColor = UIColor.blue
c.register(UICollectionViewCell.self, forCellWithReuseIdentifier: "cell")
let ds = DataSource()
c.dataSource = ds

screen.addSubview(c)

PlaygroundPage.current.needsIndefiniteExecution = true
PlaygroundPage.current.liveView = screen