Bricks.swift

import UIKit
import PlaygroundSupport

class MyViewController : UIViewController {
    
    override func loadView() {
        let view = UIView()
        
        let gradView = GradientView()
        gradView.frame = view.bounds
        gradView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        view.addSubview(gradView)
        
        let layout = Layout()
        layout.delegate = self
        let cv = UICollectionView(frame: view.bounds, collectionViewLayout: layout)
        cv.contentInset = .init(top: 10, left: 10, bottom: 10, right: 10)
        cv.backgroundColor = .clear
        cv.frame = view.bounds
        cv.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        cv.dataSource = self
        cv.register(Cell.self, forCellWithReuseIdentifier: String(describing: Cell.self))
        
        view.addSubview(cv)
        self.view = view
    }
    
    var words: [String] = {
        let text = "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum."
        return text.components(separatedBy: .whitespaces)
    }()
    
}

extension MyViewController: UICollectionViewDataSource {
    
    func numberOfSections(in collectionView: UICollectionView) -> Int {
        return 1
    }
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return words.count
    }
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: String(describing: Cell.self), for: indexPath) as! Cell
        cell.contentView.backgroundColor = .white
        cell.label.text = string(for: indexPath)
        return cell
    }
    
}

extension MyViewController: LayoutDelegate {
    
    func string(for indexPath: IndexPath) -> String {
        guard indexPath.row < words.count else { return "" }
        return words[indexPath.row]
    }
    
}

protocol LayoutDelegate: class {
    
    func string(for indexPath: IndexPath) -> String
    
}

final class Layout: UICollectionViewLayout {
        
    weak var delegate: LayoutDelegate?
    
    private var calcResults: [IndexPath : UICollectionViewLayoutAttributes] = [:]
    
    //content size
    private var contentWidth: CGFloat {
        guard let collectionView = collectionView else {
            return 0
        }
        let insets = collectionView.contentInset
        return collectionView.bounds.width - (insets.left + insets.right)
    }
    private var contentHeight: CGFloat = 0
    
    //brick size
    func minimumBrickWidth(forText text: NSString) -> CGFloat {
        let textWidth = text.boundingRect(with: .init(width: 0, height: brickHeight), attributes: [.font:UIFont.systemFont(ofSize: 14)], context: nil).width
        return ceil(textWidth + brickSidePadding * 2)
    }
    let brickHeight: CGFloat = 40
    let brickInsets: UIEdgeInsets = .init(top: 1, left: 1, bottom: 1, right: 1)
    let brickSidePadding: CGFloat = 16
    
    override func prepare() {
        super.prepare()
        
        guard let collectionView = collectionView else { return }
        if !calcResults.isEmpty { return }
        guard let delegate = delegate else { return }
        
        let section = 0
        var currentPos = CGPoint.zero
        var results: [[IndexPath : UICollectionViewLayoutAttributes]] = [[:]]
        for i in 0..<collectionView.numberOfItems(inSection: section) {
            let indexPath = IndexPath(item: i, section: section)
            let text = delegate.string(for: indexPath)
            let minBrickWidth = minimumBrickWidth(forText: text as NSString)
            let brickFrame: CGRect
            
            let surplus = contentWidth - currentPos.x
            if minBrickWidth <= surplus {
                //余白が十分にあれば、セルを横に並べる
                brickFrame = .init(origin: currentPos, size: .init(width: minBrickWidth, height: brickHeight))
            } else {
                //余白が不足していれば、
                //1. 現在行に含まれる幅をsurplus分均等に広げる
                let lastLineArray = results.last!.sorted {
                    $0.value.frame.origin.x < $1.value.frame.origin.x
                }
                let bw = surplus / CGFloat(lastLineArray.count)
                for (i, map) in lastLineArray.enumerated() {
                    let attrs = map.value
                    var newFrame = attrs.frame
                    newFrame.size.width += bw
                    newFrame.origin.x += bw * CGFloat(i)
                    attrs.frame = newFrame
                }
                //2. 改行して新しい行にセルを並べる
                currentPos.x = 0
                currentPos.y += brickHeight
                brickFrame = .init(origin: currentPos, size: .init(width: minBrickWidth, height: brickHeight))
                results.append([:])
            }
            currentPos.x += minBrickWidth
            contentHeight = max(contentHeight, brickFrame.maxY)
            
            let attrs = UICollectionViewLayoutAttributes(forCellWith: IndexPath(item: i, section: section))
            let insetFrame = brickFrame.inset(by: brickInsets)
            attrs.frame = insetFrame
            results[results.count - 1][indexPath] = attrs
        }
        calcResults = results.reduce([:], { (result, map) -> [IndexPath : UICollectionViewLayoutAttributes] in
            return result.merging(map) {(item, _) in item}
        })
    }
    
    override var collectionViewContentSize: CGSize {
        return CGSize(width: contentWidth, height: contentHeight)
    }
    
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        return calcResults.filter {
            $0.value.frame.intersects(rect)
        }.map {
            $0.value
        }
    }
    
    override func layoutAttributesForItem(at indexPath: IndexPath)
        -> UICollectionViewLayoutAttributes? {
            return calcResults[indexPath]
    }
    
    override func invalidateLayout() {
        super.invalidateLayout()
        calcResults.removeAll()
    }
    
}

final class Cell: UICollectionViewCell {
    
    static let font = UIFont.systemFont(ofSize: 14)
    let label = UILabel()
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        
        contentView.layer.cornerCurve = .continuous
        contentView.layer.cornerRadius = 3
        contentView.clipsToBounds = true
        label.frame = contentView.bounds
        label.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        label.textAlignment = .center
        label.font = type(of: self).font
        label.textColor = UIColor(white: 0.2, alpha: 1.0)
        contentView.addSubview(label)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
}

final class GradientView: UIView {
    
    let colors: [UIColor] = [.systemRed, .systemBlue]
    
    override func draw(_ rect: CGRect) {
        guard let ctx = UIGraphicsGetCurrentContext() else { return }
        let clrs = colors.map { $0.cgColor } as CFArray
        let grad = CGGradient(colorsSpace: nil, colors: clrs, locations: [0, 1])!
        ctx.drawLinearGradient(grad, start: .zero, end: .init(x: 0, y: rect.height), options: [])
    }
    
}

// Present the view controller in the Live View window
PlaygroundPage.current.liveView = MyViewController()

Result: