FullWidthSelfSizingCollectionView.swift

import UIKit
import Cartography // https://github.com/robb/Cartography

/**
 This is an example of self sizing `UICollectionView` cells using AutoLayout,
 where the width of cells is always the width of the parent, to mimic `UITableView`.
 */

fileprivate let items: [String] = (0..<100)
    .map { _ in Lorem.sentences(Int.random(min: 1, max: 8)) } // Using https://github.com/lukaskubanek/LoremSwiftum/blob/master/Sources/LoremSwiftum.swift

final class ViewController: UIViewController {
    override func loadView() {
        self.view = self.customView
    }

    private lazy var customView = View()
}

private final class View: UIView {
    init() {
        self.collectionView = UICollectionView(frame: .zero,
                                               collectionViewLayout: self.layout)
        
        super.init(frame: .zero)
        
        self.collectionView.register(Cell.self, forCellWithReuseIdentifier: View.reuseIdentifier)
        self.collectionView.dataSource = self
        
        self.addSubview(self.collectionView)
        
        constrain(self.collectionView, self) { view, superview in
            view.edges == superview.edges
        }
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    private let collectionView: UICollectionView
    
    private let layout: UICollectionViewFlowLayout = {
        let layout = UICollectionViewFlowLayout()
        layout.scrollDirection = .vertical
        layout.sectionInset = .zero
        layout.estimatedItemSize = CGSize(width: 100, height: 100)
        
        return layout
    }()
    
    var previousSize: CGSize?
    override func layoutSubviews() {
        super.layoutSubviews()
        
        if let previousSize = self.previousSize, previousSize != .zero, previousSize != self.bounds.size {
            // Need to invalidate layout AND reload data to make rotation work
            self.layout.invalidateLayout()
            self.collectionView.reloadData()
        }
        
        self.previousSize = self.bounds.size
    }
    
    fileprivate static let reuseIdentifier = "cell"
}

extension View: UICollectionViewDataSource {
    func numberOfSections(in collectionView: UICollectionView) -> Int {
        return 1
    }
    
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return items.count
    }
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: View.reuseIdentifier, for: indexPath) as! Cell
        cell.text = items[indexPath.row]
        cell.width = collectionView.bounds.size.width
        
        return cell
    }
}

private final class Cell: UICollectionViewCell {
    override init(frame: CGRect) {
        super.init(frame: frame)
        
        self.contentView.addSubview(self.label)
        self.contentView.backgroundColor = .orange
        
        constrain(self.label, self.contentView, self) { label, superview, cell in
            label.edges == inset(superview.edges, 10)
            superview.center == cell.center
        }
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    var text: String = "" {
        didSet {
            self.label.text = self.text
            
            self.setNeedsLayout()
        }
    }
    
    var width: CGFloat = 0 {
        didSet {
            self.widthGroup = constrain(self.contentView, replace: self.widthGroup) { view in
                view.width == self.width
            }
        }
    }
    
    private var widthGroup: ConstraintGroup!
    
    private let label: UILabel = {
        let label = UILabel()
        label.numberOfLines = 0
        label.textColor = .white
        label.font = UIFont.preferredFont(forTextStyle: .body)
        
        return label
    }()
}