GenericCollectionView.swift

// known-good: Swift 3
import UIKit

/// for a UIView that can be populated via `configure`
protocol ConfigurableView : class {
  associatedtype Value
  func configure(_ value:Value)
}

/// for a UICollectionViewCell wrapping a UIView
protocol ConfigurableCell : class {
  associatedtype View : ConfigurableView
  var wrappedView:View { get }
  static var classReuseIdentifier:String { get }
}

/// Generic UICollectionViewCell, for UIViews that conform to ConfigurableView
class WrappedCollectionViewCell<T:UIView> :  UICollectionViewCell, ConfigurableCell
  where T:ConfigurableView
{
  func setup()
  {
    let wrappedView = T.init(frame: self.contentView.bounds)
    self.contentView.addSubview(wrappedView)
    wrappedView.translatesAutoresizingMaskIntoConstraints = false
    
    for vfl in ["V:|[v]|","H:|[v]|"]
    {
      let cs = NSLayoutConstraint.constraints(withVisualFormat: vfl, options: [], metrics: nil, views: ["v":wrappedView])
      self.contentView.addConstraints(cs)
    }
  }
  
  var wrappedView:T {
    return self.contentView.subviews.first! as! T
  }
  
  class var classReuseIdentifier:String {
    return "\(T.self)" + "CellIdentifier"
  }
  
  override class var requiresConstraintBasedLayout:Bool { return true }
  override init(frame:CGRect)
  {
    super.init(frame:frame)
    setup()
  }
  
  required init?(coder aDecoder: NSCoder) {
    super.init(coder:aDecoder)
    setup()
  }
}


/**
 Generic single-row, horizontal scrolling collection view.
 
 This is generic over the data type and the view that displays a data item.
 
 This works, but maybe is too generic/clever for real use.
 
 How to use:
 
 1. Choose a value type V, like Int
 2. Define a UIView subtype, like MyView, adopting to ConfigurableView
 3. Instantiate the VC via HorizontalGalleryVC<Int,MyView>(items: hundred)
 
 */
class HorizontalGalleryVC<V,View:UIView> : UICollectionViewController
  where View.Value==V, View:ConfigurableView
{
  var items:[V] = []
  typealias Cell = WrappedCollectionViewCell<View>
  
  let interItemHorizontalSpace:CGFloat = 20
  let topPadding:CGFloat = 10
  let bottomPadding:CGFloat = 10
  
  init(items:[V]) {
    self.items = items
    let lay = UICollectionViewFlowLayout()
    
    lay.scrollDirection = .horizontal
    lay.minimumLineSpacing = self.interItemHorizontalSpace
    lay.minimumInteritemSpacing = CGFloat.greatestFiniteMagnitude // force one row
    lay.sectionInset = UIEdgeInsets(top: self.topPadding,
                                    left: 0,
                                    bottom: self.bottomPadding,
                                    right: 0)
    
    super.init(collectionViewLayout: lay)
  }
  
  required init?(coder aDecoder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
  }
  
  override func viewDidLoad()
  {
    super.viewDidLoad()
    self.collectionView!.isOpaque = true
    self.collectionView!.backgroundColor = .white
    
    self.collectionView!.register(Cell.self, forCellWithReuseIdentifier: Cell.classReuseIdentifier)
    
    let cv = self.collectionView!
    
    cv.allowsSelection = true
    cv.allowsMultipleSelection = false
    cv.scrollsToTop = false
    cv.isPagingEnabled = false
    
    let flowLayout = self.collectionView!.collectionViewLayout as! UICollectionViewFlowLayout
    flowLayout.itemSize = CGSize(width: 59, height: 50)
  }
  
  override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell
  {
    let cell = self.collectionView!.dequeueReusableCell(withReuseIdentifier: Cell.classReuseIdentifier,
                                                        for: indexPath)
    let configurableCell = cell as! Cell
    configurableCell.wrappedView.configure(self.items[indexPath.row])
    return cell
  }
  
  override func numberOfSections(in collectionView: UICollectionView) -> Int
  {
    return 1
  }
  
  override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int
  {
    return items.count
  }
  
  override func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath)
  {
    NSLog("tapped")
  }
}