StackView.swift

import UIKit
import Photos

protocol ImageStackViewDelegate: class {
  func imageStackViewDidPress()
}

class ImageStackView: UIView {

  struct Dimensions {
    static let imageSize: CGFloat = 58
  }

  weak var delegate: ImageStackViewDelegate?

  lazy var activityView: UIActivityIndicatorView = {
    let view = UIActivityIndicatorView()
    view.alpha = 0.0

    return view
    }()

  var views: [UIImageView] = {
    var array = [UIImageView]()
    for _ in 0...3 {
      let view = UIImageView()
      view.layer.cornerRadius = 3
      view.layer.borderColor = UIColor.white.cgColor
      view.layer.borderWidth = 1
      view.contentMode = .scaleAspectFill
      view.clipsToBounds = true
      view.alpha = 0
      array.append(view)
    }
    return array
    }()

  // MARK: - Initializers

  override init(frame: CGRect) {
    super.init(frame: frame)

    subscribe()

    views.forEach { addSubview($0) }
    addSubview(activityView)
    views.first?.alpha = 1
  }

  required init?(coder aDecoder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
  }

  deinit {
    NotificationCenter.default.removeObserver(self)
  }

  // MARK: - Helpers

  func subscribe() {
    NotificationCenter.default.addObserver(self,
      selector: #selector(imageDidPush(_:)),
      name: NSNotification.Name(rawValue: ImageStack.Notifications.imageDidPush),
      object: nil)

    NotificationCenter.default.addObserver(self,
      selector: #selector(imageStackDidChangeContent(_:)),
      name: NSNotification.Name(rawValue: ImageStack.Notifications.imageDidDrop),
      object: nil)

    NotificationCenter.default.addObserver(self,
      selector: #selector(imageStackDidChangeContent(_:)),
      name: NSNotification.Name(rawValue: ImageStack.Notifications.stackDidReload),
      object: nil)
  }

  override func layoutSubviews() {
    let step: CGFloat = -3.0
    let scale: CGFloat = 0.8
    let viewSize = CGSize(width: frame.width * scale,
      height: frame.height * scale)

    let offset = -step * CGFloat(views.count)
    var origin = CGPoint(x: offset, y: offset)

    for view in views {
      origin.x += step
      origin.y += step
      view.frame = CGRect(origin: origin, size: viewSize)
    }
  }

  func startLoader() {
    if let firstVisibleView = views.filter({ $0.alpha == 1.0 }).last {
      activityView.frame.origin.x = firstVisibleView.center.x
      activityView.frame.origin.y = firstVisibleView.center.y
    }

    activityView.startAnimating()
    UIView.animate(withDuration: 0.3, animations: {
      self.activityView.alpha = 1.0
    })
  }
}

extension ImageStackView {

  @objc func imageDidPush(_ notification: Notification) {
    let emptyView = views.filter { $0.image == nil }.first

    if let emptyView = emptyView {
      animateImageView(emptyView)
    }

    if let sender = notification.object as? ImageStack {
      renderViews(sender.assets)
      activityView.stopAnimating()
    }
  }

  @objc func imageStackDidChangeContent(_ notification: Notification) {
    if let sender = notification.object as? ImageStack {
      renderViews(sender.assets)
      activityView.stopAnimating()
    }
  }

  @objc func renderViews(_ assets: [PHAsset]) {
    if let firstView = views.first, assets.isEmpty {
      views.forEach {
        $0.image = nil
        $0.alpha = 0
      }

      firstView.alpha = 1
      return
    }

    let photos = Array(assets.suffix(4))

    for (index, view) in views.enumerated() {
      if index <= photos.count - 1 {
        AssetManager.resolveAsset(photos[index], size: CGSize(width: Dimensions.imageSize, height: Dimensions.imageSize)) { image in
          view.image = image
        }
        view.alpha = 1
      } else {
        view.image = nil
        view.alpha = 0
      }

      if index == photos.count {
        UIView.animate(withDuration: 0.3, animations: {
          self.activityView.frame.origin = CGPoint(x: view.center.x + 3, y: view.center.x + 3)
        })
      }
    }
  }

  fileprivate func animateImageView(_ imageView: UIImageView) {
    imageView.transform = CGAffineTransform(scaleX: 0, y: 0)

    UIView.animate(withDuration: 0.3, animations: {
      imageView.transform = CGAffineTransform(scaleX: 1.05, y: 1.05)
      }, completion: { _ in
        UIView.animate(withDuration: 0.2, animations: { () -> Void in
          self.activityView.alpha = 0.0
          imageView.transform = CGAffineTransform.identity
          }, completion: { _ in
            self.activityView.stopAnimating()
        })
    })
  }
}