RxSwift - UIImage+Collage.swift

// UIImage+Collage.swift
extension UIImage {

  static func collage(images: [UIImage], size: CGSize) -> UIImage {
    let rows = images.count < 3 ? 1 : 2
    let columns = Int(round(Double(images.count) / Double(rows)))
    let tileSize = CGSize(width: round(size.width / CGFloat(columns)),
                          height: round(size.height / CGFloat(rows)))

    UIGraphicsBeginImageContextWithOptions(size, true, 0)
    UIColor.white.setFill()
    UIRectFill(CGRect(origin: .zero, size: size))

    for (index, image) in images.enumerated() {
      image.scaled(tileSize).draw(at: CGPoint(
        x: CGFloat(index % columns) * tileSize.width,
        y: CGFloat(index / columns) * tileSize.height
      ))
    }

    let image = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()
    return image ?? UIImage()
  }

  func scaled(_ newSize: CGSize) -> UIImage {
    guard size != newSize else {
      return self
    }

    let ratio = max(newSize.width / size.width, newSize.height / size.height)
    let width = size.width * ratio
    let height = size.height * ratio

    let scaledRect = CGRect(
      x: (newSize.width - width) / 2.0,
      y: (newSize.height - height) / 2.0,
      width: width, height: height)

    UIGraphicsBeginImageContextWithOptions(scaledRect.size, false, 0.0);
    defer { UIGraphicsEndImageContext() }

    draw(in: scaledRect)

    return UIGraphicsGetImageFromCurrentImageContext() ?? UIImage()
  }
}