ImageScaler.swift

struct ImageScaler {
  static func scaleToFill(_ image: UIImage, in targetSize: CGSize, from fromRect: CGRect = .zero) -> UIImage {
    let rect = fromRect.isEmpty ? CGRect(origin: .zero, size: image.size) : fromRect
    let scaledRect = rect.scaled(toFillSize: targetSize)
    return scale(image, fromRect: scaledRect, targetSize: targetSize)
  }
    
  private static func scale(_ image: UIImage, fromRect: CGRect = .zero, targetSize: CGSize) -> UIImage {
    let renderer = UIGraphicsImageRenderer(size: targetSize)
    return renderer.image { context in
      // UIImage and CGContext coordinates are flipped.
      var transform = CGAffineTransform(translationX: 0.0, y: targetSize.height)
      transform = transform.scaledBy(x: 1, y: -1)
      context.cgContext.concatenate(transform)      
      // UIImage -> cropped CGImage
      let makeCroppedCGImage: (UIImage) -> CGImage? = { image in
        guard let cgImage = image.cgImage else { return nil }
        guard !fromRect.isEmpty else { return cgImage }
        return cgImage.cropping(to: fromRect)
      }
      if let cgImage = makeCroppedCGImage(image) {
        context.cgContext.draw(cgImage, in: CGRect(origin: .zero, size: targetSize))
      }
      else if let ciImage = image.ciImage {
        var transform = CGAffineTransform(translationX: 0.0, y: image.size.height)
        transform = transform.scaledBy(x: 1, y: -1)
        let adjustedFromRect = fromRect.applying(transform)
        let ciContext = CIContext(cgContext: context.cgContext, options: nil)
        ciContext.draw(ciImage, in: CGRect(origin: .zero, size: targetSize), from: adjustedFromRect)
      }
    }
  }
}