Resize.swift

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)
    }
  }
}