Improving-image-rendering-using-ImageIO.swift

import UIKit

class ImageView: UIImageView {
  
  // image data
  var data: Data? {
    didSet {
      // render image
      renderImage()
    }
  }
  
  private func renderImage() {
    // get image source from data
    guard let data = data, let source = CGImageSourceCreateWithData(data as CFData, nil) else {
      image = nil
      return
    }
    // get size of image view
    let size = bounds.size
    // move to asynchronous thread for creating image
    DispatchQueue.global(qos: .userInitiated).async { [weak self] in
      // setup option for creating CGImage from source
      let options = [
        kCGImageSourceCreateThumbnailFromImageAlways: true,
        kCGImageSourceCreateThumbnailWithTransform: true,
        kCGImageSourceShouldCacheImmediately: true,
        kCGImageSourceThumbnailMaxPixelSize: max(size.width, size.height)
        ]  as CFDictionary
      // create CGImage from source
      if let cgImage = CGImageSourceCreateThumbnailAtIndex(source, 0, options) {
        // create UIImage from CGImage
        let image = UIImage(cgImage: cgImage)
        // move to main thread for actually rendering image
        DispatchQueue.main.async { [weak self] in
          guard let strongSelf = self else {
            return
          }
          // assign and animate in image by cross-dissolving
          UIView.transition(with: strongSelf, duration: 0.2, options: .transitionCrossDissolve, animations: { [weak self] in
            self?.image = image
            }, completion: nil)
        }
      }
    }
  }
}