Saving CGImageRef to a png file?

Swift 5+ adopted version

import Foundation
import CoreGraphics
import CoreImage
import ImageIO
import MobileCoreServices

extension CIImage {
  
  public func convertToCGImage() -> CGImage? {
    let context = CIContext(options: nil)
    if let cgImage = context.createCGImage(self, from: self.extent) {
      return cgImage
    }
    return nil
  }
  
  public func data() -> Data? {
    convertToCGImage()?.pngData()
  }
}

extension CGImage {
  
  public func pngData() -> Data? {
    let cfdata: CFMutableData = CFDataCreateMutable(nil, 0)
    if let destination = CGImageDestinationCreateWithData(cfdata, kUTTypePNG as CFString, 1, nil) {
      CGImageDestinationAddImage(destination, self, nil)
      if CGImageDestinationFinalize(destination) {
        return cfdata as Data
      }
    }
    
    return nil
  }
}

The provided solutions probably still work fine but there is some newer API for this in CoreImage which does the same and is a bit more "Swifty" to use:

import CoreImage

func write(cgimage: CGImage, to url: URL) throws {
    let cicontext = CIContext()
    let ciimage = CIImage(cgImage: cgimage)
    try cicontext.writePNGRepresentation(of: ciimage, to: url, format: .RGBA8, colorSpace: ciimage.colorSpace!)
}