Get Image resolution from an URL in the background thread - swift

Question

I'm trying to get the resolution of an image from URL without actually downloading it. So I have a function for that:
public func resolutionForImage(url: String) -> CGSize? {
guard let url = URL(string: url) else { return nil }
guard let source = CGImageSourceCreateWithURL(url as CFURL, nil) else {
return nil
}
let propertiesOptions = [kCGImageSourceShouldCache: false] as CFDictionary
guard let properties = CGImageSourceCopyPropertiesAtIndex(source, 0, propertiesOptions) as? [CFString: Any] else {
return nil
}
if let width = properties[kCGImagePropertyPixelWidth] as? CGFloat,
let height = properties[kCGImagePropertyPixelHeight] as? CGFloat {
return CGSize(width: width, height: height)
} else {
return nil
}
}
It works fine, but I need to run it in the Background thread, in the main thread in block the UI
And this function is called in an another function in a collectionView cell, so in the end calculateImageHeight output should be in the main thread, could anyone help me manage it, still not in good in the threading
public func calculateImageHeight(ratio: Double = 0.0) -> CGFloat {
var calculatedRatio = ratio
if ratio == 0.0 {
if let size = resolutionForImage(url: imageUrl) {
calculatedRatio = size.height/size.width
}
}
let height = imageViewWidth * calculatedRatio
return height.isFinite ? height : 100
}