ImageAspectRatioCache.swift

import Foundation
import CoreGraphics
import SDWebImage

final class ImageAspectRatioCache {
    
    var aspectLookup: [URL:CGFloat] = [:]
    
    func produceAspectRatios(for images: [URL], completion: (() -> Void)?) {
        
        let group = DispatchGroup()
        
        for url in images {
            group.enter()
            SDWebImageDownloader.shared().downloadImage(with: url,
                                                        options: [],
                                                        progress: nil) { [weak self] (image, _, _, _) in
                
                if let size = image?.size {
                    self?.aspectLookup[url] = (size.width / size.height)
                }
                
                group.leave()
            }
        }
        
        group.notify(queue: DispatchQueue.global()) {
            completion?()
        }
    }
}