gistfile1.txt

protocol Problem1 {
    func download()
}

private let imageURLs: [URL] = [
    URL(string: "https://picsum.photos/seed/lorempixum1/400/300")!,
    URL(string: "https://picsum.photos/seed/lorempixum2/400/300")!,
    URL(string: "https://picsum.photos/seed/lorempixum3/400/300")!,
    URL(string: "https://picsum.photos/seed/lorempixum4/400/300")!,
    URL(string: "https://picsum.photos/seed/lorempixum5/400/300")!,
    URL(string: "https://picsum.photos/seed/lorempixum6/400/300")!,
    URL(string: "https://picsum.photos/seed/lorempixum7/400/300")!,
    URL(string: "https://picsum.photos/seed/lorempixum8/400/300")!,
    URL(string: "https://picsum.photos/seed/lorempixum9/400/300")!,
    URL(string: "https://picsum.photos/seed/lorempixum10/400/300")!
]

final class Problem1GCD: Problem1 {
    func download() {
        // TODO:
        // Download all images in `imageURLs`.
        // Constraint: at no point may more than TWO downloads be executing concurrently.
        // When all downloads are complete:
        // - Switch to the main thread
        // - Print "All downloads finished"
        //
        // Hint: consider `DispatchSemaphore` + `DispatchGroup`.
        print("Problem1: TODO")
        
        let group = DispatchGroup()
        

        let semaphore = DispatchSemaphore(value: 2)
        
        
        for image in imageURLs {
            group.enter()
            DispatchQueue.global(qos: .utility).async { [weak self] in 
                semaphore.wait()
                
                self?.downloadImage(url: image) {
                    semaphore.signal()
                    group.leave()
                }
            }
        }
        
        group.notify(queue: .main) {
            print("All downloads finished")
        }
    }
    
    private func downloadImage(url: URL, _ completion: @escaping () -> Void) {
        DispatchQueue.global(qos: .utility).asyncAfter(deadline: .now() + 0.3) {
            URLSession.shared.dataTask(with: url) { data, _, _ in
                DispatchQueue.main.async {
                    print("Download finished for \(url.path)")
                    completion()
                }
            }.resume()
        }
    }
}