DownloadingPhotos.swift

private func downloadPhotos() {
        //  1. Show preloading animation
        self.activityIndicator.startAnimating()
        //  2. Fetch and serialize json response
        Alamofire.request(photosURL, method: .get).validate().responseData { (data) in
            guard let data = data.result.value else { return }
            guard let photos = try? JSONDecoder().decode([Photo].self, from: data) else { return }
            //  3. Download photos
            for photo in photos {
                guard let photoUrl = URL(string: photo.url) else { return }
                guard let photoData = try? Data(contentsOf: photoUrl) else { return }
                guard let photoImage = UIImage(data: photoData) else { return }
                self.photos.append(photoImage)
            }
            // 4. Hide preloading animation and update the UI
            self.activityIndicator.stopAnimating()
            self.photoCollectionView.reloadData()
        }
    }