PromiseChain.swift

    let backgroundQueue = DispatchQueue.global(qos: .background)

    firstly {
        showLoader()
    }.then(on: backgroundQueue) {
         self.fetchJSON()
    }.then(on: backgroundQueue) { (photos) in
        self.downloadPhotos(photos: Array(photos.prefix(40)))
    }.done(on: DispatchQueue.main, flags: nil) { _ in
        self.hideLoader()
        self.photoCollectionView.reloadData()
    }.catch { (error) in
        print(error.localizedDescription)
    }