The refactored version of https://gist.github.com/NunoAlexandre/85a0b73410fdb44e776bc6a5ea58042b

downloadAfter.swift

  @discardableResult
  func attemptDownload(forItem item: Item) -> Promise<URL> {
    if !canSubmitDownload(forItem: item) {
      return Promise.reject(DownloadError.duplicatedDownload)
    }
    else if !SettingsManager.shared.canDownloadOrStream() {
      let error = DownloadError.canNotUseInternet
      self.registerFailedDownload(item, withError: error)
      return Promise.reject(error)
    }
    else {
      return
        self
        .registerStartOfDownload(item)
        .then { self.download(item: item) }
        .then { self.unpackDownloaded(item: item, at: $0) }
        .then { self.moveToDownloadsDirectory(item, from: $0) }
        .chain { _ in self.registerSuccessfulDownload(item) }
        .onErrorDo { self.registerFailedDownload(item, withError: $0) }
    }
  }