let workDir = FileManager.default.urls(
    for: .documentDirectory,
    in: .userDomainMask).first!
let dataDir = workDir.appendingPathComponent("Data")

do {
    try FileManager.default.createDirectory(
        at: dataDir,
        withIntermediateDirectories: false,
        attributes: nil)
    print("successfully created directory")
} catch let e as NSError {
    print(e.localizedDescription)
}