使用Firebase

Question

我尝试使用Firebase在swift4中检索pdf文档，我收到错误请检查下面的代码。

        let storage = Storage.storage()
        let storageRef = storage.reference()
        let paths = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true)
        let documentsDirectory = paths[0]
        print("Jun18-2018:\(documentsDirectory)")
        var pdfRef = storageRef.child("pdfs/Hybris.pdf")
        _ = pdfRef.putData(data, metadata: nil, completion: { (metadata,error ) in
            guard let metadata = metadata else{
                print(error)
                return
            }
            let downloadURL = metadata.downloadURL()!
            print(downloadURL)
            guard let url = URL(string: paths[0]) else{return}
            let downloadTask = storageRef.write(toFile: url)
            { url, error in
                if let error = error {
                    print("err###########")
                } else {
                    print("pdf downloaded...........")
                }
            }
            downloadTask.observe(.resume) { snapshot in
                print("downloaded ... resume")
            }
            downloadTask.observe(.pause) { snapshot in
                print("downloaded ... paused")
            }
            downloadTask.observe(.progress) { snapshot in
                let percentComplete = 100.0 * Double(snapshot.progress!.completedUnitCount)
                    / Double(snapshot.progress!.totalUnitCount)
                print("downloaded ... progress")
            }
            downloadTask.observe(.success) { snapshot in
                print("downloaded ... successfully")
            }

        })

错误： ＆＃34;无法打开文件，因为不支持指定的URL类型。＆＃34; 由于状态403而放弃下载 下载...进度 犯错###########