Ios 使用NMSSH库从SFTP下载文件时出现问题

我正在使用NMSSH库在SFTP上下载和上载文件。我可以上传文件，但下载文件时遇到问题。尝试读取路径处的文件内容时，我得到了0个字节

我正在使用以下代码：

        let session = NMSSHSession(host: serverHost, port: portNo, andUsername: serverUsername)
        session.connect()
        if session.isConnected{
            let privateKeypath:String = Bundle.main.path(forResource: "mykey", ofType: "")!
            let privateKey: String = try! String(contentsOfFile: privateKeypath, encoding: String.Encoding.utf8)
            
            session.authenticateBy(inMemoryPublicKey: "", privateKey: privateKey, andPassword: passwordStr)
            
            if session.isAuthorized == true {
                let sftpsession = NMSFTP(session: session)
                sftpsession.connect()
                if sftpsession.isConnected {
                    let bytes = sftpsession.contents(atPath: "/test.pdf") { (copied, total) in
                        return true
                    }
                    
                    let documentDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!
                    
                    do {
                        try bytes?.write(to: documentDirectory.appendingPathComponent("/test.pdf"))
                    }
                    catch let error {
                        print("Error: ", error)
                    }
                }
            }
        }

我正在使用Swift 5.3

任何帮助都将不胜感激。谢谢大家!