快速测量上传速度

let urlString = "https://myserver/upload.php"
      guard let url = URL(string: urlString) else {
         return
      }

     var timer: Timer?
     var uploadTask: URLSessionUploadTask!

     var urlRequest = URLRequest(url: url, cachePolicy: .reloadIgnoringLocalAndRemoteCacheData, timeoutInterval: 10)
     urlRequest.httpMethod = "POST"
     urlRequest.addValue("application/json", forHTTPHeaderField: "Content-Type")

     let emptyData = Data.init(capacity: 102454)

     let json = ["file" : emptyData]
     urlRequest.httpBody = try? JSONEncoder().encode(json)

     let sessionConfiguration = URLSessionConfiguration.ephemeral
     let session = URLSession(configuration: sessionConfiguration, delegate: self, delegateQueue: nil)
     uploadTask = session.uploadTask(with: urlRequest, from: emptyData) { (data, response, error) in
        if let err = error {
            //There's an error
        }
        else if let response = response {
            //check for response status
        }

        //Stop the timer here
        timer?.invalidate()
      }

    uploadTask.resume()
    calculateSpeed()
}


func calculateSpeed() {
    var previousBytesSent: Int64 = 0
    timer = Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true, block: { (_) in
        let bytesSent = uploadTask.countOfBytesSent
        let speed =  abs(bytesSent-previousBytesSent)
        //Here you get the speed in Bytes/sec
        previousBytesSent = bytesSent
    })
}