Measure response time of http HEAD request in Swift

Question

I am trying to construct a function in Swift that sends a http HEAD request to a specified url, and measures the response time from the server. I am not concerned with parsing the response, only that I get a 200 from the server. I can do this in python with the requests module:

import requests
def get_latency():
    r = requests.head("http://example.com")
    return r.elapsed.total_seconds()

I assume I will need to use NSURL for this, and I've been able to get this far, but can't figure out the best way to actually send the request...

let url = NSURL (string: "http://example.com")
let request = NSURLRequest(URL: url!)
let started = NSDate()
  <<<Send http HEAD request, verify response>>>  <- need help here
let interval = NSDate().timeIntervalSinceDate(started)

Answer 1

I wrote this version based on the comments above. I decided to design it as an extension of the URL class. I have tested this code with Swift 4.

extension URL {

    /** Request the http status of the URL resource by sending a "HEAD" request over the network. A nil response means an error occurred. */
    public func requestHTTPStatus(completion: @escaping (_ status: Int?) -> Void) {
        // Adapted from https://stackoverflow.com/a/35720670/7488171
        var request = URLRequest(url: self)
        request.httpMethod = "HEAD"
        let task = URLSession.shared.dataTask(with: request) { (data, response, error) in
            if let httpResponse = response as? HTTPURLResponse, error == nil {
                completion(httpResponse.statusCode)
            } else {
                completion(nil)
            }
        }
        task.resume()
    }

    /** Measure the response time in seconds of an http "HEAD" request to the URL resource. A nil response means an error occurred. */
    public func responseTime(completion: @escaping (TimeInterval?) -> Void) {
        let startTime = DispatchTime.now().uptimeNanoseconds
        requestHTTPStatus { (status) in
            if status != nil {
                let elapsedNanoseconds = DispatchTime.now().uptimeNanoseconds - startTime
                completion(TimeInterval(elapsedNanoseconds)/1e9)
            }
            else {
                completion(nil)
            }
        }
    }
}

Usage:

let testURL = URL(string: "https://www.example.com")
testURL?.responseTime { (time) in
    if let responseTime = time {
        print("Response time: \(responseTime)")
    }
}