displaying alert after server response in swift - upload

Question

I have this chunk of code which posts json object to server
func postToServer(_ jsonObject: NSObject) -> Void {
var request = URLRequest(url: URL(string:url)!)
let session = URLSession.shared
request.httpMethod = "POST"
//Note : Add the corresponding "Content-Type" and "Accept" header. In this example I had used the application/json.
request.addValue("application/json", forHTTPHeaderField: "Content-Type")
request.addValue("application/json", forHTTPHeaderField: "Accept")
request.httpBody = try! JSONSerialization.data(withJSONObject: jsonObject, options: [])
let task = session.dataTask(with: request, completionHandler: { data, response, error in
guard data != nil else {
print("no data found: \(error)")
return
}
do {
if let json = try JSONSerialization.jsonObject(with: data!, options: []) as? NSDictionary {
print("Response: \(json)")
} else {
let jsonStr = NSString(data: data!, encoding: String.Encoding.utf8.rawValue)// No error thrown, but not NSDictionary
print("Error could not parse JSON: \(jsonStr)")
}
} catch let parseError {
print(parseError)// Log the error thrown by `JSONObjectWithData`
let jsonStr = NSString(data: data!, encoding: String.Encoding.utf8.rawValue)
print("Error could not parse JSON 2: '\(jsonStr)'")
self.alert(message: "test",title: "test")
}
})
task.resume()
}
My problem is this: I want to display alert to the user after I get response from server. For that I'm using this simple function:
func alert(message: String, title: String = "") {
let alertController = UIAlertController(title: title, message: message, preferredStyle: .alert)
let OKAction = UIAlertAction(title: "OK", style: .default, handler: nil)
alertController.addAction(OKAction)
self.present(alertController, animated: true, completion: nil)}
But, my app blocks after this. Any ideas?