Wait until JSON call is finished - json

Question

I have this code in a part of my app:
let myUrl = NSURL(string: "http://aesolutions.ddns.net/data/load_tasks_calendar.php");
let request = NSMutableURLRequest(url: myUrl! as URL);
request.httpMethod = "POST";
let postString = "id_class=\(UserDefaults.standard.string(forKey: "userClass")!)";
request.httpBody = postString.data(using: String.Encoding.utf8);
MenuViewController.tasksCalendarArray.removeAll()
let task = URLSession.shared.dataTask(with: request as URLRequest){
data, response, error in
if error != nil{
print("error1=\(String(describing: error))")
return
}
var _: NSError?
do{
let json = try JSONSerialization.jsonObject(with: data!, options: .mutableContainers) as? NSArray
if let parseJSON: NSArray = json {
for index in 0...parseJSON.count-1 {
if (parseJSON[index] is NSNull){
MenuViewController.tasksCalendarArray.removeAll()
}else{
let compito = parseJSON[index] as! [String:Any]
let task = tasks.init(taskId: compito["id"] as! String,taskType: compito["type"] as! String,taskSubject: compito["subject"] as! String,taskDate: compito["date"] as! String,taskComment: compito["comment"] as! String)
MenuViewController.tasksCalendarArray.append(task)
}
}
}
}catch{
print("error2=\(error)")
return
}
DispatchQueue.main.async(execute: {
self.performSegue(withIdentifier: "loginToDiary", sender: self)
});
}
task.resume();
I want to perform the segue only when I load all the array. How can I wait until the json is terminated or is done correctly? Because I noticed that sometimes it is correct and other times it performs the segue and so the array is empty and then in the app there are errors.
Can someone help me also adding an alert with a kind of "loading message" to wait that the array is loading?