Application is modifying the autolayout engine from a background thread Error - swift

Question

Im not sure why but I have a button and textfield on my viewController,
the button is not visible when the view appears but when I click where the button should be it then appears. The pre-populated text in the text box also does not appear.
I am being presented with the following warning (im not sure if its related):
2016-12-30 11:25:26.030 wellpleased[5462:776953] This application is modifying the autolayout engine from a background thread after the engine was accessed from the main thread. This can lead to engine corruption and weird crashes.
How would I prevent this warning and have the elements appear correctly?
class events: UIViewController, UITableViewDelegate, UITableViewDataSource {
var rowID:String = ""
var value:String!
#IBOutlet var tableView: UITableView!
var tableData: [String] = []
var tableEventMonth: [String] = []
var tableEventDay: [String] = []
var tableEventCity: [String] = []
var tableEventLink: [String] = []
var tableEventID: [String] = []
#IBOutlet weak var tableTopConstraint: NSLayoutConstraint!
#IBOutlet weak var eventCodeView: UIView!
#IBAction func addButton(_ sender: Any) {
if self.eventCodeView.alpha == 1{
self.view.layoutIfNeeded()
UIView.animate(withDuration: 1, animations: {
self.view.layoutIfNeeded()
self.eventCodeView.alpha = 0
})
self.tableTopConstraint.constant = 1
UIView.animate(withDuration: 1) {
self.view.layoutIfNeeded()
}
}else{
self.view.layoutIfNeeded()
UIView.animate(withDuration: 1, animations: {
self.view.layoutIfNeeded()
self.eventCodeView.alpha = 1
})
self.tableTopConstraint.constant = 70
UIView.animate(withDuration: 1) {
self.view.layoutIfNeeded()
}
}
}
override func viewDidLoad() {
super.viewDidLoad()
let nib = UINib(nibName: "vwTblCell", bundle: nil)
tableView.register(nib, forCellReuseIdentifier: "cell")
}
override func viewDidAppear(_ animated: Bool) {
getTableData()
}
func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return self.tableData.count
}
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell: TblCell = self.tableView.dequeueReusableCell(withIdentifier: "cell") as! TblCell
cell.lblCarName.text = self.tableData[indexPath.row]
cell.calendarDay.text = self.tableEventDay[indexPath.row]
cell.calendarMonth.text = self.tableEventMonth[indexPath.row]
cell.city.text = self.tableEventCity[indexPath.row]
let defaults = UserDefaults()
let event = defaults.string(forKey: "event")
if self.tableData[indexPath.row] == event {
cell.hereLabel.isHidden = false
}
return cell
}
func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
print("Row \(indexPath.row) selected")
rowID = tableEventID[indexPath.row]
self.performSegue(withIdentifier: "goAttendees", sender: self)
}
func tableView(_ tableView: UITableView, heightForRowAt indexPath: IndexPath) -> CGFloat {
return 90
}
func getTableData(){
let defaults = UserDefaults()
let userid = defaults.string(forKey: "id")
let url = NSURL(string: "https://www.asmserver.co.uk/wellpleased/backend/eventselect.php?userid=\(userid!)")
let task = URLSession.shared.dataTask(with: url as! URL) { (data, response, error) -> Void in
if let urlContent = data {
do {
if let jsonResult = try JSONSerialization.jsonObject(with: urlContent, options: []) as? [[String:AnyObject]] {
var i = 0
while i < jsonResult.count {
self.tableData.append(jsonResult[i]["eventname"]! as! String)
self.tableEventDay.append(jsonResult[i]["eventday"]! as! String)
self.tableEventMonth.append(jsonResult[i]["eventmonth"]! as! String)
self.tableEventCity.append(jsonResult[i]["city"]! as! String)
self.tableEventLink.append(jsonResult[i]["link"]! as! String)
self.tableEventID.append(jsonResult[i]["eventid"]! as! String)
i = i + 1
}
}
} catch {
print("JSON serialization failed")
}
} else {
print("ERROR FOUND HERE")
}
DispatchQueue.main.async(execute: { () -> Void in
self.tableView.reloadData()
})
self.tableView.isUserInteractionEnabled = true
}
task.resume()
}
override func prepare(for segue: UIStoryboardSegue, sender: Any?){
if(segue.identifier == "goAttendees") {
let attendees = (segue.destination as! attendees)
attendees.value = rowID
}
}
}

Answer 1

You had better change UI only in the main thread. For swift3 refer to this post,
for more swift<2, 3 objective-c.