Swift - View loads before http request is finished in viewDidLoad() - ios

Question

I am trying to load a values from a database and put them into a UITableView in the viewDidLoad function in one of my Swift files. When debugging, at the time of the view rendering, the list of values is empty, but after the view loads, the list gets populated by the view loads. I don't have much experience with threads in Swift, so I am not exactly sure why this is happening, any ideas? I have tried to run DispatchQueue.main.async, but that did not work My code is below:
override func viewDidLoad() {
super.viewDidLoad()
// Load any saved meals, otherwise load sample data.
loadDbMeals()
}
private func loadDbMeals() {
var dbMeals = [Meal]()
let requestURL = NSURL(string: self.URL_GET)
let request = NSMutableURLRequest(url: requestURL! as URL)
request.httpMethod = "GET"
//creating a task to send the post request
let task = URLSession.shared.dataTask(with: request as URLRequest){
data, response, error in
if error != nil{
print("error is \(String(describing: error))")
return;
}
//parsing the response
do {
//converting response to NSDictionary
let myJSON = try JSONSerialization.jsonObject(with: data!, options: [.mutableContainers]) as? NSDictionary
//parsing the json
if let parseJSON = myJSON {
if let nestedDictionary = parseJSON["message"] as? NSArray {
for meal in nestedDictionary {
if let nestedMeal = meal as? NSDictionary {
let mealName = nestedMeal["name"]
let rating = nestedMeal["rating"]
dbMeals.append(Meal(name: mealName as! String, photo: UIImage(named: "defaultPhoto"), rating: rating as! Int, ingredientList: [])!)
}
}
}
}
} catch {
print(error)
}
}
meals += dbMeals
//executing the task
task.resume()
}
So, the current order of breakpoints, is the call to loadDbMeals() in the viewDidLoad() function, then it tries to add the dbMeals variables to the global meals variable, and then the http request gets executed, after the empty list has already been added. I appreciate any help!

Answer 1

Reload your table after loading data
if let parseJSON = myJSON {
if let nestedDictionary = parseJSON["message"] as? NSArray {
for meal in nestedDictionary {
if let nestedMeal = meal as? NSDictionary {
let mealName = nestedMeal["name"]
let rating = nestedMeal["rating"]
dbMeals.append(Meal(name: mealName as! String, photo: UIImage(named: "defaultPhoto"), rating: rating as! Int, ingredientList: [])!)
}
}
DispatchQueue.main.async{
self.tableView.reloadData()
}
}
}

Answer 2

the request happens asynchronously. so the view is loaded while the request may still be in progress.
move the meals += dbMeals line into the request's completion handler (after the for loop), add a self. to the meals var since you are referencing it from within a closure and reload the tableview from the main thread afterwards:
DispatchQueue.main.async{
self.tableView.reloadData()
}

Answer 3

Because dataTask is not a synchronised call, we need to use lock to wait until all fetch is finished.
Code will look something like this:
override func viewDidLoad() {
super.viewDidLoad()
DispatchQueue.global().async {
let lock = DispatchSemaphore(value: 0)
// Load any saved meals, otherwise load sample data.
self.loadDbMeals(completion: {
lock.signal()
})
lock.wait()
// finished fetching data
}
}
private func loadDbMeals(completion: (() -> Void)?) {
var dbMeals = [Meal]()
let requestURL = NSURL(string: self.URL_GET)
let request = NSMutableURLRequest(url: requestURL! as URL)
request.httpMethod = "GET"
//creating a task to send the post request
let task = URLSession.shared.dataTask(with: request as URLRequest){
data, response, error in
if error != nil{
print("error is \(String(describing: error))")
return;
}
//parsing the response
do {
//converting response to NSDictionary
let myJSON = try JSONSerialization.jsonObject(with: data!, options: [.mutableContainers]) as? NSDictionary
//parsing the json
if let parseJSON = myJSON {
if let nestedDictionary = parseJSON["message"] as? NSArray {
for meal in nestedDictionary {
if let nestedMeal = meal as? NSDictionary {
let mealName = nestedMeal["name"]
let rating = nestedMeal["rating"]
dbMeals.append(Meal(name: mealName as! String, photo: UIImage(named: "defaultPhoto"), rating: rating as! Int, ingredientList: [])!)
}
}
}
}
} catch {
print(error)
}
// call completion
completion()
}
meals += dbMeals
//executing the task
task.resume()
}
So execute loadDbMeals with completion block which will be called when fetching is finished and lock will wait until completion block is called.