Swift 4, Json data doesn't appear in cells - json

Question

I can't put data to the cells, I searched in different tutorials and it should work , I checked in debug area and data are downloaded but doesn't exist in cells, I tried also with custom cells but it doesn't work too. I have not got any error message, simply empty cells. Do you know maybe what can cause this issue? I spended much time for searching solution but I can't find anything, on every tutorial people do this similar to me.
struct Country: Decodable {
let name: String
}
class TableViewController: UIViewController, UITableViewDataSource, UITableViewDelegate {
var countries = [Country]()
var liczba = Int()
#IBOutlet var tv: UITableView!
override func viewDidLoad() {
super.viewDidLoad()
json() {
self.tv.reloadData()
}
tv.dataSource = self
tv.delegate = self
}
func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return liczba
}
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: "cell", for: indexPath)
cell.textLabel?.text = countries[indexPath.row].name
return cell
}
func json (completed: #escaping()->()) {
let jsonUrl = "https://restcountries.eu/rest/v2/all"
let url = URL(string: jsonUrl)
URLSession.shared.dataTask(with: url!) { (data, response, error) in
do {
self.countries = try JSONDecoder().decode([Country].self, from: data!)
let numer = self.countries.count
self.liczba = numer
}
catch {
print("error")
}
}.resume()
}
}

Answer 1

It seems that you are having a problem with the sequentiality of your code. You are calling reloadData on your tableview but at that moment you don't have set the datasource yet.
try this:
tv.dataSource = self
tv.delegate = self
json() {
self.tv.reloadData()
}

Answer 2

You are never calling the completed closure of your json method, so self.tv.reloadData() will never be executed.
Add completed() after self.liczba = numer.

Answer 3

Since you are not returning anything from the closure I recommend to delete the completion handler and reload the table view on the main thread within the closure.
And don't forget to handle a potential error
func json() {
let jsonUrl = "https://restcountries.eu/rest/v2/all"
let url = URL(string: jsonUrl)
URLSession.shared.dataTask(with: url!) { [unowned self] (data, response, error) in
if let error = error { print(error); return }
do {
self.countries = try JSONDecoder().decode([Country].self, from: data!)
self.liczba = self.countries.count
DispatchQueue.main.async {
self.tv.reloadData()
}
}
catch {
print("error")
}
}.resume()
}
and call the method in viewDidLoad
override func viewDidLoad() {
super.viewDidLoad()
tv.dataSource = self // I'd connect datasource and delegate in Interface Builder
tv.delegate = self
json()
}