TableView loads cell before API request can update model - ios

Question

I have a tableView controller where each line will represent the value of a specific cryptocurrency. I have a class for each different cryptocurrency the user wants to track called CryptoCurrency. Under the TableView CellforRowAt function I'm calling another function called getCryptoData, that will use AlamoFire to send api request to get the price of each cryptocurrency.
The problem is that the cellForRowAt function returns a cell with the default price of 0.00 before the getCryptoData function can finish and update the model.
I'm assuming this is because that function is run asynchronously?
How do I make it either wait for the function to finish before returning the cell or reload the cell after it is finished?
I tried adding tableView.reloaddata() at the end of the updateCryptoData function but that resulted in an endless loop.
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: "CryptoCurrencyCell", for: indexPath)
let cryptoCurrencyItem = cryptoCurrencyContainer.listOfCryptoCurrencies[indexPath.row]
getCryptoData(url: getApiString(for: cryptoCurrencyItem.id), currencyItem: cryptoCurrencyItem)
cell.textLabel?.text = cryptoCurrencyContainer.listOfCryptoCurrencies[indexPath.row].name + "\(cryptoCurrencyItem.currentPrice)"
print(cryptoCurrencyItem.currentPrice)
return cell
}
func getCryptoData(url: String, currencyItem: CryptoCurrency) {
Alamofire.request(url, method: .get).responseJSON {
response in
if response.result.isSuccess {
print("Sucess! bitcoin data")
let cryptoDataJSON : JSON = JSON(response.result.value!)
print(cryptoDataJSON)
self.updateCryptoData(json: cryptoDataJSON, currencyItem: currencyItem)
} else {
print("oops")
print("Error: \(String(describing: response.result.error))")
//self.bitcoinPriceLabel.text = "Connection Issues"
}
}
}
func updateCryptoData(json : JSON, currencyItem: CryptoCurrency) {
if let cryptoPriceResult = json["ask"].double {
//bitcoinPriceLabel.text = String(bitcoinResult)
currencyItem.updatePrice(price: cryptoPriceResult)
print(currencyItem.currentPrice)
} else {
//bitcoinPriceLabel.text = "Price Unavailable"
print("something aint right")
}
}
Under the cellForRowAt function there is a print statement:
print(cryptoCurrencyItem.currentPrice)
capturing the curent price before the cell is return. The console shows that it is still 0.00 implying that the getCryptoData function hasn't finished running.

Answer 1

Probably it is not the best solution but you could pass the cell to your method
func getCryptoData(url: String, currencyItem: CryptoCurrency, for cell: UITableViewCell) {
Alamofire.request(url, method: .get).responseJSON {
response in
if response.result.isSuccess {
print("Sucess! bitcoin data")
let cryptoDataJSON : JSON = JSON(response.result.value!)
print(cryptoDataJSON)
self.updateCryptoData(json: cryptoDataJSON, currencyItem: currencyItem, for: cell)
} else {
print("oops")
print("Error: \(String(describing: response.result.error))")
//self.bitcoinPriceLabel.text = "Connection Issues"
}
}
}
func updateCryptoData(json : JSON, currencyItem: CryptoCurrency, for cell: UITableViewCell) {
if let cryptoPriceResult = json["ask"].double {
//bitcoinPriceLabel.text = String(bitcoinResult)
currencyItem.updatePrice(price: cryptoPriceResult)
print(currencyItem.currentPrice)
cell.textLabel?.text = "\(cryptoPriceResult)" // your cell
} else {
//bitcoinPriceLabel.text = "Price Unavailable"
cell.textLabel?.text = "\(cryptoPriceResult)" // your cell
print("something aint right")
}
}
Since UITableViewCell is a reference type, once the async call is finished, it will update the cell text label.

Answer 2

Your request is running in a background thread. It means your tableView runs before you get data from request. So when you get success in updateCryptoData method you should call tableView.reloadData() in main thread.
DispatchQueue.main.async {
tableView.reloadData()
}
This code will run tableView cellForRow method

Answer 3

Yes tableView.reloaddata() at the end of the updateCryptoData function will rsult in infinite loop because in every reload after api call you again calling api and cycle continues.
From one of many approach, You can have a data structure of key-value based.
In your cellForRowAt function, you will be assigning value from data sctructure if available to your Label. if not value is available in your data structure then show some default value like "loading" or tiny loading wheel UI.
cell.priceLabel?.text = dataStructre["key"]
Now this key could be your some unique id that you identify your currency items.
Before calling your api from cell, check beforehand if value for "key" is available in your data structure. If value is unavailable then only you will call the api else simple fetch from data structure and set it to your Label.
if let value = dataStructre["key"]{
cell.priceLabel?.text = value
}
else{
callApi()
}
Finally In success method of your api call simple store the value that you got from server in you data structure with the "key identifier". After saving to your data structure, simply call "reloadData".
function callApi(){
if(success){
dataStructure["key"] = value from server
table.reloadData()
}
}