Decoding API data - ios

Question

I'm trying to fetch data from an API but I can't get it right and I don't know the issue here:
struct BTCData : Codable {
let close : Double
let high : Double
let low : Double
private enum CodingKeys : Int, CodingKey {
case close = 3
case high = 4
case low = 5
}
}
func fetchBitcoinData(completion: #escaping (BTCData?, Error?) -> Void) {
let url = URL(string: "https://api.bitfinex.com/v2/candles/trade:30m:tBTCUSD/hist")!
let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
guard let data = data else { return }
do {
if let bitcoin = try JSONDecoder().decode([BTCData].self, from: data).first {
print(bitcoin)
completion(bitcoin, nil)
}
} catch {
print(error)
}
}
task.resume()
}
I'd like to be able to access close in every dict and iterate like that:
var items : BTCData!
for idx in 0..<15 {
let diff = items[idx + 1].close - items[idx].close
upwardMovements.append(max(diff, 0))
downwardMovements.append(max(-diff, 0))
}
I get nil. I don't understand how to decode this kind of API where I need to iterate something which is not inside another dict.
EDIT: The above was solved and I'm now struggling to use [BTCData] in another function.
I am trying to use it here :
func fetchBitcoinData(completion: #escaping ([BTCData]?, Error?) -> Void) {
let url = URL(string: "https://api.bitfinex.com/v2/candles/trade:30m:tBTCUSD/hist")!
let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
guard let data = data else {
completion(nil, error ?? FetchError.unknownNetworkError)
return
}
do {
let bitcoin = try JSONDecoder().decode([BTCData].self, from: data); completion(bitcoin, nil)
//let close52 = bitcoin[51].close
//print(bitcoin)
//print(close52)
} catch let parseError {
completion(nil, parseError)
}
}
task.resume()
}
class FindArray {
var items = [BTCData]()
func findArray() {
let close2 = items[1].close
print(close2)
}
}
fetchBitcoinData() { items, error in
guard let items = items,
error == nil else {
print(error ?? "Unknown error")
return
}
let call = FindArray()
call.items = items
call.findArray()
}
EDIT 2: Solved it with [BTCData](). var items : [BTCData] = [] works too

Answer 1

To decode an array of arrays into a struct with Decodable you have to use unkeyedContainer. Since there is no dictionary CodingKeys are useless.
struct BTCData : Decodable {
let timestamp : Int
let open, close, high, low, volume : Double
init(from decoder: Decoder) throws {
var container = try decoder.unkeyedContainer()
timestamp = try container.decode(Int.self)
open = try container.decode(Double.self)
close = try container.decode(Double.self)
high = try container.decode(Double.self)
low = try container.decode(Double.self)
volume = try container.decode(Double.self)
}
}
You don't have to change your JSONDecoder() line.
...
if let bitcoin = try JSONDecoder().decode([BTCData].self, from: data).first {
print(bitcoin)
completion(bitcoin, nil)
}
Just by adding two lines it's even possible to decode the timestamp into a Date value
struct BTCData : Decodable {
let timestamp : Date
let open, close, high, low, volume : Double
init(from decoder: Decoder) throws {
var container = try decoder.unkeyedContainer()
timestamp = try container.decode(Date.self)
...
let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .millisecondsSince1970
if let bitcoin = try decoder.decode([BTCData].self, from: data).first {
print(bitcoin)
completion(bitcoin, nil)
}
To decode the array and get a value at specific index
do {
let bitcoins = try JSONDecoder().decode([BTCData].self, from: data)
let close52 = bitcoins[51].close
print(close52)
...

Answer 2

You need to use JSONSerialization and cast to [[NSNumber]] to get the result needed
UPDATE
Checking this https://docs.bitfinex.com/v2/reference#rest-public-candles I think this is what you are searching for
Try using this
func fetchBitcoinData(completion: #escaping ([BTCData]?, Error?) -> Void) {
let url = URL(string: "https://api.bitfinex.com/v2/candles/trade:30m:tBTCUSD/hist")!
let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
guard let data = data else { return }
do {
if let array = try JSONSerialization.jsonObject(with: data, options: .allowFragments) as? [[NSNumber]]{
var arrayOfCoinData : [BTCData] = []
for currentArray in array{
arrayOfCoinData.append(BTCData(close: currentArray[2].doubleValue, high: currentArray[3].doubleValue, low: currentArray[4].doubleValue))
}
debugPrint(arrayOfCoinData)
completion(arrayOfCoinData, nil)
}
} catch {
print(error)
completion(nil, error)
}
}
task.resume()
}
Log Result
[BitcoinApiExample.BTCData(close: 7838.8999999999996,...]