JSON Decode Function From File Versus Web - ios

Question

SwiftUI is supposed to simplify things- I am bit frustrated as I have been working on the URLSession+JSONDecoder for weeks, I really need some help!
I have a function to load JSON data from a file in Swift and it works as expected. I copy/pasted the function and updated it to get the data via an API, however I receive a compile time error: "Unexpected non-void return value in void function". Is my approach wrong to use a function for JSON over the web?
JSON response:
{
"T":"CSU",
"v":468303,
"vw":1.2838,
"o":1.31,
"c":1.24,
"h":1.38,
"l":1.2001,
"t":1607374800000,
"n":994
}
struct Root2: Codable {
var T: String
var v: Double
var vw: Double
var o: String
var c: String
var h: Double
var l: Double
var t: Double
}
This file-based function works as expected:
let symbolData: [Root2] = load("symbolData.json")
func load<T: Decodable>(_ filename: String) -> T {
let data: Data
guard let file = Bundle.main.url(forResource: filename, withExtension: nil)
else {
fatalError("Couldn't find \(filename) in main bundle.")
}
do {
data = try Data(contentsOf: file)
} catch {
fatalError("Couldn't load \(filename) from main bundle:\n\(error)")
}
do {
let decoder = JSONDecoder()
return try decoder.decode(T.self, from: data)
} catch {
fatalError("Couldn't parse \(filename) as \(T.self):\n\(error)")
}
}
For the web version, I receive compile time error: "Unexpected non-void return value in void function".
Line: return try decoder.decode(T.self, from: data)
func loadURL<T: Decodable>() -> T {
guard let url = URL(string: """)
else {
fatalError("Invalid URL in main bundle.")
}
let request = URLRequest(url: url)
URLSession.shared.dataTask(with: request) { data, response, error in
do {
if let data = data {
let stringData = String(decoding: data, as: UTF8.self)
print("1 Fetched: \(url)")
print("2 Response: \(stringData)")
let decoder = JSONDecoder()
return try decoder.decode(T.self, from: data)
}
}
catch {
fatalError("Couldn't parse as :\n\(error)")
}
}.resume()
}
Working version after Leo's help!
class Manager: ObservableObject {
#Published var symbols: [Symbol] = []
func loadURL<T: Decodable>(using decoder: JSONDecoder = .msSince1970, completion: #escaping ((T?, Error?) -> Void)) {
let url = URL(string: """)!
URLSession.shared.dataTask(with: url) { data, response, error in
guard let data = data else {
print("ops")
completion(nil, error)
return
}
print("1 Fetched: \(url)")
print("2 Response:", String(data: data, encoding: .utf8) ?? "")
_ = Data("""
[
{
"open": {
"price": 124.02,
"time": 1657105851499
},
"close": {
"price": 124.96,
"time": 1618647822184
},
"high": 124.64,
"low": 124.65,
"volume": 75665274,
"symbol": "AAPL"
}
]
""".utf8)
do {
completion(try decoder.decode(T.self, from: data), nil)
//completion(try decoder.decode(T.self, from: tempDataForTesting), nil)
} catch {
completion(nil, error)
}
}.resume()
}
}
extension JSONDecoder {
static let msSince1970: JSONDecoder = {
let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .millisecondsSince1970
return decoder
}()
}

Answer 1

You can't wait for an asynchronous task to finish and return the result. What you need is a completion handler. You would need also to explicitly set the resulting type if you don't pass the resulting type to your decode method and you need to call resume to start your url session data task:
import SwiftUI
struct ContentView: View {
#ObservedObject var manager = Manager()
#State var string: String = "Hello, world!"
var body: some View {
Text(manager.symbol)
.padding()
.onAppear {
manager.load(symbol: manager.symbol) { (symbols: [Symbol]?, error: Error?) in
guard let symbols = symbols else {
print("error:", error ?? "")
string = "JSON could not be parsed"
return
}
for symbol in symbols {
print(symbol.open.price)
print(symbol.open.time)
print(symbol.close.price)
print(symbol.close.time)
print(symbol.high)
print(symbol.low)
print(symbol.volume)
print(symbol.symbol)
DispatchQueue.main.async {
manager.symbols = symbols
}
}
string = "JSON was successufly parsed"
}
}
}
}
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView()
}
}
class Manager: ObservableObject {
#Published var symbols: [Symbol] = []
#Published var symbol: String = "IBM"
func load<T: Decodable>(symbol: String, using decoder: JSONDecoder = .msSince1970, completion: #escaping ((T?, Error?) -> Void)) {
guard let url = URLComponents(symbol: symbol).url else {
completion(nil, URL.Error.invalidURL)
return
}
URLSession.shared.dataTask(with: url) { data, response, error in
guard let data = data else {
print("ops")
completion(nil, error)
return
}
print("1 Fetched: \(url)")
print("2 Symbol: \(symbol)")
print("3 Response:", String(data: data, encoding: .utf8) ?? "")
do {
completion(try decoder.decode(T.self, from: data), nil)
} catch {
completion(nil, error)
}
}.resume()
}
}
struct Symbol: Codable {
let open, close: Price
let high, low: Double
let volume: Int
let symbol: String
}
struct Price: Codable {
let price: Double
let time: Date
}
extension JSONDecoder {
static let msSince1970: JSONDecoder = {
let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .millisecondsSince1970
return decoder
}()
}
extension URLComponents {
init(scheme: String = "https",
host: String = "sandbox.iexapis.com",
path: String = "/stable/stock/market/ohlc",
symbol: String,
token: String = "YOUR_API_TOKEN") {
self.init()
self.scheme = scheme
self.host = host
self.path = path
self.queryItems = [URLQueryItem(name: "symbols", value: symbol),
URLQueryItem(name: "token", value: token)]
}
}
extension URL {
enum Error: String, Swift.Error {
case invalidURL = "Invalid URL"
}
}
This will print
1 Fetched: https://sandbox.iexapis.com/stable/stock/market/ohlc?symbols=IBM&token=YOUR_API_TOKEN
2 Symbol: IBM
3 Response: [{"open":{"price":128.9,"time":1636600302693},"close":{"price":131.44,"time":1662259300134},"high":132.517,"low":130.074,"volume":3403359,"symbol":"IBM"}]
128.9
2021-11-11 03:11:42 +0000
131.44
2022-09-04 02:41:40 +0000
132.517
130.074
3403359
IBM