SwiftUI JSON Decoder not Working (using async await) - ios

Question

I am creating a new version of an existing app and want to use the new async await
format for a web request. Placing a break at the JSONDecoder().decode line I see that I do have data -
but the decoding does not work. (The url and my key DO work in the old version)
Here's the JSON format of the web source (shortened - there are many more items in
a fuel_station):
{
"station_locator_url":"https://afdc.energy.gov/stations/",
"total_results":110,
"station_counts":{},
"fuel_stations":[
{
"access_code":"public",
"access_days_time":"24 hours daily; call 866-809-4869 for Clean Energy card",
"access_detail_code":"KEY_ALWAYS",
"cards_accepted":"CleanEnergy",
"date_last_confirmed":"2021-09-10",
}
]
}
I created the following models from the above:
enum CodingKeys: String, CodingKey {
case fuelStations = "fuel_stations"
case accessCode = "access_code"
case accessDaysTime = "access_days_time"
case accessDetailCode = "access_detail_code"
case cardsAccepted = "cards_accepted"
case dateLastConfirmed = "date_last_confirmed"
}
struct TopLevel: Codable {
let fuelStations: [FuelStation]
}
struct FuelStation: Codable {
let accessCode, accessDaysTime, accessDetailCode, cardsAccepted: String
let dateLastConfirmed: String
let id: String
}
I put a simplified version of the initial view in one file for testing:
struct SiteListView: View {
#State private var fuelStations: [FuelStation] = []
#State private var topLevel: TopLevel = TopLevel(fuelStations: [])
var body: some View {
NavigationView {
VStack {
List(fuelStations, id: \.id) { item in
VStack {
Text(item.accessCode)
Text(item.accessDaysTime)
}
}
}
.navigationTitle("Site List View")
.task {
await loadData()
}
}//nav
}
func loadData() async {
//I believe the DEMO_KEY in the url will allow limited retrievals
guard let url = URL(string: "https://developer.nrel.gov/api/alt-fuel-stations/v1.json?api_key=DEMO_KEY") else {
print("Invalid URL")
return
}
do {
let (data, response) = try await URLSession.shared.data(from: url)
guard (response as? HTTPURLResponse)?.statusCode == 200 else { return }
print("response status code is 200")
if let decodedResponse = try? JSONDecoder().decode(TopLevel.self, from: data) {
topLevel = decodedResponse
print("in decoding: topLevel.fuelStations.count is \(topLevel.fuelStations.count)")
//I would iterate through topLevel here and add to the fuelStations
//array but I never get here
}
} catch {
print("Invalid Data")
}
}//load data
}//struct
Any guidance would be appreciated. Xcode 13.2.1 iOS 15.2

Answer 1

First you should remove ? from try? for the catch to work when there is a problem in decoding like this
func loadData() async {
//I believe the DEMO_KEY in the url will allow limited retrievals
guard let url = URL(string: "https://developer.nrel.gov/api/alt-fuel-stations/v1.json?api_key=DEMO_KEY") else {
print("Invalid URL")
return
}
do {
let (data, response) = try await URLSession.shared.data(from: url)
guard (response as? HTTPURLResponse)?.statusCode == 200 else { return }
print("response status code is 200")
let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromSnakeCase
let decodedResponse = try decoder.decode(TopLevel.self, from: data)
print("in decoding: topLevel.fuelStations.count is \(decodedResponse.fuelStations.count)")
//I would iterate through topLevel here and add to the fuelStations
//array but I never get here
} catch {
print(error)
}
}
After you do this , you'll find that some attributes in your struct are coming null in response so you should change string to string? to finally be
struct TopLevel: Codable {
let fuelStations: [FuelStation]
}
struct FuelStation: Codable {
let accessCode, accessDaysTime, accessDetailCode, cardsAccepted,dateLastConfirmed: String?
let id: Int
}
In addition note use of
let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromSnakeCase
instead of hard-coding the enum