Issues with Swift valueNotFound during API Call - ios

Question

I am looking for some help working with a Swift project. I am building an app that pulls aviation weather data from an API and this situation is common:
User wants data from airport weather station KDAB - current report says:
Wind 10 kt
Scattered Clouds 2500 ft
Visibility 10 SM
Light Rain
User wants data from airport weather station KJAX - current report says:
Wind 16 kt
Wind Gust 24 kt
Broken Clouds 1400 ft
Scattered Clouds 1900 ft
Few clouds 2400 ft
In this simple example, you may notice that there is no wind gust data supplied for KJAX during this reporting period and no "special weather" (ie rain, haze, fog) specified for KDAB. My app needs to be able to handle "nil" or not provided data without just telling me that there is a valueNotFound or that the index is out of range.
Here are the API Docs: https://avwx.docs.apiary.io/#reference/0/metar/get-metar-report
Here is my code:
import Foundation
struct WeatherManager {
let weatherURL = "https://avwx.rest/api/metar/"
func fetchWeather (stationICAO: String) {
let urlString = "\(weatherURL)\(stationICAO)?token=OVi45FiTDo1LmyodShfOfoizNe5m9wyuO6Mkc95AN-c"
performRequest(urlString: urlString)
}
func performRequest (urlString: String) {
if let url = URL(string: urlString) {
let session = URLSession(configuration: .default)
let task = session.dataTask(with: url) { (data, response, error) in
if error != nil {
print(error!)
return
}
if let safeData = data {
self.parseJSON(weatherData: safeData)
}
}
task.resume()
print(urlString)
}
}
func parseJSON(weatherData: Data) {
do {
let decoder = JSONDecoder()
let decodedData = try decoder.decode(WeatherData.self, from: weatherData)
let lowCloudsType = decodedData.clouds[0].type
let midCloudsType = decodedData.clouds[1].type
let highCloudsType = decodedData.clouds[2].type
let lowCloudsAlt = decodedData.clouds[0].altitude
let midCloudsAlt = decodedData.clouds[1].altitude
let highCloudsAlt = decodedData.clouds[2].altitude
let reportingStationVar = decodedData.station
let windGustValue = decodedData.wind_gust.value
let windSpeedValue = decodedData.wind_speed.value
let windDirectionValue = decodedData.wind_direction.value
let visibilityValue = decodedData.visibility.value
let flightRulesValue = decodedData.flight_rules
let weather = WeatherModel(lowestCloudsType: lowCloudsType, lowestCloudsAlt: lowCloudsAlt, middleCloudsType: midCloudsType, middleCloudsAlt: midCloudsAlt, highestCloudsType: highCloudsType, highestCloudsAlt: highCloudsAlt, reportingStation: reportingStationVar, windGust: windGustValue, windSpeed: windSpeedValue, windDirection: windDirectionValue, visibility: visibilityValue, flightRules: flightRulesValue)
print(weather.flightConditions)
} catch {
print(error)
}
}
}
import Foundation
struct WeatherModel {
let lowestCloudsType: String
let lowestCloudsAlt: Int
let middleCloudsType: String
let middleCloudsAlt: Int
let highestCloudsType: String
let highestCloudsAlt: Int
let reportingStation: String
let windGust: Int
let windSpeed: Int
let windDirection: Int
let visibility: Int
let flightRules: String
var flightConditions: String {
switch flightRules {
case "VFR":
return "green"
case "MVFR":
return "blue"
case "IFR":
return "red"
case "LIFR":
return "purple"
default:
return "gray"
}
}
}
Last One:
import Foundation
struct WeatherData: Decodable {
let clouds: [Clouds]
let flight_rules: String
let remarks: String
let wind_speed: WindSpeed
let wind_gust: WindGust
let wind_direction: WindDirection
let visibility: Visibility
let station: String
}
struct Clouds: Decodable {
let type: String
let altitude: Int
}
struct WindSpeed: Decodable {
let value: Int
}
struct WindGust: Decodable {
let value: Int
}
struct WindDirection: Decodable {
let value: Int
}
struct Visibility: Decodable {
let value: Int
}
Depending on what I play with, I get the following errors when entering a station that does not have all of that given information that I need to be able to present to the user if reported by the weather service.
2020-09-22 02:47:58.930421-0400 AvWx Pro[66612:4483807] libMobileGestalt MobileGestaltCache.c:38: No persisted cache on this platform.
KDAB
https://avwx.rest/api/metar/KDAB?token=(mySecretToken)
2020-09-22 02:48:02.943231-0400 AvWx Pro[66612:4483809] [] nw_protocol_get_quic_image_block_invoke dlopen libquic failed
valueNotFound(Swift.KeyedDecodingContainer<AvWx_Pro.WindGust.(unknown context at $1053fb3b8).CodingKeys>,
Swift.DecodingError.Context(codingPath:
[CodingKeys(stringValue: "wind_gust", intValue: nil)],
debugDescription: "Cannot get keyed decoding container
-- found null value instead.", underlyingError: nil))
A different error when I use an airport that doesn't have all three of the possible cloud layers reported:
2020-09-22 03:06:02.398628-0400 AvWx Pro[66736:4497432] libMobileGestalt MobileGestaltCache.c:38: No persisted cache on this platform.
KJAX
https://avwx.rest/api/metar/KJAX?token=(mySecretKey)
2020-09-22 03:06:07.955064-0400 AvWx Pro[66736:4497429] [] nw_protocol_get_quic_image_block_invoke dlopen libquic failed
Fatal error: Index out of range: file /Library/Caches/com.apple.xbs/Sources/swiftlang/swiftlang-1200.2.22.2/swift/stdlib/public/core/ContiguousArrayBuffer.swift, line 444
2020-09-22 03:06:08.908826-0400 AvWx Pro[66736:4497429] Fatal error: Index out of range: file /Library/Caches/com.apple.xbs/Sources/swiftlang/swiftlang-1200.2.22.2/swift/stdlib/public/core/ContiguousArrayBuffer.swift, line 444
(lldb)
I've spent a few hours now trying various solutions I've found online, including using optionals and force unwrapping, using guard let, using if let, and a few others. I'm pretty lost at the moment.
I am new to this platform (as a poster) and would really appreciate any insight anyone can offer! Thanks for the help in advance.

Answer 1

To avoid crashes while decoding json you should make correct configuration of your structs and check the fields before access values:
Optional fields
[CodingKeys(stringValue: "wind_gust", intValue: nil)],
debugDescription: "Cannot get keyed decoding container -- found null
value instead.", underlyingError: nil))
wind_gust can be empty so you should make it optional:
If a field can be empty or null in responses you should make it optional in your struct e.g.:
struct WeatherData: Decodable {
...
let wind_gust: WindGust?
...
}
Then in your code just use optional binding to extract a value if wind_gust exists:
if let value = decodedData.wind_gust?.value {
print(value)
}
Arrays
Fatal error: Index out of range
You must always check an array's bounds before access to items e.g.:
let clouds = decodedData.clouds
let lowCloudsType = clouds.count > 0 ? clouds[0].type : nil
let midCloudsType = clouds.count > 1 ? clouds[1].type : nil
let highCloudsType = clouds.count > 2 ? clouds[2].type : nil
if let low = lowCloudsType, let mid = midCloudsType, let high = highCloudsType {
print(low)
print(mid)
print(high)
}

Answer 2

The answer is to use Optionals
Consider the below playground example, the age and location properties are optional, the location property is missing from the JSON.
import Foundation
struct TestModel: Decodable {
let name: String
let age: Int?
let location: String?
enum CodingKeys: String, CodingKey {
case name
case age
case location
}
}
let jsonData = """
{
"name": "John Smith",
"age": 28
}
""".data(using: .utf8)!
let result = try JSONDecoder().decode(TestModel.self, from: jsonData)
print(result)
Output
TestModel(name: "John Smith", age: Optional(28), location: nil)