Why does this JSON decoding crash my app? - swift

Question

I have this function that is triggered by keyboard input, if the input is correct the application proceeds correctly; if not the application essentially freezes and the keyboard input cannot be changed (requires restart).
This is what calls the function:
override func viewDidLoad() {
super.viewDidLoad()
makeGetCall()
repeat{
RunLoop.current.run(until: Date(timeIntervalSinceNow: 0.1))
}while !done
Function Being Called:
func makeGetCall() {
let jsonUrlString = "http://api.openweathermap.org/data/2.5/weather?q=" + city + ",us&appid=f0d10597634568abee813f68138452fd&units=imperial"
guard let url = URL(string: jsonUrlString) else {
print("Error: cannot create URL")
return
}
URLSession.shared.dataTask(with: url) { (data, response, err) in
guard let data = data else {
print("Error: did not receive data")
return
}
do {
self.document = try JSONDecoder().decode(WeatherDocument.self, from: data)
self.done = true
print(self.document!)
print("========================INIT===============================")
print(self.document?.main?.temp! ?? "No temp")
print(self.document?.name! ?? "No temp")
print(self.document?.weather![0].description ?? "No info")
print(self.document?.wind?.speed ?? "No wind")
print("==========================END===============================")
print(self.document?.weather![0].main ?? "No main info")
} catch let jsonErr {
print("Error serializing json:", jsonErr)
}
}.resume()
}
Any ideas why this happening?
This is the error message that is displayed in the console:
"Error serializing json: typeMismatch(Swift.Double, Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "cod", intValue: nil)], debugDescription: "Expected to decode Double but found a string/data instead.", underlyingError: nil)"
WeatherDocument
struct WeatherDocument: Decodable {
let coord: Coordinates?
let weather: [Weather]?
let base: String?
let main: Main?
let visibility: Double?
let wind: Wind?
let clouds: Clouds?
let dt: Double?
let sys: Sys?
let id: Double?
let name: String?
let cod: Double?
}
Now the application does a breakpoint at the following statements I have made:
let tempe = (self.document?.main?.temp!)!
let humiditye = (self.document?.main?.humidity!)!
let pressurePow = (self.document?.main?.pressure!)! * 0.295300 * 0.10
let tempeMax = (self.document?.main?.temp_max!)! - 273.15
let tempeMin = (self.document?.main?.temp_min!)! - 273.15
//let clouding = (self.precip?.threeHours!)!
let name = (self.document?.name!)!
Why does the completion handler have a problem with this?

Answer 1

Two issues:
The error clearly states that the type of cod is String. And you can declare all struct members as non-optional. Openweathermap sends reliable data. There are only a few values (for example Rain in the forecast API) which are optional
let cod : String
Never use such a repeat loop. You block the thread. Use a completion handler. And it's highly recommended to use URLComponents which adds percent encoding implicitly.
var city = "New York,us"
let apiKey = <your api key>
...
func makeGetCall(completion: #escaping (WeatherDocument?, Error?)->Void) {
var urlComponents = URLComponents(string: "https://api.openweathermap.org/data/2.5/weather")!
let queryItems = [URLQueryItem(name: "q", value: city),
URLQueryItem(name: "appid", value: apiKey),
URLQueryItem(name: "units", value: "imperial")]
guard let url = urlComponents.url else {
print("Error: cannot create URL")
return
}
URLSession.shared.dataTask(with: url) { (data, response, err) in
guard let data = data else {
print("Error: did not receive data")
completion(nil, error!)
}
do {
let document = try JSONDecoder().decode(WeatherDocument.self, from: data)
print(document)
print("========================INIT===============================")
print(document.main.temp) // main and temp can be non-optional
print(document.name) // name can be non-optional
print(document.weather[0].description) // weather and description can be non-optional
print(document.wind.speed) // wind and speed can be non-optional
print("==========================END===============================")
print(document.weather[0].main) // weather and main can be non-optional
completion(document, nil)
} catch {
print("Error serializing json:", error)
completion(nil, error)
}
}.resume()
}
and call it
makeGetCall() { doc, error in
if let doc = doc {
self.document = doc
} else {
print(error!)
}
}
PS: You are mixing up the forecast and the weather API. Your struct belongs to the forecast API – where cod is actually Int – but the code (and the decoding error) belongs to weather API. The struct in the question will never work with this code / URL.

Answer 2

The problem is this:
makeGetCall()
repeat{
RunLoop.current.run(until: Date(timeIntervalSinceNow: 0.1))
} while !done
You're only calling makeGetCall() once. If it fails, you'll repeat that RunLoop.current.run(until: Date(timeIntervalSinceNow: 0.1)) call forever, blocking the main thread. If you want to re-attempt the makeGetCall until it works, move it down into the repeat block.
It sounds like there's also a decoding issue that will prevent makeGetCall() from completing, but I leave that for a separate answer.