How to convert JSON structure into dictionary or structure - json

Question

I'm extracting exchange rates for different currencies. I have a solution that works, but it doesn't seem elegant. I use a JSON input file and I want to convert it into a dictionary so I can access easily the exchange rate for a currency for a specific day.
My goal is to have the best way to do it without too much code and flexibility.
It's in Swift 5, with a JSON input file. The input file looks like:
{
"terms":{
"url": "https://www.banqueducanada.ca/conditions-utilisation-avis/"
},
"seriesDetail":{
"FXEURCAD":{"label":"EUR/CAD","description":"Taux de change quotidien de l’euro en dollars canadiens"},
"FXGBPCAD":{"label":"GBP/CAD","description":"Taux de change quotidien de la livre sterling en dollars canadiens"},
"FXMXNCAD":{"label":"MXN/CAD","description":"Taux de change quotidien du peso mexicain en dollars canadiens"},
"FXUSDCAD":{"label":"USD/CAD","description":"Taux de change quotidien du dollar américain en dollars canadiens"}
},
"observations":[
{"d":"2019-04-29","FXUSDCAD":{"v":1.3456}, "FXEURCAD":{"v":1.5029}, "FXGBPCAD":{"v":1.7391}, "FXMXNCAD":{"v":0.07083}},
{"d":"2019-04-30","FXUSDCAD":{"v":1.3423}, "FXEURCAD":{"v":1.5055}, "FXGBPCAD":{"v":1.7493}, "FXMXNCAD":{"v":0.07071}},
{"d":"2019-05-01","FXUSDCAD":{"v":1.3416}, "FXEURCAD":{"v":1.5070}, "FXGBPCAD":{"v":1.7540}, "FXMXNCAD":{"v":0.07105}},
{"d":"2019-05-02","FXUSDCAD":{"v":1.3462}, "FXEURCAD":{"v":1.5055}, "FXGBPCAD":{"v":1.7543}, "FXMXNCAD":{"v":0.07053}},
{"d":"2019-05-03","FXUSDCAD":{"v":1.3429}, "FXEURCAD":{"v":1.5021}, "FXGBPCAD":{"v":1.7611}, "FXMXNCAD":{"v":0.07069}}
]
}
I'm extracting the "observations" section.
The list of currencies for a date can vary. It can have more or less. I have a minimum of one currency. I would like to have a solution that doesn't require that I manually code the different exchange rates.
func getCurrencies (){
var currencies = [String : [String:Double]]()
if let urlJSON = URL(string: "https://www.banqueducanada.ca/valet/observations/FXUSDCAD,FXEURCAD,FXGBPCAD,FXMXNCAD/json?recent=5"){
do {
let contents = try String(contentsOf: urlJSON)
print(contents)
let data = Data(contents.utf8)
if let json = try JSONSerialization.jsonObject(with: data, options: []) as? [String: Any] {
let observations = json["observations"] as! [[String: Any]]
for (x,part) in observations.enumerated() {
let date = part["d"] as! String
currencies [date] = [String:Double]()
// get the currencies in the JSON structure
currencies [date]!["FXEURCAD"] = (part["FXEURCAD"] as! [String : Double])["v"]
currencies [date]!["FXGBPCAD"] = (part["FXGBPCAD"] as! [String : Double])["v"]
currencies [date]!["FXMXNCAD"] = (part["FXMXNCAD"] as! [String : Double])["v"]
currencies [date]!["FXUSDCAD"] = (part["FXUSDCAD"] as! [String : Double])["v"]
} // for
print (currencies)
} // if let json
} catch {}
} // if
} // getCurrencies
The currencies dictionary produced is fine:
["2019-05-02": ["FXGBPCAD": 1.7543, "FXMXNCAD": 0.07053, "FXEURCAD": 1.5055, "FXUSDCAD": 1.3462],
"2019-05-01": ["FXGBPCAD": 1.754, "FXEURCAD": 1.507, "FXMXNCAD": 0.07105, "FXUSDCAD": 1.3416],
"2019-05-03": ["FXEURCAD": 1.5021, "FXUSDCAD": 1.3429, "FXMXNCAD": 0.07069, "FXGBPCAD": 1.7611],
"2019-04-30": ["FXEURCAD": 1.5055, "FXMXNCAD": 0.07071, "FXUSDCAD": 1.3423, "FXGBPCAD": 1.7493],
"2019-04-29": ["FXGBPCAD": 1.7391, "FXEURCAD": 1.5029, "FXUSDCAD": 1.3456, "FXMXNCAD": 0.07083]]
I can extract a value using that command:
print (currencies ["2019-05-03"]!["FXUSDCAD"]!)
Is there a better way to do it? Is the JSONSerialization.jsonObject the right way to do it?

Answer 1

In this case JSONSerialization is a reasonable way however never load data synchronously from a remote URL.
This is a more generic version with a parameter for the currencies and with an asynchronous network request.
func getCurrencies(_ currencies : [String]){
var result = [String : [String:Double]]()
if let urlJSON = URL(string: "https://www.banqueducanada.ca/valet/observations/" + currencies.joined(separator:",") + "/json?recent=5"){
URLSession.shared.dataTask(with: urlJSON) { (data, _, error) in
if let error = error { print(error); return }
do {
if let json = try JSONSerialization.jsonObject(with: data!) as? [String: Any] {
let observations = json["observations"] as! [[String: Any]]
for part in observations {
let date = part["d"] as! String
result[date] = [String:Double]()
// get the currencies in the JSON structure
for currency in currencies {
result[date]![currency] = (part[currency] as! [String : Double])["v"]
}
} // for
print(result)
} // if let json
} catch {
print(error)
}
}.resume()
} // if
} // getC
getCurrencies(["FXUSDCAD","FXEURCAD","FXGBPCAD","FXMXNCAD"])