Can't modify variable of a class from closure - swift

Question

I want decodedResult variable assigns to exchangeInformations variable:
self.exchangeInformations = decodedResult
I created an instance of exchangeInfo() class. I try print the value of the exchangeInformations variable but I got nil. Why?
import Cocoa
class exchangeInfo {
var exchangeInformations: exchangeInformation? //Exchange information
let exchangeInformationURL: String = "https://api.binance.com/api/v3/exchangeInfo"
struct exchangeInformation: Codable {
var timezone: String //UTC
var serverTime: Int //1565246363776
var rateLimits: [rateLimits] = []//Rate limits
var exchangeFilters: [exchangeFilters] = [] //Exchange filters
var symbols: [symbols] //Symbols
}
struct rateLimits: Codable { //Rate limits
var rateLimitType: String //REQUEST_WEIGHT, ORDERS, RAW_REQUESTS
var interval: String //interval
var intervalNum: UInt16 //1
var limit: UInt32 //1200
}
struct exchangeFilters: Codable {
var filterType: String
var minPrice, maxPrice, tickSize, multiplierUp, multiplierDown, minQty, maxQty, stepSize, minNotional, maxPosition: String?
var avgPriceMins, limit, maxNumOrders, maxNumAlgoOrders, maxNumIcebergOrders: UInt16?
var applyToMarket: Bool?
}
struct symbols: Codable { //Symbols
var symbol: String //ETHBTC
var status: String //TRADING
var baseAsset: String //ETH
var baseAssetPrecision: UInt16 //8
var quoteAsset: String //BTC
var quotePrecision: UInt16 //8
var quoteAssetPrecision: UInt16 //8
var baseCommissionPrecision: UInt16 //8
var quoteCommissionPrecision: UInt16 //8
var orderTypes: [String] //orderTypes
var icebergAllowed: Bool //true
var ocoAllowed: Bool //true
var quoteOrderQtyMarketAllowed: Bool //true
var isSpotTradingAllowed: Bool //true
var isMarginTradingAllowed: Bool //true
var filters: [exchangeFilters] = [] //Filters
var permissions: [String] //Permissions
}
init() {
guard let url = URL(string: exchangeInformationURL) else {
print("URL is not valid")
return
}
let request = URLRequest(url: url)
let configuration = URLSessionConfiguration.ephemeral
let session = URLSession(configuration: configuration)
session.dataTask(with: request) { data, response, error in
do {
if let adat = data {
if let decodedResult = try? JSONDecoder().decode(exchangeInformation.self, from: adat) {
DispatchQueue.main.async {
self.exchangeInformations = decodedResult
}
}
}
} catch {
print("Error: \(error.localizedDescription)")
}
}.resume()
}
}
var informacio = exchangeInfo()
if let serverTime = informacio.exchangeInformations?.serverTime {
print (serverTime)
}
I got nothing.