currency-api | demo project on how to test a node/express app | Runtime Evironment library

import SwiftUI
//You can't use the standard Codable for this. You have to make your own.
class BaseCurrency: Codable {
    let id = UUID()
    var baseCurrencies: [String : [String: Double]] = [:]
    required public init(from decoder: Decoder) throws {
        do{
            print(#function)
            let baseContainer = try decoder.singleValueContainer()
            let base = try baseContainer.decode([String : [String: Double]].self)
            for key in base.keys{
                baseCurrencies[key] = base[key]
            }
        }catch{
            print(error)
            throw error
        }
    }
    //@State should never be used outside a struct that is a View
}
struct CurrencyView: View {
    @StateObject var vm: CurrencyViewModel = CurrencyViewModel()
    
    var body: some View {
        VStack{
            List{
                if vm.results != nil{
                    ForEach(vm.results!.baseCurrencies.sorted{$0.key < $1.key}, id: \.key) { key, baseCurrency in
                        DisclosureGroup(key){
                            ForEach(baseCurrency.sorted{$0.key < $1.key}, id: \.key) { key, rate in
                                HStack{
                                    Text(key)
                                    Text(rate.description)
                                }
                            }
                        }
                    }
                }else{
                    Text("waiting...")
                }
            }
            //To select another rate to go fetch
            RatesPickerView().environmentObject(vm)
        }.onAppear(){
            vm.UpdateRates()
        }
    }
}
struct RatesPickerView: View {
    @EnvironmentObject var vm: CurrencyViewModel
    var body: some View {
        if vm.results != nil{
            //You can probaly populate this picker with the keys in
            // baseCurrency.baseCur.baseS
            Picker("rates", selection: $vm.selectedBaseCurrency){
                ForEach((vm.results!.baseCurrencies.first?.value.sorted{$0.key < $1.key})!, id: \.key) { key, rate in
                    Text(key).tag(key)
                }
            }
        }else{
            Text("waiting...")
        }
    }
}
class CurrencyViewModel: ObservableObject{
    
    @Published var results: BaseCurrency?
    @Published var selectedBaseCurrency: String = "usd"{
        didSet{
            UpdateRates()
        }
    }
    init() {
        //If you can .onAppear you don't need it here
        //UpdateRates()
    }
    func UpdateRates() {
        print(#function)
        let baseUrl = "https://cdn.jsdelivr.net/gh/fawazahmed0/currency-api@1/latest/currencies/"
        let baseCur = selectedBaseCurrency   // usd
        let requestType = ".json"
        
        guard let url = URL(string: baseUrl + baseCur + requestType) else {
            print("Invalid URL")
            return
        }
        let request = URLRequest(url: url)
        URLSession.shared.dataTask(with: request) { data, response, error in
            if let data = data {
                do{
                    let decodedResponse = try JSONDecoder().decode(BaseCurrency.self, from: data)
                    
                    DispatchQueue.main.async {
                        
                        if self.results == nil{
                            //Assign a new base currency
                            self.results = decodedResponse
                        }else{ //merge the existing with the new result
                            for base in decodedResponse.baseCurrencies.keys{
                                self.results?.baseCurrencies[base] = decodedResponse.baseCurrencies[base]
                            }
                        }
                        //update the UI
                        self.objectWillChange.send()
                    }
                    
                }catch{
                    //Error thrown by a try
                    print(error)//much more informative than error?.localizedDescription
                }
            }
            if error != nil{
                //data task error
                print(error!)
            }
        }.resume()
    }
}
struct CurrencyView_Previews: PreviewProvider {
    static var previews: some View {
        CurrencyView()
    }
}