Fetching JSON from URL in SwiftUI

Question

the following is the code I am trying to get to work, all I want to do is display the current price and the currency it is in.

the link returns a format of : {"currency":"USD","rate":"178.0466666666667"}

import SwiftUI
import Combine

struct BitcoinPrice: Decodable, Identifiable {
    var id = UUID()
    let currency: String
    let rate: String
}


struct BSVPrice: View {
    @State private var requests = Set<AnyCancellable>()
    @State private var exchangeRate = [BitcoinPrice]()
    
    var body: some View {
        NavigationView {
            List(exchangeRate) { rate in
                VStack(alignment: .leading) {
                    Text(rate.currency)
                    Text(rate.rate)
                }
            }
            .navigationTitle("Bitcoin Price")
        }
        
        .onAppear {
            let bitcoinPriceURL = URL(string: "https://api.whatsonchain.com/v1/bsv/main/exchangerate")!
            //let bitcoinPriceTask = fetch(bitcoinPriceURL, defaultValue: [BitcoinPrice]() )
            fetch(bitcoinPriceURL, defaultValue: [BitcoinPrice]() ) {
                exchangeRate = $0
            }
        }
    }
    
    func fetch<T: Decodable>(_ url: URL, defaultValue: T, completion: @escaping (T) -> Void) {
        let decoder = JSONDecoder()
        
        URLSession.shared.dataTaskPublisher(for: url)
            .retry(1)
            .map(\.data)
            .decode(type: T.self, decoder: decoder)
            .replaceError(with: defaultValue)
            .receive(on: DispatchQueue.main)
            .sink(receiveValue: completion)
            .store(in: &requests)
    }
    
    func fetch<T: Decodable>(_ url: URL, defaultValue: T) -> AnyPublisher<T, Never> {
        let decoder = JSONDecoder()
        
        return URLSession.shared.dataTaskPublisher(for: url)
            .retry(1)
            .map(\.data)
            .decode(type: T.self, decoder: decoder)
            .replaceError(with: defaultValue)
            .receive(on: DispatchQueue.main)
            .eraseToAnyPublisher()
    }
}


struct BSVPrice_Previews: PreviewProvider {
    static var previews: some View {
        BSVPrice()
    }
}

Does anyone know why the JSON is not loading into the application? Any help would be great, I have got the code to work with a different API so a bit confused.

You should get and print the Decoding error somehow. There is an error. — vadian
– vadian, Commented Jul 21, 2020 at 16:27

pawello2222 · Accepted Answer · 2020-07-21 16:39:36Z

1

First you're suppressing your error by using .replaceError(with: defaultValue) which will hide any errors and replace them with a default value (here an empty list).

In

func fetch<T: Decodable>(_ url: URL, defaultValue: T, completion: @escaping (T) -> Void)

you're trying to decode T.self:

.decode(type: T.self, decoder: decoder)

which should be of BitcoinPrice type to work.

Note that the response from your link is a single object, not an array.

However, in your view, you pass [BitcoinPrice] as the default value:

fetch(bitcoinPriceURL, defaultValue: [BitcoinPrice]() ) { ...

which infers T to be of type [BitcoinPrice].

Also remove var id = UUID() from BitcoinPrice:

struct BitcoinPrice: Decodable {
    let currency: String
    let rate: String
}

and use it like this:

List(exchangeRate, id: \.currency) { rate in

I also recommend to use plural names for collections, ie. exchangeRates instead of exchangeRate.

edited Jul 21, 2020 at 16:39

answered Jul 21, 2020 at 16:34

pawello2222

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4 Comments

Henry Hudson Over a year ago

Thanks for your help! with the T.self I was trying to make the code generic so that it can work with anything, are you saying I should replace the T.self with BitcoinPrice? I have tried to incorporate your suggestions into my code but its saying im missing some parameters `

pawello2222 Over a year ago

@HenryHudson There's no problem with your code being generic. But if you want to decode BitcoinPrice you need to make your generic function correctly infer the T type. You can do it by passing some BitcoinPrice variable as the default value.

Henry Hudson Over a year ago

I see, I have added the defaultValue of BitcoinPrice() now, I am still getting some errors though, think the main one is it says I am missing a parameter from @State private var exchangeRates = BitcoinPrice(), it wants a from parameter in the braces I tried to post the code into this comment but am limited by the number of characters that comments allow

pawello2222 Over a year ago

@HenryHudson exchangeRate is an array. When you assign the decoded value: exchangeRate = $0 replace it with exchangeRate = [$0]

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