How to display JSON data from API in SwiftUI?

Question

I have an JSON that comes from an API. I need to show as a text a value from it.

This is a piece of it.

]
    {
        "d": "2019-09-20",
        "v": 56.62
    },
    {
        "d": "2019-09-23",
        "v": 56.93
    }
]

Now, I created the model for it, called Dolar.swift.

struct Dolar: Decodable {
    var d: String?
    var v: Double?
}

And also a class called WebService.swift that will handle the call:

class WebService {
    func getCurrency(completion: @escaping (Dolar?) -> ()) {
        guard let url = URL(string: "https://api.estadisticasbcra.com/usd_of") else { return }

        URLSession.shared.dataTask(with: url) { (data, response, error) in
            guard let data = data, error == nil else {
                return
            }

            let currencyResponse = try? JSONDecoder().decode(Dolar.self, from: data)

            if let currencyResponse = currencyResponse {
                let dolar = currencyResponse
                print(dolar)
                completion(dolar)
            } else {
                completion(nil)
            }
        }.resume()
    }
}

The documentation from the API says it requires a TOKEN be added to the request:

Authorization: BEARER {TOKEN}

How do I do that? I have the TOKEN, but don't know how to implement it. That's my first question. Second, once I get the value, how do I show it on the ContentView.swift? Any help is appreciated.

The decoding will not work regardless of if This is a piece of it. or not. Print the error — vadian
– vadian, Commented Sep 24, 2019 at 16:16
If the response is indeed an array of Dolar, then the decode statement should be JSONDecoder().decode([Dolar].self, from: data), but your completion expects a singular Dolar so there is some confusion that needs clearing up. — Chris
– Chris, Commented Sep 24, 2019 at 17:55
Sorry, I can't get the hand of this JSON decoding, could you help me out? If it helps, I only need the last object from the JSON (the last "d" and "v") but that may be a problem for another day. How can I decode this? — Nachot93
– Nachot93, Commented Sep 25, 2019 at 14:43

Chris · Accepted Answer · 2020-03-11 19:46:34Z

For the first part of your question, you need to use the variant of URLSession.dataTask which takes a URLRequest. Then you need to check the response had payload, that it can be decoded as your type and that it has at least one value in it. Something like:

let request = URLRequest(url: url)
request.setValue(token, forHTTPHeaderField: "Authorization")
// Or maybe:
// request.setValue("BEARER {\(token)}", forHTTPHeaderField: "Authorization")

URLSession.shared.dataTask(with: request) { (data, response, error) in
    guard let data = data else {
        return // Couldn't get data from service
    }

    guard let values = JSONDecoder().decode([Dolar].self, from: data) else {
        return  // Couldn't decode data as array of `Dolar`s
    }

    guard let finalValue = values.last else {
        return // No values in data
    }

    // `finalValue` now describes the last element in your array
}.resume()

See the docs for URLSession and URLRequest

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