Expected to decode Array<Any> but found a dictionary instead

Question

I was fetching data from an API returning an array but needed to replace it by an API that has "sub levels":

RAW:
    ETH:
        USD:
             TYPE:              "5"
             MARKET:            "CCCAGG"
             FROMSYMBOL:        "ETH"
             TOSYMBOL:          "USD"
             PRICE:             680.89
             CHANGEPCT24HOUR    :   -9.313816893529749

Here is my struct:

struct Ethereum: Codable {

    let percentChange24h: String
    let priceUSD: String

    private enum CodingKeys: String, CodingKey {
        case priceUSD = "PRICE", percentChange24h = "CHANGEPCT24HOUR"
    }
}

And the implementation:

    func fetchEthereumInfo(completion: @escaping (Ethereum?, Error?) -> Void) {
    let url = URL(string: "https://min-api.cryptocompare.com/data/pricemultifull?fsyms=ETH&tsyms=USD")!

    let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
        guard let data = data else { return }
        do {
            if let ethereumUSD = try JSONDecoder().decode([Ethereum].self, from: data).first {
                print(ethereumUSD)
                completion(ethereumUSD, nil)
            }
        } catch {
            print(error)
        }
    }
    task.resume()
}

The console prints typeMismatch(Swift.Array<Any>, Swift.DecodingError.Context(codingPath: [], debugDescription: "Expected to decode Array<Any> but found a dictionary instead.", underlyingError: nil))

I can't really figure out what to update in my code or what this form of API is

Solution

First of all the JSON does not contain any array. It's very very easy to read JSON. There are only 2 (two!) collection types, array [] and dictionary {}. As you can see there are no square brackets at all in the JSON string.

Any (sub)dictionary {} has to be decoded to its own type, so it's supposed to be

struct Root : Decodable {
    private enum CodingKeys : String, CodingKey { case raw = "RAW" }
    let raw : RAW
}

struct RAW : Decodable {
    private enum CodingKeys : String, CodingKey { case eth = "ETH" }
    let eth : ETH
}

struct ETH : Decodable {
    private enum CodingKeys : String, CodingKey { case usd = "USD" }
    let usd : USD
}

struct USD : Decodable {

    private enum CodingKeys : String, CodingKey {
        case type = "TYPE"
        case market = "MARKET"
        case price = "PRICE"
        case percentChange24h = "CHANGEPCT24HOUR"
    }
    let type : String
    let market : String
    let price : Double
    let percentChange24h : Double
}

To decode the JSON and and print percentChange24h you have to write

 let result = try JSONDecoder().decode(Root.self, from: data)
 print("percentChange24h", result.raw.eth.usd.percentChange24h)

Comments

[Wizzardzz]

Thank you very much Vadian! This is exactly what I was looking for. Is there a simpler syntax for result.raw.eth.usd.percentChange24h ?

[vadian]

You could declare a computed variable in Root to get raw.eth.usd

[Wizzardzz]

Great, by the way I am getting Initializer for conditional binding must have Optional type, not 'Root' on the let result line. I'd like to keep my do-catch method so how can I get rid of the error?

[vadian]

Don't if let, if the decoding succeeds the result is a non-optional

[Leo Dabus]

Don't forget to tell OP to call completion inside the catch and inside the guard else bracket completion(nil, error)