Generic JSON response from API request

Question

Created Jan ’21

Replies 3

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Views 5.2k

Participants 3

Hello guys. I've been stuck two days with this issue, hope you can understand my question below because I'm still new with Swift.

I want to create a generic function that request some JSON data from an API, decode the response using the Codable protocol and save it in a @State variable to update the View

Here is what I've done so far.

Code Block swiftstruct Response: Codable {
    var results: [Result]
}
struct Result: Codable {
    var trackId: Int
    var trackName: String
    var collectionName: String
}

Here is the function

Code Block swift    func loadData<T: Decodable>(model: T.Type) {
        guard let url = URL(string: "https://itunes.apple.com/search?term=taylor+swift&entity=song") else {
            print("Invalid URL")
            return
        }
        let request = URLRequest(url: url)
        URLSession.shared.dataTask(with: request) { data, response, error in
            if let data = data {
                if let decodedResponse = try? JSONDecoder().decode(model, from: data) {
                    DispatchQueue.main.async {
                        print(decodedResponse)
                    }
                    return
                }
            }
            print("Fetch failed: \(error?.localizedDescription ?? "Unknown error")")
        }
        .resume()
    }

The thing is, so far I'm only able to print the response, but I can't do something like this:

Code Block swiftDispatchQueue.main.async {
	self.results = decodedResponse.results
}

Because it shows me this error Value of type 'T' has no member ' results'

Here is the View

Code Block swiftstruct TestView: View {
    @State private var results = [Result]()
   	var body: some View {
        List(results, id: \.trackId) { item in
            VStack(alignment: .leading) {
                Text(item.trackName)
                    .font(.headline)
                Text(item.collectionName)
            }
        }
        .onAppear {
            loadData(model: Response.self)
        }
    }
// Here is the function defined
}

Somebody could give a hint? I really appreciate it

Thank you.
Dennis

Boost

Answer 1

OOPer OP

Jan ’21

Accepted Answer

If you want to make your loadData generic and access the property results of decodedResponse,
You need to tell that type T has a property named results.

For example:

Code Block struct Response: Codable {
    var results: [Result]
}
struct Result: Codable {
    var trackId: Int
    var trackName: String
    var collectionName: String
}
protocol HoldingResults {
    associatedtype R
    var results: [R] {get}
}
extension Response: HoldingResults {}

As you see, the protocol HoldingResults is used to tell Swift that the type conforming to it has a property named results.
You can define your loadData using the protocol:

Code Block     func loadData<T: Decodable>(model: T.Type, completion: @escaping ([T.R])->Void)
    where T: HoldingResults
    {
        guard let url = URL(string: "https://itunes.apple.com/search?term=taylor+swift&entity=song") 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(model, from: data)
                    DispatchQueue.main.async {
                        print(decodedResponse)
                        completion(decodedResponse.results)
                    }
                    return
                } catch {
                    print(error)
                }
            }
            print("Fetch failed: \(error?.localizedDescription ?? "Unknown error")")
        }
        .resume()
    }

And use it as:

Code Block         loadData(model: Response.self) {
            self.results = $0
        }

(I replaced if-let-try? to do-try-catch, that's my preference and not required.)

2

Answer 2

Claude31 OP

Jan ’21

Because it shows me this error Value of type 'T' has no member ' results'

That's true ! load() does not know anything yet about T.

Why do you want a generic here ? And not directly Response ?

Otherwise, you need to test is T is Response and cast it to Response.
But what's the interest in doing so ?

0

Answer 3

DennisCM OP

Jan ’21

It works perfectly, thank you very much.

0