Swift: Loading JSON data from a URL

Question

I am attempting to modify Apple's code from the Landmarks tutorial to load JSON data from a remote URL. The url is a php script which returns plaintext JSON data.

Apple's code:

func loadLocalJSON<T: Decodable>(_ filename: String) -> T {
    let data: Data
    
    guard let file = Bundle.main.url(forResource: filename, withExtension: nil)
        else {
            fatalError("Couldn't find \(filename) in main bundle")
    }
    
    do {
        data = try Data(contentsOf: file)
    } catch {
        fatalError("Couldn't load \(filename) from main bundle:\n\(error)")
    }
    
    do {
        let decoder = JSONDecoder()
        return try decoder.decode(T.self, from: data)
    } catch {
        fatalError("Couldn't parse \(filename) from main bundle:\n\(error)")
    }
}

And my code as currently modified:

func loadRemoteJSON<T: Decodable>(_ urlString: String) -> T {
    let data: Data
    
    guard let url = URL(string: urlString) else {
        fatalError("Invalid URL")
    }
    
    let request = URLRequest(url: url)
    URLSession.shared.dataTask(with: request) { data, response, error in
        guard let data = data else {
            fatalError(error?.localizedDescription ?? "Unknown Error")
        }
        
        do {
            let decoder = JSONDecoder()
            return try decoder.decode(T.self, from: data) // <--ERROR HERE
        } catch {
            fatalError("Couldn't parse data from \(urlString)\n\(error)")
        }
    }
}

The error I am getting is Unexpected non-void return value in void function

I thought the function was supposed to be returning an instance of T. Where am I going wrong?

"The error I am getting is Unexpected non-void return value in void function" Where!? — El Tomato, Jun 21 '21 at 05:36
Does this answer your question? [Unexpected Non-Void Return Value In Void Function (Swift 2.0)](https://stackoverflow.com/questions/32121108/unexpected-non-void-return-value-in-void-function-swift-2-0) — Joakim Danielson, Jun 21 '21 at 05:42
I think you need a completion block instead of a return. If you go with a new async-await then return is fine but this code is not for that. — Raja Kishan, Jun 21 '21 at 05:44
I agree with @RajaKishan-you shouldn't use a return if you're running an async task, you risk having the value of your variable initialized with `loadRemoteJSON` be nil since Swift will move forward through your code without awaiting the initialization of that variable. — The Swift Coder, Jun 21 '21 at 05:47

Raja Kishan · Accepted Answer · 2021-06-21T05:56:25.687

You need a completion block instead of a return type. You are doing the async task. URLSession.shared.dataTask is async type.

func loadRemoteJSON<T: Decodable>(_ urlString: String, completion: @escaping  ((T) -> Void)) {
    let data: Data
    
    guard let url = URL(string: urlString) else {
        fatalError("Invalid URL")
    }
    
    let request = URLRequest(url: url)
    URLSession.shared.dataTask(with: request) { data, response, error in
        guard let data = data else {
            fatalError(error?.localizedDescription ?? "Unknown Error")
        }
        
        do {
            let decoder = JSONDecoder()
            let data = try decoder.decode(T.self, from: data)
            completion(data)
        } catch {
            fatalError("Couldn't parse data from \(urlString)\n\(error)")
        }
    }
}

Usage:

struct TestModel: Decodable {
    var name: String
}

loadRemoteJSON("urlstring") { (data: TestModel) in
    print(data.name)
}

If you are using Swift 5.5 then you can return your data by using Async Await. There many articles and videos are available on the net. You can check this.

score 1 · Answer 2 · answered Jun 21 '21 at 05:54

func loadRemoteJSON<T: Decodable>(_ urlString: String,completion: @escaping (T)->Void)

Using the completion block is simple, just decode the JSON and pass it into your completion handler.

let decodedJSON = try decoder.decode(T.self, from: data)
completion(decodedJSON)

Now, when you call loadRemoteJSON, you'll be able to access the JSON inside of the function's completion block, like:

loadRemoteJSON(urlString: someString){ data in
 //data is your returned value of the generic type T.
}

Swift: Loading JSON data from a URL

2 Answers2

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