Swift: Loading JSON data from a URL - json

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?

Answer 1

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.

Answer 2

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.
}