Call a JSON with a general func

@narcisG

Aug '21

Swift, storyboards

I can acces to information in a JSON in a url. It works well.

My question is: can I have the func outside any class? In the example I provide, If I put func getData outside the class ViewController, is there a way I can call let result2 when I click the button check?

struct User: Codable {
    let name: String
    let email: String
}

func getData() {
    let url = URL(string: "https://jsonplaceholder.typicode.com/users")!
    URLSession.shared.dataTask(with: url) { data, _, _ in
        if let data = data {
            let result = try? JSONDecoder().decode([User].self, from: data)
            let result2 = result![0].name
            //print (result2)
        }
    }.resume()
}

class ViewController: NSViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

    }

    @IBAction func check(_ sender: NSButton) {
        getData()
        //How can I call result2 here?
    }

}

3

@roosterboy HWS+

Aug '21

You would need to return a value from your getData function.

3

@narcisG

Aug '21

I do not know how to do it. I tried many things. For instance:

func getData() -> (result2:String, result3:String) {
    let url = URL(string: "https://jsonplaceholder.typicode.com/users")!
    URLSession.shared.dataTask(with: url) { data, _, _ in
        if let data = data {
            let result = try? JSONDecoder().decode([User].self, from: data)
            let result2 = result![0].name
            let result3 = result![0].email
            //print (result2)
            return (result2, result3) // Error: Unexpected non-vioid return value in void function
        }
    }.resume()
}

3

@roosterboy HWS+

Aug '21

The usual way to return a value from an asynchronous action (such as using URLSession to query a URL) is to provide a completion handler. Something like this:

func getData(completion: (User?) -> Void) {
    let url = URL(string: "https://jsonplaceholder.typicode.com/users")!
    URLSession.shared.dataTask(with: url) { data, _, _ in
        if let data = data {
            do {
                let result = try JSONDecoder().decode([User].self, from: data)
                completion(result[0])
            } catch {
                completion(nil)
            }
        }
    }.resume()
}

And you would call it like this:

@IBAction func check(_ sender: NSButton) {
    getData { user in
       //user is a User?, so either a valid User or nil
       //do whatever you want with it
    }
}

Even better would be to supply a Result type to the completion handler:

enum MyErrorType: Error {
    case invalidUser
    //and any other errors you might receive
}

func getData(completion: (Result<User, MyErrorType>) -> Void) {
    let url = URL(string: "https://jsonplaceholder.typicode.com/users")!
    URLSession.shared.dataTask(with: url) { data, _, _ in
        if let data = data {
            do {
                let result = try JSONDecoder().decode([User].self, from: data)
                completion(.success(result[0]))
            } catch {
                completion(.failure(.invalidUser))
            }
        }
    }.resume()
}

@IBAction func check(_ sender: NSButton) {
    getData { result in
        switch result {
        case .success(let user):
            //do whatever you want with the user data
        case .failure(let error):
            //handle the error
        }
    }
}

3

@narcisG

Aug '21

It gives me an error: Escaping closure captures non-escaping parameter 'completion'

I do not understand what does it mean.

3

@roosterboy HWS+

Aug '21

Change:

func getData(completion: (Result<User, MyErrorType>) -> Void)

to:

func getData(completion: @escaping (Result<User, MyErrorType>) -> Void)

An escaping closure is one which isn't called immediately but may outlive the life of the function in which it is called. Since getData is asynchronous, the completion handler will eventually be called after getData has already run and exited, so it has to be marked as @escaping so the compiler knows to hang onto it until it is needed. My bad for not including it.

3

@narcisG

Aug '21

All this is very complicated for a beginner like me. There are several things I do not fully understand. That is two levels ahead to what I know. Anyway, it works well.

3

	@narcisG Aug '21 Swift, storyboards I can acces to information in a JSON in a url. It works well. My question is: can I have the func outside any class? In the example I provide, If I put func getData outside the class ViewController, is there a way I can call let result2 when I click the button check? struct User: Codable { let name: String let email: String } func getData() { let url = URL(string: "https://jsonplaceholder.typicode.com/users")! URLSession.shared.dataTask(with: url) { data, _, _ in if let data = data { let result = try? JSONDecoder().decode([User].self, from: data) let result2 = result![0].name //print (result2) } }.resume() } class ViewController: NSViewController { override func viewDidLoad() { super.viewDidLoad() } @IBAction func check(_ sender: NSButton) { getData() //How can I call result2 here? } } 3
	@roosterboy HWS+ Aug '21 You would need to return a value from your `getData` function. 3
	@narcisG Aug '21 I do not know how to do it. I tried many things. For instance: `func getData() -> (result2:String, result3:String) { let url = URL(string: "https://jsonplaceholder.typicode.com/users")! URLSession.shared.dataTask(with: url) { data, _, _ in if let data = data { let result = try? JSONDecoder().decode([User].self, from: data) let result2 = result![0].name let result3 = result![0].email //print (result2) return (result2, result3) // Error: Unexpected non-vioid return value in void function } }.resume() }` 3
	@roosterboy HWS+ Aug '21 The usual way to return a value from an asynchronous action (such as using `URLSession` to query a `URL`) is to provide a completion handler. Something like this: `func getData(completion: (User?) -> Void) { let url = URL(string: "https://jsonplaceholder.typicode.com/users")! URLSession.shared.dataTask(with: url) { data, _, _ in if let data = data { do { let result = try JSONDecoder().decode([User].self, from: data) completion(result[0]) } catch { completion(nil) } } }.resume() }` And you would call it like this: `@IBAction func check(_ sender: NSButton) { getData { user in //user is a User?, so either a valid User or nil //do whatever you want with it } }` Even better would be to supply a `Result` type to the completion handler: enum MyErrorType: Error { case invalidUser //and any other errors you might receive } func getData(completion: (Result<User, MyErrorType>) -> Void) { let url = URL(string: "https://jsonplaceholder.typicode.com/users")! URLSession.shared.dataTask(with: url) { data, _, _ in if let data = data { do { let result = try JSONDecoder().decode([User].self, from: data) completion(.success(result[0])) } catch { completion(.failure(.invalidUser)) } } }.resume() } @IBAction func check(_ sender: NSButton) { getData { result in switch result { case .success(let user): //do whatever you want with the user data case .failure(let error): //handle the error } } } 3
	@narcisG Aug '21 It gives me an error: Escaping closure captures non-escaping parameter 'completion' I do not understand what does it mean. 3
	@roosterboy HWS+ Aug '21 Change: `func getData(completion: (Result<User, MyErrorType>) -> Void)` to: `func getData(completion: @escaping (Result<User, MyErrorType>) -> Void)` An escaping closure is one which isn't called immediately but may outlive the life of the function in which it is called. Since `getData` is asynchronous, the completion handler will eventually be called after `getData` has already run and exited, so it has to be marked as `@escaping` so the compiler knows to hang onto it until it is needed. My bad for not including it. 3
	@narcisG Aug '21 All this is very complicated for a beginner like me. There are several things I do not fully understand. That is two levels ahead to what I know. Anyway, it works well. 3

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