Writing a JSON serialization function - json

Question

I'd like to make a function that takes in a few parameters and then outputs the data I need from a web API. Obviously a good deal of the time I'll need to customize it to suit the use case but just for fun I'm trying to figure out a super basic function the successfully parses JSON, as about half of the lines of code in the function below are generic error handling.
For example if I generally use something like
func getJSON(completionHandler: #escaping (Bool) -> ()) {
let jsonUrlString = "https://api.nytimes.com/svc/topstories/v1/business.json?api-key=f4bf2ee721031a344b84b0449cfdb589:1:73741808"
guard let url = URL(string: jsonUrlString) else {return}
URLSession.shared.dataTask(with: url) { (data, response, err) in
guard let data = data, err == nil else {
print(err!)
return
}
do {
let response = try
JSONDecoder().decode(TopStoriesResponse.self, from: data)
self.storyData = response.results
completionHandler(true)
} catch let jsonErr {
print("Error serializing JSON", jsonErr)
}
}.resume()
}
The only three things that will change from case to case (again, in the most absolutely basic of scenarios) are the url link to the API, the Struct that I set up to look for the pieces of data I need, and the array that I output the results to once the data request is finished.
Could I trim the fat on that and do something like
func jsonFetcher(apiLink: String, structToDecode: String, arrayThatHoldsResponse: [String], completionHandler: #escaping (Bool) -> ()) {
let jsonUrlString = apiLink
guard let url = URL(string: jsonUrlString) else {return}
URLSession.shared.dataTask(with: url) { (data, response, err) in
guard let data = data, err == nil else {
print(err!)
return
}
do {
let response = try
JSONDecoder().decode(structToDecode, from: data)
arrayThatHoldsResponse = response.results
completionHandler(true)
} catch let jsonErr {
print("Error serializing JSON", jsonErr)
}
}.resume()
}
I'm just not sure about the data types of structToDecode and arrayThatHoldsResponse (in the example function above I just using String as a placeholder), assuming they look like
Struct(s)
struct TopStoriesResponse: Decodable {
let status: String
let results: [Story]
}
struct Story: Decodable {
let title: String
let abstract: String
let url: String
let multimedia: [Multimedia]
private enum CodingKeys: String, CodingKey {
case title
case abstract
case url
case multimedia
}
init(from decoder:Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
title = try container.decode(String.self, forKey: .title)
abstract = try container.decode(String.self, forKey: .abstract)
url = try container.decode(String.self, forKey: .url)
multimedia = (try? container.decode([Multimedia].self, forKey: .multimedia)) ?? []
}
}
Array
var storyData = [Story]()
This way I can just call
jsonFetcher(apiLink: link, structToDecode: myStruct, arrayThatHoldsResponse: myArray, completionHandler: <#T##(Bool) -> ()#>)
Thanks for any help!

Answer 1

The power of generics. You can make a generic function, where the parameter is the urlString. The T inheritance the Decodable protocol.
This way you can call this function everytime as long as your Model inheritance the Decodable protocol.
func fetchData<T: Decodable>(urlString: String, completion: #escaping (T) -> ()) {
let url = URL(string: urlString)!
URLSession.shared.dataTask(with: url) { (data, response, error) in
if let error = error {
print(error.localizedDescription)
}
guard let data = data else { return }
do {
let object = try JSONDecoder().decode(T.self, from: data)
completion(object)
} catch let jsonErr {
print("Failed to decode json:", jsonErr)
}
}.resume()
}
How to call the function:
struct User: Decodable { }
fetchData(urlString: "yourUrl") { (User: User) in
// Handle result
}
struct Animal: Decodable { }
fetchData(urlString: "yourUrl") { (animal: Animal) in
// Handle result
}
// Or if you want to fetch an array of users instead
fetchData(urlString: "yourUrl") { (users: [User]) in
// Handle result
}
In your case
var storiesData: [Story] = []
fetchData(urlString: "https://api.nytimes.com/svc/topstories/v1/business.json?api-key=f4bf2ee721031a344b84b0449cfdb589:1:73741808") { (stories: [Story] in
storiesData = stories
DispatchQueue.main.async {
self.tableView.reloadData()
}
}

Answer 2

Based on Jacob's answer I recommend to return also a possible error.
To keep the generic layout declare an – also generic – enum as return type
enum FetchResult<T> {
case success(T), failure(Error)
}
and return FetchResult with the passed static type
func fetchData<T: Decodable>(url: URL, completion: #escaping (FetchResult<T>) -> Void) {
URLSession.shared.dataTask(with: url) { (data, response, error) in
guard let data = data else {completion(.failure(error!)); return }
do {
let object = try JSONDecoder().decode(T.self, from: data)
completion(.success(object))
} catch {
completion(.failure(error))
}
}.resume()
}
and use it
let jsonUrl = URL(string: "https://api.nytimes.com/svc/topstories/v1/business.json?api-key=••••••••••••••••••:1:73741808")!
fetchData(url: jsonUrl) { (result : FetchResult<TopStoriesResponse>) in
switch result {
case .success(let object): print(object) // do something with object
case .failure(let error): print(error) // handle the error
}
}