How can i make my Networking class generic in Swift? - ios

Question

Here i am extracting data as DataModel. But i want to make this class generic and pass the model myself so that i can use it to parse data from multiple API's. Can Anyone Help?
import Foundation
struct NetworkManager {
func fetchData(url : String, completion : #escaping (DataModel?) -> ()) {
print("Neeraj here")
let sessionURL = URL(string: url)
let session = URLSession(configuration: .default)
let dataTask = session.dataTask(with: sessionURL!) { (data, response, error) in
if error == nil {
if let safeData = data {
if let parsedData = self.parseData(data : safeData) {
print("got data")
completion(parsedData)
}
else {
debugPrint("failed to fetch data")
completion(nil)
}
}
}
else {
print("error in data task is \(String(describing: error))")
completion(nil)
}
}
dataTask.resume()
}
func parseData(data : Data) -> DataModel? {
let decoder = JSONDecoder()
do {
let decodedData = try decoder.decode(DataModel.self, from: data)
return decodedData
} catch {
print("error while parsing data \(error)")
return nil
}
}
}

Answer 1

With the convenient Result type you can write a quite tiny generic method, it returns the decoded type on success and any error on failure
func fetchData<T: Decodable>(urlString: String, completion: #escaping (Result<T,Error>) -> Void) {
guard let url = URL(string: urlString) else { return } // or throw an error
URLSession.shared.dataTask(with: url) { (data, _, error) in
if let error = error { completion(.failure(error)); return }
completion( Result{ try JSONDecoder().decode(T.self, from: data!) })
}.resume()
}
Note: Force unwrapping data! is 100% safe if no error occurs
Be aware that you have to specify the concrete type when you are going to call the method
fetchData(urlString: "https://example.com/api") { (result : Result<MyModel,Error>) in
switch result {
case .success(let model): print(model)
case .failure(let error): print(error)
}
}

Answer 2

You can add a generic type constraint (called Model) which conforms Decodable like below:
struct NetworkManager {
func fetchData<Model: Decodable>(url : String, completion : #escaping (Model?) -> ()) {
let sessionURL = URL(string: url)
let session = URLSession(configuration: .default)
let dataTask = session.dataTask(with: sessionURL!) { (data, response, error) in
if error == nil {
if let safeData = data {
do {
let decodedData = try JSONDecoder().decode(Model.self, from: safeData)
completion(decodedData)
} catch {
print("error while parsing data \(error)")
}
} else {
debugPrint("failed to fetch data")
completion(nil)
}
}
else {
print("error in data task is \(String(describing: error))")
completion(nil)
}
}
dataTask.resume()
}
}
Usage
struct SampleModel: Decodable {
let name: String
}
NetworkManager().fetchData(url: "") { (data: SampleModel?) in
print(data)
}

Answer 3

You can write a generic function to fetch data like this one :
func fetchGenericData<T: Decodable>(urlString: String, completion: #escaping (T) -> ()) {
let url = URL(string: urlString)
URLSession.shared.dataTask(with: url!) { (data, resp, err) in
if let err = err {
print("Failed to fetch data:", err)
return
}
guard let data = data else { return }
do {
let obj = try JSONDecoder().decode(T.self, from: data)
completion(obj)
} catch let jsonErr {
print("Failed to decode json:", jsonErr)
}
}.resume()
}
}
I suppose that you have a data model, if you have not, you should create for your every object. Also by using a dummy URL i will make a request and fetch the JSON includes some users name and ids with JSON format.
Let`s define a data model for this:
struct StackUser: Decodable {
let id: Int
let name: String
}
fetchGenericData(urlString: "https://api.stackoverexample.com/stackusers") { (stackUsers: [StackUser]) in
stackUsers.forEach({print($0.name)})
}
Finally you will be parse the data and prints like this:
Rob
Matt
Vadian