Improved error handling on top of tundsdev async/await example on YouTube: https://youtu.be/53lbqYYAPLw

CharactersService.swift

//
//  CharactersService.swift
//  RickAndMortyApp
//
//  Created by Andreas Hård on 2021-09-27.
//

import Foundation

struct CharactersService {
    let decoder: JSONDecoder
    let endpointURL: URL = .init(string: "https://rickandmortyapi.com/api/character")!
    
    init(decoder: JSONDecoder) {
        self.decoder = decoder
    }
    
    func fetchCharacters() async -> FetchCharactersResult {
        do {
            let (data, response) = try await URLSession.shared.data(from: endpointURL)
            
            guard let response: HTTPURLResponse = response as? HTTPURLResponse else {
                let error: FetchCharactersError = .failedToGetHTTPURLResponse
                return .failure(error)
            }
            
            guard response.statusCode == 200 else {
                let error: FetchCharactersError = .invalidStatusCode(statusCode: response.statusCode)
                return .failure(error)
            }
            
            do {
                let response: CharactersResponseModel = try decoder.decode(CharactersResponseModel.self, from: data)
                return .success(response.results)
            } catch {
                let error: FetchCharactersError = .decodeError(error: error)
                return .failure(error)
            }
        } catch {
            let error: FetchCharactersError = .urlSessionError(error: error)
            return .failure(error)
        }
    }
}

extension CharactersService {
    typealias FetchCharactersResult = Result<[CharactersResponseModel.Character], FetchCharactersError>
    
    enum FetchCharactersError: Error {
        case urlSessionError(error: Error)
        case failedToGetHTTPURLResponse
        case invalidStatusCode(statusCode: Int)
        case decodeError(error: Error)
    }
}

Improvements to the code:

• Adding the JSONDecoder to the struct so it doesn't have to be instantiated each time the function is called
• Removing the throwing of the fetchCharacters() method to have a clear category of what types of errors can occur in the dedicated FetchCharactersError returned in the FetchCharactersResult