This file is a walkthrough of an article by Donny Wals at https://www.donnywals.com/architecting-a-robust-networking-layer-with-protocols/

NetworkLayer.swift

import Foundation

struct Feed: Decodable {
}

class FeedViewModel {
	let service: FeedProviding
	var feed: Feed?
	var onFeedUpdate: () -> Void = {}

	init(service: FeedProviding) {
		self.service = service
	}

	func fetch() {
		service.getFeed { result in
			do {
				self.feed = try result.get()
				self.onFeedUpdate()
			} catch {
				// handle error
			}

		}
	}
}

protocol FeedProviding {
	var network: Networking { get }

	func getFeed(_ completion: @escaping (Result<Feed, Error>) -> Void)
}

extension FeedProviding {
	func getFeed(_ completion: @escaping (Result<Feed, Error>) -> Void) {
		network.execute(Endpoint.feed, completion: completion)
	}
}

protocol Networking {
	func execute<T: Decodable>(_ requestProvider: Endpoint, completion: @escaping (Result<T, Error>) -> Void)
}

enum Endpoint {
	case feed
}

extension Networking {
	func execute<T: Decodable>(_ requestProvider: Endpoint, completion: @escaping (Result<T, Error>) -> Void) {
		let urlRequest = requestProvider.urlRequest

		URLSession.shared.dataTask(with: urlRequest) { data, response, error in
			do {
				if let error = error {
					completion(.failure(error))
					return
				}

				guard let data = data else {
					preconditionFailure("No error was received but we also don't have data...")
				}

				let decodedObject = try JSONDecoder().decode(T.self, from: data)

				completion(.success(decodedObject))
			} catch {
				completion(.failure(error))
			}
		}.resume()
	}
}

protocol RequestProviding {
	var urlRequest: URLRequest { get }
}

extension Endpoint: RequestProviding {
	var urlRequest: URLRequest {
		switch self {
		case .feed:
			guard let url = URL(string: "https://mydomain.com/feed") else {
				preconditionFailure("Invalid URL used to create URL instance")
			}

			return URLRequest(url: url)
		}
	}
}