Wrapper around `dataTask(with, completionHandler:)` that uses swifts result type in the completion handler.

URLSession+Result.swift

//
//  URLSession+Result.swift
//  HttpRequestSample
//
//  Created by Felix Mau on 08/16/19.
//  Copyright © 2019 Felix Mau. All rights reserved.
//

import Foundation

extension URLSession {
    // MARK: - Types

    /// Custom error type, used to describe the missing data.
    enum DataTaskError: Error {
        /// The response object is missing.
        case invalidResponse

        /// The data object is missing.
        case invalidData
    }

    /// Tuple combining the response object (e.g. HTTP headers) and
    /// data of a successfully URLSession `dataTask`.
    typealias DataTaskResponse = (URLResponse, Data)

    /// Result type for a data task.
    typealias DataTaskResult = Result<DataTaskResponse, Error>

    // MARK: - Public methods

    /// Wrapper around `dataTask(with:URL, completionHandler:...)` that uses swift's result type in the completion handler.
    ///
    /// > Creates a task that retrieves the contents of the specified URL,
    /// > then calls a handler upon completion.
    ///
    /// - Parameters:
    ///   - url: The URL to be retrieved.
    ///   - completion: The completion handler to call when the request is completed.
    ///
    /// - Returns: The new session data task.
    ///
    /// - SeeAlso: [Apple documentation](https://developer.apple.com/documentation/foundation/urlsession/1410330-datatask)
    func dataTask(with url: URL,
                  completion: @escaping (DataTaskResult) -> Void) -> URLSessionDataTask {
        return dataTask(with: url) { [weak self] data, response, error in
            self?.handleDataTaskCompleted(data: data,
                                          response: response,
                                          error: error,
                                          completion: completion)
        }
    }

    /// Wrapper around `dataTask(with:URLRequest, completionHandler:...)` that uses swift's result type in the completion handler.
    ///
    /// > Creates a task that retrieves the contents of the specified URL,
    /// > then calls a handler upon completion.
    ///
    /// - Parameters:
    ///   - request: A URL request object that provides the URL, cache policy, request type, body data or body stream, and so on.
    ///   - completion: The completion handler to call when the load request is completed.
    ///
    /// - Returns: The new session data task.
    ///
    /// - SeeAlso: [Apple documentation](https://developer.apple.com/documentation/foundation/urlsession/1407613-datatask)
    func dataTask(with request: URLRequest,
                  completion: @escaping (DataTaskResult) -> Void) -> URLSessionDataTask {
        return dataTask(with: request) { [weak self] data, response, error in
            self?.handleDataTaskCompleted(data: data,
                                          response: response,
                                          error: error,
                                          completion: completion)
        }
    }

    // MARK: - Private methods

    private func handleDataTaskCompleted(data: Data?,
                                         response: URLResponse?,
                                         error: Error?,
                                         completion: (DataTaskResult) -> Void) {
        if let error = error {
            completion(.failure(error))
            return
        }

        guard let response = response else {
            completion(.failure(DataTaskError.invalidResponse))
            return
        }

        guard let data = data else {
            completion(.failure(DataTaskError.invalidData))
            return
        }

        completion(.success((response, data)))
    }
}