An example on how enums can clean up your code

Enums.swift

//
//  ViewController.swift
//  LambdaEnums
//
//  Created by Fernando Olivares on 11/1/19.
//  Copyright © 2019 Fernando Olivares. All rights reserved.
//

import UIKit

class ViewController: UIViewController {

	override func viewDidLoad() {
		super.viewDidLoad()
		
		// Registering a User
		let api = API()
		api.registerUser(username: "fernando", password: "password") { result in

			switch result {

			case .fail(let error):
				print("Error: \(error)")

			case .success(let userDictionary):
				print("User: \(userDictionary)")
			}
		}

		// Getting all events
//		api.getEvents { result in
//
//			switch result {
//
//			case .fail(let error):
//				print("Error: \(error)")
//
//			case .success(let events):
//				print("Events: \(events)")
//			}
//		}
	}
}

class API {
	
	enum Endpoint {
		case registerUser(username: String, password: String)
		case getEvents
		
		var url: URL {
			
			let endpoint: String
			switch self {
				
			case .getEvents:
				endpoint = "events"
				
			case .registerUser:
				endpoint = "user"
			}
			
			return URL(string: "https://demo0638130.mockable.io/\(endpoint)")!
		}
		
		var request: URLRequest {
			
			switch self {
			case .getEvents:
				return URLRequest(url: url)
				
			case .registerUser(let username, let password):
				var request = URLRequest(url: url)
				request.httpMethod = "POST"
				
				let requestDataDictionary = [
					"username": username,
					"password": password,
				]
				request.httpBody = try! JSONSerialization.data(withJSONObject: requestDataDictionary)
				return request
			}
			
		}
	}
	
	func registerUser(username: String, password: String, completion: @escaping (DuplicateCodeResult) -> () ) {
		duplicateCode(endpoint: .registerUser(username: username,
											  password: password),
					  completion: completion)
	}
	
	enum GetEventsResult {
		case fail(Error)
		case success([String: Any])
	}
	func getEvents(completion: @escaping (GetEventsResult) -> () ) {
		
		let url = URL(string: "https://demo0638130.mockable.io/events")!
		let request = URLRequest(url: url)
		
		let task = URLSession.shared.dataTask(with: request) { (possibleData, possibleResponse, possibleError) in
			
			// possibleData = nil, possibleResponse = nil, possibleError = nil (doesn't make sense)
			// possibleData != nil, possibleResponse = nil, possibleError = nil (successful)
			guard possibleError == nil else {
				completion(.fail(possibleError!))
				return
			}
			
			guard let existingData = possibleData else {
				// There's no error, but no data either.
				completion(.fail(NSError(domain: "Invalid State", code: 1, userInfo: nil)))
				return
			}
			
			guard let parsedJSON = try! JSONSerialization.jsonObject(with: existingData) as? [String: Any]  else {
				completion(.fail(NSError(domain: "Invalid Data", code: 2, userInfo: nil)))
				return
			}
			
			completion(.success(parsedJSON))
		}
		
		task.resume()
	}
	
	enum DuplicateCodeResult {
		case fail(Error)
		case success([String: Any])
	}
	func duplicateCode(endpoint: Endpoint, completion: @escaping (DuplicateCodeResult) -> () ) {
		
		let task = URLSession.shared.dataTask(with: endpoint.request) { (possibleData, possibleResponse, possibleError) in
			
			// possibleData = nil, possibleResponse = nil, possibleError = nil (doesn't make sense)
			// possibleData != nil, possibleResponse = nil, possibleError = nil (successful)
			guard possibleError == nil else {
				completion(.fail(possibleError!))
				return
			}
			
			guard let existingData = possibleData else {
				// There's no error, but no data either.
				completion(.fail(NSError(domain: "Invalid State", code: 1, userInfo: nil)))
				return
			}
			
			guard let parsedJSON = try! JSONSerialization.jsonObject(with: existingData) as? [String: Any]  else {
				completion(.fail(NSError(domain: "Invalid Data", code: 2, userInfo: nil)))
				return
			}
			
			completion(.success(parsedJSON))
		}
		
		task.resume()
	}
}

Awesome! Thanks for doing so!