De error a alerta con acciones

• Agregar funcionalidad al tipo de error
• Convierta los errores en alertas fáciles de usar
• Mostramos las posibles acciones adicionales en la interfaz y procesamos sus clics

final class FeedViewController: UIViewController {
  // -   

	func handleFeedResponse(...) {
	/// -    
	if let error = error {
		let alertVC = UIAlertController(
			title: "Error",
			message: "Error connecting to the server",
			preferredStyle: .alert)
		let action = UIAlertAction(title: "OK", style: .default, handler: nil)
		alertVC.addAction(action)
		self.present(alertVC, animated: true, completion: nil)
	}
}

• No le dimos la oportunidad de cambiar de alguna manera un escenario erróneo a uno exitoso. OK en el caso actual, solo oculta la alerta, y esta no es una solución
• Desde el punto de vista de la experiencia del usuario, el texto debe hacerse más claro y neutral. Para que el usuario no tenga miedo y no se ejecute para poner una estrella en la AppStore para su aplicación. En este caso, un texto detallado nos sería útil al depurar
• Y, para ser sincero, las alertas están algo desactualizadas como solución (cada vez más, aparecen pantallas ficticias o brindis en las aplicaciones). Pero esta es una pregunta que debe discutirse por separado con el equipo.

• Debe ser extensible. Todos sabemos acerca de la variabilidad del diseño inherente. Nuestro mecanismo debe estar listo para cualquier cosa.
• Se agrega al objeto (y se elimina) en un par de líneas de código
• Bien probado

Error en Swift

public protocol Error {
}

public enum DecodingError : Error {
    
        /// ,     . 
    	///  ,    
        public struct Context {
    
    	    ///      
            public let codingPath: [CodingKey]
            public let debugDescription: String
    
    	    /// ,    . 
            ///      .   
            public let underlyingError: Error?
    
            public init(codingPath: [CodingKey], debugDescription: String, underlyingError: Error? = nil)
        }
    
    	/// N    
        case typeMismatch(Any.Type, DecodingError.Context)
        case valueNotFound(Any.Type, DecodingError.Context)
    
    ...

enum NetworkError: Error {
       //  500 
	case serverError
        //   ,   
	case responseError
        //  ,   ,  
	case internetError
}

func blah() -> String throws {
    	throw NetworkError.serverError
    }
    
    do {
    	let string = try blah()
    	//     ,      
    	let anotherString = try blah()
    } catch {
    	//  NetworkError.serverError
    	print(error)
    }
    
    //  string = nil
    let string = try? blah()

func blah<ResultType>(handler: @escaping (Swift.Result<ResultType, Error>) -> Void) {
	handler(.failure(NetworkError.serverError)
}

blah<String>(handler { result in 
	switch result {
		case .success(let value):
			print(value)
		case .failure(let error):
			print(error)
	}
})

• Errores en Swift es un protocolo
• Es conveniente presentar errores en forma de enumeración
• Hay dos maneras de lidiar con los errores: sincrónico (capturar) y asincrónico (su propia competencia o resultado)

Texto de error

enum NetworkError: Error {
        //  500 
	case serverError
        //   ,   
	case responseError
        //  ,   ,  
	case internetError
}

protocol LocalizedError : Error {
    var errorDescription: String? { get }
    var failureReason: String? { get }
    var recoverySuggestion: String? { get }
    var helpAnchor: String? { get }
}

extension NetworkError: LocalizedError {
    	var errorDescription: String? {
            switch self {
            case .serverError, .responseError:
                return "Error"
    	    case .internetError:
                return "No Internet Connection"
            }
        }
    
        var failureReason: String? {
            switch self {
            case .serverError, .responseError:
                return "Something went wrong"
    	    case .internetError:
                return nil
            }
        }
    
        var recoverySuggestion: String? {
            switch self {
            case .serverError, .responseError:
                return "Please, try again"
    	    case .internetError:
                return "Please check your internet connection and try again"
            }
        }
    }

	if let error = error {
		let errorMessage = [error.failureReason, error.recoverySuggestion].compactMap({ $0 }).joined(separator: ". ")
		let alertVC = UIAlertController(
			title: error.errorDescription,
			message: errorMessage,
			preferredStyle: .alert)
		let action = UIAlertAction(title: "OK", style: .default) { (_) -> Void in }
		alertVC.addAction(action)
		self.present(alertVC, animated: true, competion: nil)

Error de recuperación

struct RecovableAction {
    	let title: String
    	let action: () -> Void
    }
    
    func showRecovableOptions(actions: [RecovableAction], from viewController: UIViewController) {
    	let alertActions = actions.map { UIAlertAction(name: $0.title, action: $0.action) }
    	let cancelAction = UIAlertAction(name: "Cancel", action: nil)
    	let alertController = UIAlertController(actions: alertActions)
    	viewController.present(alertController, complition: nil)
    }

func handleError(error: Error) {
	if error is RecovableError {
		showRecovableOptions(actions: error.actions, from: viewController)
		return
	}
	showErrorAlert(...)
} 

struct RecovableError: Error {
	let recovableACtions: [RecovableAction]
	let context: Context
}

Implementación nativa?

// A specialized error that may be recoverable by presenting several potential recovery options to the user.
protocol RecoverableError : Error {
    var recoveryOptions: [String] { get }

    func attemptRecovery(optionIndex recoveryOptionIndex: Int, resultHandler handler: @escaping (Bool) -> Void)
    func attemptRecovery(optionIndex recoveryOptionIndex: Int) -> Bool
}

• recoveryOptions: [String] - una propiedad que almacena opciones de recuperación
• func intentRecovery (optionIndex: Int) -> Bool - restaura de un error, sincrónicamente. Verdadero: en caso de éxito
• func intentRecovery (optionIndex: Int, resultHandler: (Bool) -> Void) - Opción asincrónica, la idea es la misma

• El mecanismo está pensado para aplicaciones MacOs y muestra un cuadro de diálogo
• Originalmente fue construido alrededor de NSError.
• El objeto RecoveryAttempter está encapsulado dentro del error en userInfo, que conoce las condiciones del error y puede elegir la mejor solución al problema. El objeto no debe ser nulo
• RecoveryAttempter debe admitir el protocolo informal NSErrorRecoveryAttempting
• También en userInfo debería haber una opción de recuperación
• Y todo está relacionado con la llamada al método presentError, que solo está en el SDK de macOS. Muestra una alerta
• Si la alerta se muestra a través de presentError, cuando selecciona una opción en la ventana emergente en AppDelegate, una función interesante se contrae:

func attemptRecovery(fromError error: Error, optionIndex recoveryOptionIndex: Int, delegate: Any?, didRecoverSelector: Selector?, contextInfo: UnsafeMutableRawPointer?)

Construir una bicicleta

struct RecoverableError: Foundation.RecoverableError {
	let error: Error
	var recoveryOptions: [String] {
		return ["Try again"]s
	}
	
	func attemptRecovery(optionIndex recoveryOptionIndex: Int) -> Bool {
		//  ,    
		return true
	}
	
	func attemptRecovery(optionIndex: Int, resultHandler: (Bool) -> Void) {
		//      . 
               //      
		switch optionIndex {
			case 0:
				resultHandler(true)
			default: 
				resultHandler(false)
	}
}

struct RecoveryAttemper {
    	//   
    	private let _recoveryOptions: [RecoveryOptions]
    
    	var recoveryOptionsText: [String] {
    		return _recoveryOptions.map({ $0.title })
    	}
    
    	init(options: [RecoveryOptions] {
    		_recoveryOptions = recoveryOptions
    	}
    
    	//    
    	func attemptRecovery(fromError error: Error, optionIndex: Int) -> Bool {
    		let option = _recoveryOptions[optionIndex]
    				switch option {
    				case .tryAgain(let action)
    					action()
    					return true
    				case .cancel:
    					return false
    				}
    		}
    }
    
    //  enum,       
    enum RecoveryOptions {
    	//      (,     )
    	case tryAgain(action: (() -> Void))
    	case cancel
    }

protocol ErrorAlertCreatable: class, ErrorReasonExtractable {
    	//     
    	func createAlert(for error: Error) -> UIAlertController
    }
    
    // MARK: - Default implementation
    extension ErrorAlertCreatable where Self: UIViewController {
    	func createAlert(for error: Error) -> UIAlertController {
    		//      
    		if let recoverableError = error as? RecoverableError {
    			return createRecoverableAlert(for: recoverableError)
    		}
    		let defaultTitle = "Error"
    		let description = errorReason(from: error)
    
    		//          
    		if let localizedError = error as? LocalizedError {
    			return createAlert(
    				title: localizedError.errorDescription ?? defaultTitle,
    				message: description,
    				actions: [.okAction],
    				aboveAll: aboveAll)
    		}
    
    		return createAlert(title: defaultTitle, message: description, actions: [.okAction])
    	}
    
    	fileprivate func createAlert(title: String?, message: String?, actions: [UIAlertAction]) -> UIAlertController {
    		let alertViewController = UIAlertController(title: title, message: message, preferredStyle: .alert)
    		actions.forEach({ alertViewController.addAction($0) })
    		return alertViewController
    	}
    
    	fileprivate func createRecoverableAlert(for recoverableError: RecoverableError) -> UIAlertController {
    		let title = recoverableError.errorDescription
    		let message = recoverableError.recoverySuggestion
    		//     . 
    		let actions = recoverableError.recoveryOptions.enumerated().map { (element) -> UIAlertAction in
    		let style: UIAlertAction.Style = element.offset == 0 ? .cancel : .default
    		return UIAlertAction(title: element.element, style: style) { _ in
    			recoverableError.attemptRecovery(optionIndex: element.offset)
    		      }
    		}
    		return createAlert(title: title, message: message, actions: actions)
    	}
    
    	func createOKAlert(with text: String) -> UIAlertController {
    		return createAlert(title: text, message: nil, actions: [.okAction])
    	}
    }
    
    extension ERror
    
    //     ok
    extension UIAlertAction {
    	static let okAction = UIAlertAction(title: "OK", style: .cancel) { (_) -> Void in }
    }
    
    //    
    protocol ErrorReasonExtractable {
    	func errorReason(from error: Error) -> String?
    }
    
    // MARK: - Default implementation
    extension ErrorReasonExtractable {
    	func errorReason(from error: Error) -> String? {
    		if let localizedError = error as? LocalizedError {
    			return localizedError.recoverySuggestion
    		}
    		return "Something bad happened. Please try again"
    	}
    }

protocol ErrorAlertPresentable: class {
	func presentAlert(from error: Error)
}

// MARK: - Default implementation
extension ErrorAlertPresentable where Self: ErrorAlertCreatable & UIViewController {
	func presentAlert(from error: Error) {
		let alertVC = createAlert(for: error)
		present(alertVC, animated: true, completion: nil)
	}
}

protocol ViewControllerInput: class {
     //  
    }
    extension ViewControllerInput: ErrorAlertPresentable { }
    
    extension ViewController: ErrorAlertCreatable { }
    //  ,         ,    
    //    
    
    //       "",    ErrorAlertPresentable      . 
    extension ViewController: ErrorToastCreatable { }

func requestFeed(...) {
    		service.requestObject { [weak self] (result) in
    			guard let `self` = self else { return }
    			switch result {
    			case .success:
    				break
    			case .failure(let error):
    				//           
    				// -     (   viewController)  ,
    				//    .     tryAgainOption 
    				let tryAgainOption = RecoveryOptions.tryAgain {
    					self.requestFeed(...)
    				}
    				let recoveryOptions = [tryAgainOption]
    				let attempter = RecoveryAttemper(recoveryOptions: recoveryOptions)
    				let recovableError = RecoverableError(error: error, attempter: attempter)
    				self.presentAlert(from: recovableError)
    			}
    		}
    	}
    
    // MARK: - ErrorAlertCreatable
    extension ViewController: ErrorAlertCreatable { }
    
    // MARK: - ErrorAlertPresentable
    extension ViewController: ErrorAlertPresentable { }

struct RecoveryAttemper {
    	//
    	...
    	//
    	static func tryAgainAttempter(block: @escaping (() -> Void)) -> Self {
    		return RecoveryAttemper(recoveryOptions: [.cancel, .tryAgain(action: block)])
    	}
    }
    
    
    func requestFeed() {
    		service.requestObject { [weak self] (result) in
    			guard let `self` = self else { return }
    			switch result {
    			case .success:
    				break
    			case .failure(let error):
    				//    
    				let recovableError = RecoverableError(error: error, attempter: .tryAgainAttempter(block: {
    					self.requestFeed()
    				}))
    				self.presentAlert(from: recovableError)
    			}
    		}
    	}

¿Qué pasa si tenemos varios escenarios de salida?

//  
    func runOutOfSpace() {
    		service.runOfSpace { [weak self] (result) in
    			guard let `self` = self else { return }
    			switch result {
    			case .success:
    				break
    			case .failure(let error):
    				let notEnoughSpace = RecoveryOptions.freeSpace {
    					self.freeSpace()
    				}
    
    				let buyMoreSpace = RecoveryOptions.buyMoreSpace {
    					self.buyMoreSpace()
    				}
    				let options = [notEnoughSpace, buyMoreSpace]
    				let recovableError = RecoverableError(error: error, attempter: .cancalableAttemter(options: options))
    				self.presentAlert(from: recovableError)
    			}
    		}
    	}
    
    	func freeSpace() {
    		let alertViewController = createOKAlert(with: "Free space selected")
    		present(alertViewController, animated: true, completion: nil)
    	}
    
    	func buyMoreSpace() {
    		let alertViewController = createOKAlert(with: "Buy more space selected")
    		present(alertViewController, animated: true, completion: nil)
    	}
    
    
    struct RecoveryAttemper {
    	//
    	...
    	//
    	static func cancalableAttemter(options: [RecoveryOptions]) -> Self {
    		return RecoveryAttemper(recoveryOptions: [.cancel] + options)
    	}
    }

Si queremos mostrar no una alerta, sino una vista de información en el medio de la pantalla?

protocol ErrorViewCreatable {
    	func createErrorView(for error: Error) -> ErrorView
    }
    
    // MARK: - Default implementation
    extension ErrorViewCreatable {
    	func createErrorView(for error: Error) -> ErrorView {
    		if let recoverableError = error as? RecoverableError {
    			return createRecoverableAlert(for: recoverableError)
    		}
    
    		let defaultTitle = "Error"
    		let description = errorReason(from: error)
    		if let localizedError = error as? LocalizedError {
    			return createErrorView(
    				title: localizedError.errorDescription ?? defaultTitle,
    				message: description)
    		}
    		return createErrorView(title: defaultTitle, message: description)
    		}
    
    	fileprivate func createErrorView(title: String?, message: String?, actions: [ErrorView.Action] = []) -> ErrorView {
    		//  ErrorView        . 
                //     github
    		return ErrorView(title: title, description: message, actions: actions)
    	}
    
    	fileprivate func createRecoverableAlert(for recoverableError: RecoverableError) -> ErrorView {
    		let title = recoverableError.errorDescription
    		let message = errorReason(from: recoverableError)
    		let actions = recoverableError.recoveryOptions.enumerated().map { (element) -> ErrorView.Action in
    			return ErrorView.Action(title: element.element) {
    				recoverableError.attemptRecovery(optionIndex: element.offset)
    			}
    		}
    		return createErrorView(title: title, message: message, actions: actions)
    	}
    }

protocol ErrorViewAddable: class {
    	func presentErrorView(from error: Error)
    
    	var errorViewSuperview: UIView { get }
    }
    
    // MARK: - Default implementation
    extension ErrorViewAddable where Self: ErrorViewCreatable {
    	func presentErrorView(from error: Error) {
    		let errorView = createErrorView(for: error)
    		errorViewSuperview.addSubview(errorView)
    		errorView.center = errorViewSuperview.center
    	}
    }
    
    
    //    
    
    // MARK: - ErrorAlertCreatable
    extension ViewController: ErrorViewCreatable { }
    
    // MARK: - ErrorAlertPresentable
    extension ViewController: ErrorViewAddable {
    	var errorViewSuperview: UIView {
    		return self
    	}
    }

Si no hay acceso a la vista?

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

	//   –   . 
	//              DI
	static private(set) var errorWindow: UIWindow = {
		let alertWindow = UIWindow.init(frame: UIScreen.main.bounds)
		alertWindow.backgroundColor = .clear

		//  rootViewController,    present   viewController
		let viewController = UIViewController()
		viewController.view.backgroundColor = .clear
		alertWindow.rootViewController = viewController

		return alertWindow
	}()

final class AboveAllAlertController: UIAlertController {
	var alertWindow: UIWindow {
		return AppDelegate.alertWindow
	}

	func show() {
		let topWindow = UIApplication.shared.windows.last
		if let topWindow = topWindow {
			alertWindow.windowLevel = topWindow.windowLevel + 1
		}

		alertWindow.makeKeyAndVisible()
		alertWindow.rootViewController?.present(self, animated: true, completion: nil)
	}

	override func viewWillDisappear(_ animated: Bool) {
		super.viewWillDisappear(animated)

		alertWindow.isHidden = true
	}
}

protocol ErrorAlertCreatable: class {
    	//      
    	func createAlert(for error: Error, aboveAll: Bool) -> UIAlertController
    }
    
    // MARK: - Default implementation
    extension ErrorAlertCreatable where Self: UIViewController {
    	...
    	//     
    	fileprivate func createAlert(title: String?, message: String?, actions: [UIAlertAction], aboveAll: Bool) -> UIAlertController {
    		let alertViewController = aboveAll ?
    			AboveAllAlertController(title: title, message: message, preferredStyle: .alert) :
    			UIAlertController(title: title, message: message, preferredStyle: .alert)
    		actions.forEach({ alertViewController.addAction($0) })
    		return alertViewController
    	}
    }
    
    //     
    protocol ErrorAlertPresentable: class {
    	func presentAlert(from error: Error)
    	func presentAlertAboveAll(from error: Error)
    }
    
    // MARK: - Default implementation
    extension ErrorAlertPresentable where Self: ErrorAlertCreatable & UIViewController {
    	func presentAlert(from error: Error) {
    		let alertVC = createAlert(for: error, aboveAll: false)
    		present(alertVC, animated: true, completion: nil)
    	}
    
    	func presentAlertAboveAll(from error: Error) {
    		let alertVC = createAlert(for: error, aboveAll: true)
    		//         
    		if let alertVC = alertVC as? AboveAllAlertController {
    			alertVC.show()
    			return
    		}
    		//    ,  - 
    		assert(false, "Should create AboveAllAlertController")
    		present(alertVC, animated: true, completion: nil)
    	}
    }

Tiempo mínimo de solicitud

final class DelayOperation: AsyncOperation {
	private let _delayTime: Double

	init(delayTime: Double = 0.3) {
		_delayTime = delayTime
	}
	override func main() {
		super.main()

		DispatchQueue.global().asyncAfter(deadline: .now() + _delayTime) {
			self.state = .finished
		}
	}
}
		
// -  
let flowListOperation = flowService.list(for: pageID, path: path, limiter: limiter)
let handler = createHandler(for: flowListOperation)
let delayOperation = DelayOperation(delayTime: 0.5)
///  >>>  addDependency. 
[flowListOperation, delayOperation] >>> handler
operationQueue.addOperations([flowListOperation, delayOperation, handler])

//  global      
DispatchQueue.global().asyncAfter(deadline: .now() + 0.15) {
    // your code here
}

struct Task {
    	let closure: () -> Void
    
    	private var _delayTime: Double?
    
    	init(closure: @escaping () -> Void) {
    		self.closure = closure
    	}
    
    	fileprivate init(closure: @escaping () -> Void, time: Double) {
    		self.closure = closure
    		_delayTime = time
    	}
    
    	@discardableResult
    	func run() -> Self {
    		if let delayTime = _delayTime {
    			DispatchQueue.global().asyncAfter(deadline: .now() + delayTime) {
    				self.closure()
    			}
    			return self
    		}
    		closure()
    		return self
    	}
    
    	func delayedTask(time: Double) -> Self {
    		return Task(closure: closure, time: time)
    	}
    }
    
    //    
    func requestObject(completionHandler: @escaping ((Result<Bool, Error>) -> Void)) -> Task {
    		return Task {
    			completionHandler(.failure(NetworkError.internetError))
    		}
    			.delayedTask(time: 0.5)
    		.run()
    	}

Testabilidad

func requestFeed() {
		service.requestObject { [weak self] (result) in
			guard let `self` = self else { return }
			switch result {
			case .success:
				break
			case .failure(let error):
				DispatchQueue.main.async {
					let recoverableError = RecoverableError(error: error, attempter: .tryAgainAttempter(block: {
						self.requestFeed()
					}))
					//     
					self.viewInput?.presentAlert(from: recoverableError)
				}
			}
		}
	}

// -  
func testRequestFeedFailed() {
	// Put out mock that conform to AlertPresntable protocol
	controller.viewInput = ViewInputMock()

	//  .    ,    
	//    expectation
	controller.requestFeed()

	// Our mocked object should save to true to bool variable when method called
	XCTAssert(controller.viewInput.presentAlertCalled)
	// Next we could compare recoverable error attempter to expected attempter
}

• Hecho un mecanismo conveniente para mostrar alertas
• Dio a los usuarios la opción de volver a intentar una operación fallida
• Y trató de mejorar la experiencia del usuario con nuestra aplicación