Ejemplos de uso: El principio del patrón Memento puede cumplirse utilizando la serialización, que es bastante habitual en Swift. Aunque no es la única forma ni la más efectiva de realizar instantáneas del estado de un objeto, permite almacenar copias de seguridad del estado mientras protege de otros objetos la estructura del originador.
Este ejemplo ilustra la estructura del patrón de diseño Memento y se centra en las siguientes preguntas:
¿De qué clases se compone?
¿Qué papeles juegan esas clases?
¿De qué forma se relacionan los elementos del patrón?
Después de conocer la estructura del patrón, será más fácil comprender el siguiente ejemplo basado en un caso de uso real de Swift.
Example.swift: Ejemplo conceptual
import XCTest
/// The Originator holds some important state that may change over time. It also
/// defines a method for saving the state inside a memento and another method
/// for restoring the state from it.
class Originator {
/// For the sake of simplicity, the originator's state is stored inside a
/// single variable.
private var state: String
init(state: String) {
self.state = state
print("Originator: My initial state is: \(state)")
}
/// The Originator's business logic may affect its internal state.
/// Therefore, the client should backup the state before launching methods
/// of the business logic via the save() method.
func doSomething() {
print("Originator: I'm doing something important.")
state = generateRandomString()
print("Originator: and my state has changed to: \(state)")
}
private func generateRandomString() -> String {
return String(UUID().uuidString.suffix(4))
}
/// Saves the current state inside a memento.
func save() -> Memento {
return ConcreteMemento(state: state)
}
/// Restores the Originator's state from a memento object.
func restore(memento: Memento) {
guard let memento = memento as? ConcreteMemento else { return }
self.state = memento.state
print("Originator: My state has changed to: \(state)")
}
}
/// The Memento interface provides a way to retrieve the memento's metadata,
/// such as creation date or name. However, it doesn't expose the Originator's
/// state.
protocol Memento {
var name: String { get }
var date: Date { get }
}
/// The Concrete Memento contains the infrastructure for storing the
/// Originator's state.
class ConcreteMemento: Memento {
/// The Originator uses this method when restoring its state.
private(set) var state: String
private(set) var date: Date
init(state: String) {
self.state = state
self.date = Date()
}
/// The rest of the methods are used by the Caretaker to display metadata.
var name: String { return state + " " + date.description.suffix(14).prefix(8) }
}
/// The Caretaker doesn't depend on the Concrete Memento class. Therefore, it
/// doesn't have access to the originator's state, stored inside the memento. It
/// works with all mementos via the base Memento interface.
class Caretaker {
private lazy var mementos = [Memento]()
private var originator: Originator
init(originator: Originator) {
self.originator = originator
}
func backup() {
print("\nCaretaker: Saving Originator's state...\n")
mementos.append(originator.save())
}
func undo() {
guard !mementos.isEmpty else { return }
let removedMemento = mementos.removeLast()
print("Caretaker: Restoring state to: " + removedMemento.name)
originator.restore(memento: removedMemento)
}
func showHistory() {
print("Caretaker: Here's the list of mementos:\n")
mementos.forEach({ print($0.name) })
}
}
/// Let's see how it all works together.
class MementoConceptual: XCTestCase {
func testMementoConceptual() {
let originator = Originator(state: "Super-duper-super-puper-super.")
let caretaker = Caretaker(originator: originator)
caretaker.backup()
originator.doSomething()
caretaker.backup()
originator.doSomething()
caretaker.backup()
originator.doSomething()
print("\n")
caretaker.showHistory()
print("\nClient: Now, let's rollback!\n\n")
caretaker.undo()
print("\nClient: Once more!\n\n")
caretaker.undo()
}
}
Output.txt: Resultado de la ejecución
Originator: My initial state is: Super-duper-super-puper-super.
Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: 1923
Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: 74FB
Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: 3681
Caretaker: Here's the list of mementos:
Super-duper-super-puper-super. 11:45:44
1923 11:45:44
74FB 11:45:44
Client: Now, let's rollback!
Caretaker: Restoring state to: 74FB 11:45:44
Originator: My state has changed to: 74FB
Client: Once more!
Caretaker: Restoring state to: 1923 11:45:44
Originator: My state has changed to: 1923
Ejemplo del mundo real
Example.swift: Ejemplo del mundo real
import XCTest
class MementoRealWorld: XCTestCase {
/// State and Command are often used together when the previous state of the
/// object should be restored in case of failure of some operation.
///
/// Note: UndoManager can be used as an alternative.
func test() {
let textView = UITextView()
let undoStack = UndoStack(textView)
textView.text = "First Change"
undoStack.save()
textView.text = "Second Change"
undoStack.save()
textView.text = (textView.text ?? "") + " & Third Change"
textView.textColor = .red
undoStack.save()
print(undoStack)
print("Client: Perform Undo operation 2 times\n")
undoStack.undo()
undoStack.undo()
print(undoStack)
}
}
class UndoStack: CustomStringConvertible {
private lazy var mementos = [Memento]()
private let textView: UITextView
init(_ textView: UITextView) {
self.textView = textView
}
func save() {
mementos.append(textView.memento)
}
func undo() {
guard !mementos.isEmpty else { return }
textView.restore(with: mementos.removeLast())
}
var description: String {
return mementos.reduce("", { $0 + $1.description })
}
}
protocol Memento: CustomStringConvertible {
var text: String { get }
var date: Date { get }
}
extension UITextView {
var memento: Memento {
return TextViewMemento(text: text,
textColor: textColor,
selectedRange: selectedRange)
}
func restore(with memento: Memento) {
guard let textViewMemento = memento as? TextViewMemento else { return }
text = textViewMemento.text
textColor = textViewMemento.textColor
selectedRange = textViewMemento.selectedRange
}
struct TextViewMemento: Memento {
let text: String
let date = Date()
let textColor: UIColor?
let selectedRange: NSRange
var description: String {
let time = Calendar.current.dateComponents([.hour, .minute, .second, .nanosecond],
from: date)
let color = String(describing: textColor)
return "Text: \(text)\n" + "Date: \(time.description)\n"
+ "Color: \(color)\n" + "Range: \(selectedRange)\n\n"
}
}
}
