Swift Example

/// Memento Design Pattern
///
/// Intent: Lets you save and restore the previous state of an object without
/// revealing the details of its implementation.

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()
    }
}

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