Computer Science Design Patterns/Memento

reading 25 22 May 2013 Add more illustrations. Mediator Model–view–controller Briefly, the Originator (the object to be saved) creates a snap-shot of itself as a Memento object, and passes that reference to the Caretaker object. The Caretaker object keeps the Memento until such a time as the Originator may want to revert to a previous state as recorded in the Memento object. Implementation in Java The following Java program illustrates the "undo" usage of the Memento Pattern. class Originator { private String state; // The class could also contain additional data that is not part of the // state saved in the memento. public void set(String state) { System.out.println("Originator: Setting state to " + state); this.state = state; } public Memento saveToMemento() { System.out.println("Originator: Saving to Memento."); return new Memento(state); } public void restoreFromMemento(Memento memento) { state = memento.getSavedState(); System.out.println("Originator: State after restoring from Memento: " + state); } public static class Memento { private final String state; private Memento(String stateToSave) { state = stateToSave; } private String getSavedState() { return state; } } } import java.util.List; import java.util.ArrayList; class Caretaker { public static void main(String[] args) { List<Originator.Memento> savedStates = new ArrayList<Originator.Memento>(); Originator originator = new Originator(); originator.set("State1"); originator.set("State2"); savedStates.add(originator.saveToMemento()); originator.set("State3"); // We can request multiple mementos, and choose which one to roll back to. savedStates.add(originator.saveToMemento()); originator.set("State4"); originator.restoreFromMemento(savedStates.get(1)); } } The output is: Originator: Setting state to State1 Originator: Setting state to State2 Originator: Saving to Memento. Originator: Setting state to State3 Originator: Saving to Memento. Originator: Setting state to State4 Originator: State after restoring from Memento: State3 Implementation in C# ==C# Example== using System; namespace DoFactory.GangOfFour.Memento.Structural { // MainApp startup class for Structural // Memento Design Pattern. class MainApp { // Entry point into console application. static void Main() { Originator o = new Originator(); o.State = "On"; // Store internal state Caretaker c = new Caretaker(); c.Memento = o.CreateMemento(); // Continue changing originator o.State = "Off"; // Restore saved state o.SetMemento(c.Memento); // Wait for user Console.ReadKey(); } } // The 'Originator' class class Originator { private string _state; // Property public string State { get { return _state; } set { _state = value; Console.WriteLine("State = " + _state); } } // Creates memento public Memento CreateMemento() { return (new Memento(_state)); } // Restores original state public void SetMemento(Memento memento) { Console.WriteLine("Restoring state..."); State = memento.State; } } // The 'Memento' class class Memento { private readonly string _state; // Constructor public Memento(string state) { this._state = state; } // Gets or sets state public string State { get { return _state; } } } // The 'Caretaker' class class Caretaker { private Memento _memento; // Gets or sets memento public Memento Memento { set { _memento = value; } get { return _memento; } } } } ==Another way to implement memento in C#== public interface IOriginator { IMemento GetState(); } public interface IShape : IOriginator { void Draw(); void Scale(double scale); void Move(double dx, double dy); } public interface IMemento { void RestoreState(); } public class CircleOriginator : IShape { private class CircleMemento : IMemento { private readonly double x; private readonly double y; private readonly double r; private readonly CircleOriginator originator; public CircleMemento(CircleOriginator originator) { this.originator = originator; x = originator.x; y = originator.y; r = originator.r; } public void RestoreState() { originator.x = x; originator.y = y; originator.r = r; } } double x; double y; double r; public CircleOriginator(double x, double y, double r) { this.x = x; this.y = y; this.r = r; } public void Draw() { Console.WriteLine("Circle with radius {0} at ({1}, {2})", r, x, y); } public void Scale(double scale) { r *= scale; } public void Move(double dx, double dy) { x += dx; y += dy; } public IMemento GetState() { return new CircleMemento(this); } } public class RectOriginator : IShape { private class RectMemento : IMemento { private readonly double x; private readonly double y; private readonly double w; private readonly double h; private readonly RectOriginator originator; public RectMemento(RectOriginator originator) { this.originator = originator; x = originator.x; y = originator.y; w = originator.w; h = originator.h; } public void RestoreState() { originator.x = x; originator.y = y; originator.w = w; originator.h = h; } } double x; double y; double w; double h; public RectOriginator(double x, double y, double w, double h) { this.x = x; this.y = y; this.w = w; this.h = h; } public void Draw() { Console.WriteLine("Rectangle {0}x{1} at ({2}, {3})", w, h, x, y); } public void Scale(double scale) { w *= scale; h *= scale; } public void Move(double dx, double dy) { x += dx; y += dy; } public IMemento GetState() { return new RectMemento(this); } } public class Caretaker { public void Draw(IEnumerable<IShape> shapes) { foreach(IShape shape in shapes) { shape.Draw(); } } public void MoveAndScale(IEnumerable<IShape> shapes) { foreach(IShape shape in shapes) { shape.Scale(10); shape.Move(3, 2); } } public IEnumerable<IMemento> SaveStates(IEnumerable<IShape> shapes) { List<IMemento> states = new List<IMemento>(); foreach(IShape shape in shapes) { states.Add(shape.GetState()); } return states; } public void RestoreStates(IEnumerable<IMemento> states) { foreach(IMemento state in states) { state.RestoreState(); } } public static void Main() { IShape[] shapes = { new RectOriginator(10, 20, 3, 5), new CircleOriginator(5, 2, 10) }; //Outputs: // Rectangle 3x5 at (10, 20) // Circle with radius 10 at (5, 2) Draw(shapes); //Save states of figures IEnumerable<IMemento> states = SaveStates(shapes); //Change placement of figures MoveAndScale(shapes); //Outputs: // Rectangle 30x50 at (13, 22) // Circle with radius 100 at (8, 4) Draw(shapes); //Restore old placement of figures RestoreStates(states); //Outputs: // Rectangle 3x5 at (10, 20) // Circle with radius 10 at (5, 2) Draw(shapes); } } Implementation in Pascal {$apptype console} program Memento; uses SysUtils, Classes; type TMemento = class private _state: string; function GetSavedState: string; public constructor Create(stateToSave: string); property SavedState: string read GetSavedState; end; TOriginator = class private _state: string; // The class could also contain additional data that is not part of the // state saved in the memento. procedure SetState(const state: string); public function SaveToMemento: TMemento; procedure RestoreFromMemento(Memento: TMemento); property state: string read _state write SetState; end; TCaretaker = class private _memento: TMemento; public property Memento: TMemento read _memento write _memento; end; { TMemento } constructor TMemento.Create(stateToSave: string); begin _state := stateToSave; end; function TMemento.GetSavedState: string; begin writeln('restoring state from Memento'); result := _state; end; { TOriginator } procedure TOriginator.RestoreFromMemento(Memento: TMemento); begin _state := Memento.SavedState; writeln('Originator: State after restoring from Memento: ' + state); end; function TOriginator.SaveToMemento: TMemento; begin writeln('Originator: Saving to Memento.'); result := TMemento.Create(state); end; procedure TOriginator.SetState(const state: string); begin writeln('Originator: Setting state to ' + state); _state := state; end; var originator: TOriginator; c1,c2: TCaretaker; begin originator := TOriginator.Create; originator.SetState('State1'); originator.SetState('State2'); // Store internal state c1 := TCaretaker.Create(); c1.Memento := originator.SaveToMemento(); originator.SetState('State3'); // We can request multiple mementos, and choose which one to roll back to. c2 := TCaretaker.Create(); c2.Memento := originator.SaveToMemento(); originator.SetState('State4'); // Restore saved state originator.RestoreFromMemento(c1.Memento); originator.RestoreFromMemento(c2.Memento); c1.Memento.Free; c1.Free; c2.Memento.Free; c2.Free; originator.Free; end. Implementation in Scala import java.util.Date import common.patterns.Originator.Memento import scala.collection.immutable.TreeMap object Originator { case class Memento(x: Int, y: Int, time: Date) } case class Originator(var x: Int, var y: Int) { def setState(x: Int, y: Int) { this.x = x this.y = y } def createMemento(): Memento = Memento(x, y, new Date()) def restoreFromMemento(restore: Memento) { this.x = restore.x this.y = restore.y } } object Caretaker extends App { var states: TreeMap[Date, Memento] = new TreeMap val originator = new Originator(0, 0) var memento = originator.createMemento() states += (memento.time -> memento) Thread.sleep(100) originator.setState(5, 4) memento = originator.createMemento() states += (memento.time -> memento) Thread.sleep(100) originator.setState(10, 10) memento = originator.createMemento() states += (memento.time -> memento) originator.restoreFromMemento(states(states.drop(1).firstKey)) //restore second state println(originator) } Output: Originator(5,4)