意圖

在不破壞封裝性的前提下涂滴，捕獲一個(gè)對象的內(nèi)部狀態(tài)，并在該對象之外保存這個(gè)狀態(tài)缔杉。這樣以后就可以將該對象恢復(fù)到原先保存的狀態(tài)搁料。

結(jié)構(gòu)

備忘錄結(jié)構(gòu)圖

以及它們之間的協(xié)作關(guān)系：

備忘錄協(xié)作圖

動機(jī)

記錄一個(gè)對象的內(nèi)部狀態(tài)。允許用戶在必要的時(shí)候霸琴，通過恢復(fù)其狀態(tài)來取消不確定的操作或從錯誤中恢復(fù)過來昭伸。

備忘錄模式用一個(gè)備忘錄（Memento）對象存儲原發(fā)器（Originator）對象在某個(gè)瞬間的內(nèi)部狀態(tài)。在具體實(shí)現(xiàn)上庐杨，盡量（有些語言不支持）做到只有原發(fā)器可以向備忘錄中存取信息，備忘錄對其他對象 “不可見”仁堪。

適用性

• 必須保存一個(gè)對象在某一個(gè)時(shí)刻的狀態(tài)（或部分狀態(tài)）, 這樣以后需要時(shí)它才能恢復(fù)到先前的狀態(tài)各吨；
• 如果通過接口讓其它對象直接得到這些私密的狀態(tài)，又會暴露對象的實(shí)現(xiàn)細(xì)節(jié)并破壞對象的封裝性揭蜒；

注意事項(xiàng)

• 使用備忘錄可以避免暴露那些只應(yīng)由原發(fā)器管理卻又必須存儲在原發(fā)器之外的信息；
• 原先徙融，原發(fā)器需保留所有請求的內(nèi)部狀態(tài)版本。現(xiàn)在欺冀，只需保留當(dāng)前請求的內(nèi)部狀態(tài)版本萨脑，簡化了原發(fā)器的設(shè)計(jì)；
• 如何保證只有原發(fā)器才能訪問備忘錄的狀態(tài)渤早，避免信息變相泄露；
• 能否通過增量式（存儲）解決備忘錄的各種開銷悴灵，如存儲開銷骂蓖、復(fù)制開銷等。

示例

模擬一個(gè)圖形編輯器登下，它使用MoveCommand命令對象來執(zhí)行（或回退）一個(gè)Graphic圖形對象從一個(gè)位置到另一個(gè)位置的變換。

實(shí)現(xiàn)（C#）

結(jié)構(gòu)圖

using System;
using System.Collections.Generic;


// 位置
public struct Point
{
    public int X { get; set; }
    public int Y { get; set; }

    public override string ToString()
    {
        return string.Format("({0},{1})", this.X, this.Y);
    }

    public static Point operator -(Point obj)
    {
        return new Point{ X = -obj.X, Y = -obj.Y };
    }

    public static Point operator -(Point obj1, Point obj2)
    {
        return new Point{ X = obj1.X - obj2.X, Y = obj1.Y - obj2.Y };
    }

    public static bool operator ==(Point obj1, Point obj2)
    {
        if(obj1 != null && obj2 != null)
        {
            return obj1.Equals(obj2);
        }

        return false;
    }

    public static bool operator !=(Point obj1, Point obj2)
    {
        if(obj1 != null && obj2 != null)
        {
            return !obj1.Equals(obj2);
        }

        return false;
    }

    public override bool Equals(object obj)
    {
        if(obj is Point)
        {
            Point obj2 = (Point)obj;

            return this.X == obj2.X && this.Y == obj2.Y;
        }

        return false;
    }

    public override int GetHashCode()
    {
        return this.X.GetHashCode() ^ this.Y.GetHashCode();
    }
}

// 圖塊類
public class Graphic
{
    // 圖塊名稱
    public string Name { get; private set;}
    // 當(dāng)前位置
    public Point Position {get; private set;}

    public Graphic(string name, Point position)
    {
        this.Name = name;
        this.Position = position;
    }

    public void Move(Point delta)
    {
        bool flag = delta.X < 0 || delta.Y < 0;

        delta.X += Position.X;
        delta.Y += Position.Y;
        Console.WriteLine("{0} 從 {1} {3}到 {2}", this.Name, this.Position, delta, (flag ? "撤銷" : "移動"));
        Position = delta;

        ConstraintSolverMemento.ConstraintSolver solver = ConstraintSolverMemento.ConstraintSolver.GetInstance();
        solver.Update(this);
    }


    public static bool operator ==(Graphic obj1, Graphic obj2)
    {
        if(obj1 != null && obj2 != null)
        {
            return obj1.Equals(obj2);
        }

        return false;
    }

    public static bool operator !=(Graphic obj1, Graphic obj2)
    {
        if(obj1 != null && obj2 != null)
        {
            return !obj1.Equals(obj2);
        }

        return false;
    }

    public override bool Equals(object obj)
    {   

        if(obj != null && obj is Graphic) 
        {
            Graphic obj2 = (obj as Graphic) ;

            return this.Name == obj2.Name && this.Position == obj2.Position;
        }
        
        return false;

    }

    public override string ToString()
    {
        return string.Format("{0}{1}", this.Name, this.Position);
    }

    public override int GetHashCode()
    {
        return this.Name.GetHashCode() ^ this.Position.GetHashCode();
    }

    public Graphic Clone()
    {
        return new Graphic(this.Name, this.Position);
    }
}

// 客戶端移動命令，負(fù)責(zé)在外部保存?zhèn)渫浶畔ⅰ?public sealed class MoveCommand
{
    private ConstraintSolverMemento memento;
    private Point delta;
    private Graphic target;

    public MoveCommand(Graphic target, Point delta)
    {
        this.target = target;
        this.delta = delta;
    }

    public void Execute()
    {
        ConstraintSolverMemento.ConstraintSolver solver = ConstraintSolverMemento.ConstraintSolver.GetInstance();
        this.memento = solver.CreateMemento();
        this.target.Move(delta);
        solver.Solve();
    }

    public void Unexecute()
    {
        ConstraintSolverMemento.ConstraintSolver solver = ConstraintSolverMemento.ConstraintSolver.GetInstance();
        target.Move(-this.delta);
        solver.SetMemento(this.memento);
        solver.Solve();
    }

}


// 備忘錄類
public class ConstraintSolverMemento
{
    // 只允許原發(fā)器（ConstraintSolver）訪問備忘錄（ConstraintSolverMemento）的內(nèi)部信息揩瞪。
    private List<Tuple<Graphic,Graphic>> Paths { get; set;}

    // 原發(fā)器類（圖塊之間的線路關(guān)系）
    public class ConstraintSolver
    {
        private static ConstraintSolver instance;
        private List<Tuple<Graphic,Graphic>> Paths { get; set;}

        // 單例模式（不考慮線程安全）篓冲。
        public static ConstraintSolver GetInstance()
        {
            if (instance == null)
            {
                instance = new ConstraintSolver();
            }

            return instance;
        }

        private ConstraintSolver() {}

        public void Update(Graphic graphic)
        {
            Tuple<Graphic,Graphic> find = null;
            foreach(Tuple<Graphic,Graphic> item in Paths)
            {
                if(item.Item1.Name == graphic.Name || item.Item2.Name == graphic.Name)
                {
                    find = item;
                    break;
                }
            }

            if(find != null)
            {
                this.Paths.Remove(find);
                if(find.Item1.Name == graphic.Name)
                {
                    this.Paths.Add(new Tuple<Graphic,Graphic>(graphic.Clone(), find.Item2.Clone()));
                }
                else
                    this.Paths.Add(new Tuple<Graphic,Graphic>(find.Item1.Clone(), graphic.Clone()));
            }
        }

        public void AddConstraint(Graphic start, Graphic end)
        {
            if(this.Paths == null) this.Paths = new List<Tuple<Graphic,Graphic>> ();

            foreach(Tuple<Graphic,Graphic> item in Paths)
            {
                if(item.Item1 == start && item.Item2 == end) return;
            }

            Paths.Add(new Tuple<Graphic,Graphic>(start.Clone(),end.Clone()));
        }

        public void RemoveConstraint(Graphic start, Graphic end)
        {
            foreach(Tuple<Graphic,Graphic> item in Paths)
            {
                if(item.Item1 == start && item.Item2 == end) Paths.Remove(item);
            }
        }

        public ConstraintSolverMemento CreateMemento()
        {       
            return new ConstraintSolverMemento { Paths = this.Paths };
        }

        public void SetMemento(ConstraintSolverMemento memento)
        {
            this.Paths = memento.Paths;
        }

        // 繪畫圖塊之間的線路
        public void Solve()
        {
            foreach(Tuple<Graphic,Graphic> item in this.Paths)
            {
                Console.WriteLine("   路線打右冀：{0} 到 {1} 有一條連接線.", item.Item1, item.Item2);
            }
            Console.WriteLine();
        }

    }

}


// 測試。
public class App
{
    public static void Main(string[] args)
    {
        Graphic g1 = new Graphic("A", new Point { X = 0, Y = 0});
        Graphic g2 = new Graphic("B", new Point { X = 20, Y = 0});
        MoveCommand command1 =  new MoveCommand(g1, new Point { X = 15, Y = 0 });
        MoveCommand command2 =  new MoveCommand(g2, new Point { X = 45, Y = 0 });

        ConstraintSolverMemento.ConstraintSolver solver = ConstraintSolverMemento.ConstraintSolver.GetInstance();
        solver.AddConstraint(g1,g2);

        Console.WriteLine("初始位置：圖塊{0}妇菱，圖塊{1}\n", g1,g2);
        command1.Execute();
        command2.Execute();

        command2.Unexecute();
        command1.Unexecute();
    }
}

// 控制臺輸出：
//  初始位置：圖塊A(0,0)，圖塊B(20,0)

//  A 從 (0,0) 移動到 (15,0)
//     路線打哟惩拧：A(15,0) 到 B(20,0) 有一條連接線.

//  B 從 (20,0) 移動到 (65,0)
//     路線打印：A(15,0) 到 B(65,0) 有一條連接線.

//  B 從 (65,0) 撤銷到 (20,0)
//     路線打臃拷弧：A(15,0) 到 B(20,0) 有一條連接線.

//  A 從 (15,0) 撤銷到 (0,0)
//     路線打印：A(0,0) 到 B(20,0) 有一條連接線.