备忘录模式 (Memento Pattern) - 撤销功能的实现

结构图

• Originator（原发器）：它是一个普通类，可以创建一个备忘录，并存储它的当前内部状态，也可以使用备忘录来恢复其内部状态，一般将需要保存内部状态的类设计为原发器。
• Memento（备忘录）：存储原发器的内部状态，根据原发器来决定保存哪些内部状态。备忘录的设计一般可以参考原发器的设计，根据实际需要确定备忘录类中的属性。需要注意的是，除了原发器本身与负责人类之外，备忘录对象不能直接供其他类使用，原发器的设计在不同的编程语言中实现机制会有所不同。
• Caretaker（负责人）：负责人又称为管理者，它负责保存备忘录，但是不能对备忘录的内容进行操作或检查。在负责人类中可以存储一个或多个备忘录对象，它只负责存储对象，而不能修改对象，也无须知道对象的实现细节。

示例

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;

namespace DesignPatterns.Memento
{
    // 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 string _state;

        public Originator(string state)
        {
            this._state = state;
            Console.WriteLine("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.
        public void DoSomething()
        {
            Console.WriteLine("Originator: I'm doing something important.");
            this._state = this.GenerateRandomString(30);
            Console.WriteLine($"Originator: and my state has changed to: {_state}");
        }

        private string GenerateRandomString(int length = 10)
        {
            string allowedSymbols = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
            string result = string.Empty;

            while (length > 0)
            {
                result += allowedSymbols[new Random().Next(0, allowedSymbols.Length)];

                Thread.Sleep(12);

                length--;
            }

            return result;
        }

        // Saves the current state inside a memento.
        public IMemento Save()
        {
            return new ConcreteMemento(this._state);
        }

        // Restores the Originator's state from a memento object.
        public void Restore(IMemento memento)
        {
            if (!(memento is ConcreteMemento))
            {
                throw new Exception("Unknown memento class " + memento.ToString());
            }

            this._state = memento.GetState();
            Console.Write($"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.
    public interface IMemento
    {
        string GetName();

        string GetState();

        DateTime GetDate();
    }

    // The Concrete Memento contains the infrastructure for storing the
    // Originator's state.
    class ConcreteMemento : IMemento
    {
        private string _state;

        private DateTime _date;

        public ConcreteMemento(string state)
        {
            this._state = state;
            this._date = DateTime.Now;
        }

        // The Originator uses this method when restoring its state.
        public string GetState()
        {
            return this._state;
        }
        
        // The rest of the methods are used by the Caretaker to display
        // metadata.
        public string GetName()
        {
            return $"{this._date} / ({this._state.Substring(0, 9)})...";
        }

        public DateTime GetDate()
        {
            return this._date;
        }
    }

    // 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 List<IMemento> _mementos = new List<IMemento>();

        private Originator _originator = null;

        public Caretaker(Originator originator)
        {
            this._originator = originator;
        }

        public void Backup()
        {
            Console.WriteLine("\nCaretaker: Saving Originator's state...");
            this._mementos.Add(this._originator.Save());
        }

        public void Undo()
        {
            if (this._mementos.Count == 0)
            {
                return;
            }

            var memento = this._mementos.Last();
            this._mementos.Remove(memento);

            Console.WriteLine("Caretaker: Restoring state to: " + memento.GetName());

            try
            {
                this._originator.Restore(memento);
            }
            catch (Exception)
            {
                this.Undo();
            }
        }

        public void ShowHistory()
        {
            Console.WriteLine("Caretaker: Here's the list of mementos:");

            foreach (var memento in this._mementos)
            {
                Console.WriteLine(memento.GetName());
            }
        }
    }
    
    class Program
    {
        static void Main(string[] args)
        {
            // Client code.
            Originator originator = new Originator("Super-duper-super-puper-super.");
            Caretaker caretaker = new Caretaker(originator);

            caretaker.Backup();
            originator.DoSomething();

            caretaker.Backup();
            originator.DoSomething();

            caretaker.Backup();
            originator.DoSomething();

            Console.WriteLine();
            caretaker.ShowHistory();

            Console.WriteLine("\nClient: Now, let's rollback!\n");
            caretaker.Undo();

            Console.WriteLine("\n\nClient: Once more!\n");
            caretaker.Undo();

            Console.WriteLine();
        }
    }
}

运行结果

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: oGyQIIatlDDWNgYYqJATTmdwnnGZQj

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: jBtMDDWogzzRJbTTmEwOOhZrjjBULe

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: exoHyyRkbuuNEXOhhArKccUmexPPHZ

Caretaker: Here's the list of mementos:
12.06.2018 15:52:45 / (Super-dup...)
12.06.2018 15:52:46 / (oGyQIIatl...)
12.06.2018 15:52:46 / (jBtMDDWog...)

Client: Now, let's rollback!

Caretaker: Restoring state to: 12.06.2018 15:52:46 / (jBtMDDWog...)
Originator: My state has changed to: jBtMDDWogzzRJbTTmEwOOhZrjjBULe

Client: Once more!

Caretaker: Restoring state to: 12.06.2018 15:52:46 / (oGyQIIatl...)
Originator: My state has changed to: oGyQIIatlDDWNgYYqJATTmdwnnGZQj

总结

备忘录模式在很多软件的使用过程中普遍存在，但是在应用软件开发中，它的使用频率并不太高，因为现在很多基于窗体和浏览器的应用软件并没有提供撤销操作。如果需要为软件提供撤销功能，备忘录模式无疑是一种很好的解决方案。在一些字处理软件、图像编辑软件、数据库管理系统等软件中备忘录模式都得到了很好的应用。

优点

1. 它提供了一种状态恢复的实现机制，使得用户可以方便地回到一个特定的历史步骤，当新的状态无效或者存在问题时，可以使用暂时存储起来的备忘录将状态复原。
2. 备忘录实现了对信息的封装，一个备忘录对象是一种原发器对象状态的表示，不会被其他代码所改动。备忘录保存了原发器的状态，采用列表、堆栈等集合来存储备忘录对象可以实现多次撤销操作。

缺点

资源消耗过大，如果需要保存的原发器类的成员变量太多，就不可避免需要占用大量的存储空间，每保存一次对象的状态都需要消耗一定的系统资源。

适用场景

1. 保存一个对象在某一个时刻的全部状态或部分状态，这样以后需要时它能够恢复到先前的状态，实现撤销操作。
2. 防止外界对象破坏一个对象历史状态的封装性，避免将对象历史状态的实现细节暴露给外界对象。