备忘录模式

概念

Memento is a behavioral design pattern that lets you save and restore the previous state of an object without revealing the details of its implementation.

备忘录模式是一种行为设计模式， 允许在不暴露对象实现细节的情况下保存和恢复对象之前的状态。

场景

在支持 Undo/Redo 的场景下面，如果需要撤销某一次行为的时候，在执行行为之前需要将数据保存下来。

备忘录模式指的是专门构造一个类似Snapshot的类，保存当时的一些数据信息，这些信息不能修改。

在撤销的时候，拿出来这些数据，以便恢复现场。

案例

首先实现记录现场的数据类，主要记录了当时的时间和文本

    public interface IMemento
    {
        string GetName();

        string GetState();

        DateTime GetDate();
    }

    class ConcreteMemento : IMemento
    {
        private string _state;

        private DateTime _date;

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

        public string GetState()
        {
            return this._state;
        }

        public string GetName()
        {
            return $"{this._date} / ({this._state.Substring(0, 9)})...";
        }

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

 接着负责行为的类，会在恰当的时机，保存现场，生成这样一个对象

    class Originator
    {
        private string _state;

        public Originator(string state)
        {
            this._state = state;
            Console.WriteLine("Originator: My initial state is: " + state);
        }

        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;
        }

        public IMemento Save()
        {
            return new ConcreteMemento(this._state);
        }

        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}");
        }
    }

 然后实现Undo的manager类会把每一步行为都保存

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());
            }
        }
    }

 最终调用如下：

            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();