设计模式（17）备忘录模式

模式介绍

备忘录模式就是在不破坏封装的前提下，捕获一个对象的内部状态，并在该对象之外保存这个状态，这样可以在以后将对象恢复到原先保存的状态。

示例

想象一下，一家真正高端的餐厅可能直接从当地农场订购，并且餐厅需要跟踪哪些配料来自哪些供应商。
在我们的系统中，我们需要跟踪关于特定供应商输入的信息量，并且如果我们意外地输入了错误的地址，就能够将信息恢复到以前的状态。

/// <summary>
/// The Originator class, which is the class for which we want to save Mementos for its state.
/// </summary>
class FoodSupplier
{
    private string _name;
    private string _phone;
    private string _address;

    public string Name
    {
        get { return _name; }
        set
        {
            _name = value;
            Console.WriteLine("Proprietor:  " + _name);
        }
    }

    public string Phone
    {
        get { return _phone; }
        set
        {
            _phone = value;
            Console.WriteLine("Phone Number: " + _phone);
        }
    }

    public string Address
    {
        get { return _address; }
        set
        {
            _address = value;
            Console.WriteLine("Address: " + _address);
        }
    }

    public FoodSupplierMemento SaveMemento()
    {
        Console.WriteLine("
Saving current state
");
        return new FoodSupplierMemento(_name, _phone, _address);
    }

    public void RestoreMemento(FoodSupplierMemento memento)
    {
        Console.WriteLine("
Restoring previous state
");
        Name = memento.Name;
        Phone = memento.PhoneNumber;
        Address = memento.Address;
    }
}

备忘录类：

/// <summary>
/// The Memento class
/// </summary>
class FoodSupplierMemento
{
    public string Name { get; set; }
    public string PhoneNumber { get; set; }
    public string Address { get; set; }

    public FoodSupplierMemento(string name, string phone, string address)
    {
        Name = name;
        PhoneNumber = phone;
        Address = address;
    }
}

守护者类：

/// <summary>
/// The Caretaker class.  This class never examines the contents of any Memento and is
/// responsible for keeping that memento.
/// </summary>
class SupplierMemory
{
    private FoodSupplierMemento _memento;

    public FoodSupplierMemento Memento
    {
        set { _memento = value; }
        get { return _memento; }
    }
}

客户端调用：

static void Main(string[] args)
{
    //Here's a new supplier for our restaurant
    FoodSupplier s = new FoodSupplier();
    s.Name = "Harold Karstark";
    s.Phone = "(482) 555-1172";

    // Let's store that entry in our database.
    SupplierMemory m = new SupplierMemory();
    m.Memento = s.SaveMemento();

    // Continue changing originator
    s.Address = "548 S Main St. Nowhere, KS";

    // Crap, gotta undo that entry, I entered the wrong address
    s.RestoreMemento(m.Memento);

    Console.ReadKey();
}

总结

备忘录模式试图将对象的状态封装为另一个对象（称为Memento），并允许从该Memento恢复对象的状态。

源代码

https://github.com/exceptionnotfound/DesignPatterns/tree/master/Memento

原文

https://www.exceptionnotfound.net/the-daily-design-pattern-memento/