设计模式--状态模式

状态模式是一种行为设计模式，它允许在对象内部状态发生改变时改变其行为。该模式将状态封装在独立的类中，使得每个状态对象可以处理不同的对象行为，从而实现对象行为的随状态改变而改变。

在状态模式中，有三种角色：

1. Context（上下文）：上下文是一个拥有状态的对象。它维护了一个指向具体状态的引用，以便于上下文内部状态的切换。
2. State（状态）：状态是一个接口，它定义了一个在特定状态下执行的操作（行为）。
3. ConcreteState（具体状态）：实现State接口的类。

下面是一个使用C#实现状态模式的例子，假设有一个订单类，它有以下三种状态：新建状态、已支付状态和已取消状态。通过状态模式，我们可以在订单状态切换时改变订单对象的行为，比如不同状态下能否对订单进行修改等：

代码语言：javascript复制

// State接口定义订单状态的操作
public interface IOrderState
{
    void HandlePayment();
    void ShipOrder();
}

// Order类作为上下文，维护着State状态
public class Order
{
    public string Name { get; set; }
    public decimal TotalPrice { get; set; }
    public bool IsPaid { get; set; }
    public bool IsCancelled { get; set; }
    private IOrderState _state;

    // Constructor 
    public Order()
    {
        _state = new NewOrderState(this);
    }

    // Property to get or set the current Order state
    public IOrderState State
    {
        get { return _state; }
        set { _state = value; }
    }

    // Behavior method to handle payment
    public void HandlePayment()
    {
        _state.HandlePayment();
    }

    // Behavior method to ship the order
    public void ShipOrder()
    {
        _state.ShipOrder();
    }
}

// ConcreteState1：新建订单状态
public class NewOrderState : IOrderState
{
    private readonly Order _order;
    public NewOrderState(Order order)
    {
        _order = order;
    }

    public void HandlePayment()
    {
        _order.State = new PaidOrderState(_order);
    }
    public void ShipOrder()
    {
        Console.WriteLine("Cannot ship the order as it is not yet paid.");
    }
}

// ConcreteState2：已支付订单状态
public class PaidOrderState : IOrderState
{
    private readonly Order _order;
    public PaidOrderState(Order order)
    {
        _order = order;
    }

    public void HandlePayment()
    {
        Console.WriteLine("Payment has already been received.");
    }
    public void ShipOrder()
    {
        Console.WriteLine("Order shipped.");
        _order.State = new ShippedOrderState(_order);
    }
}

// ConcreteState3：已取消订单状态
public class CancelledOrderState : IOrderState
{
    private readonly Order _order;
    public CancelledOrderState(Order order)
    {
        _order = order;
    }

    public void HandlePayment()
    {
        Console.WriteLine("Cannot receive payment as the order has been cancelled.");
    }
    public void ShipOrder()
    {
        Console.WriteLine("Cannot ship the order as it has been cancelled.");
    }
}

// ConcreteState4：已发货订单状态
public class ShippedOrderState : IOrderState
{
  private readonly Order _order;
  public ShippedOrderState(Order order)
  {
      _order = order;
  }

  public void HandlePayment()
  {
      Console.WriteLine("Cannot receive payment as the order has already been shipped.");
  }
  public void ShipOrder()
  {
      Console.WriteLine("Cannot ship the order as it has already been shipped.");
  }
}

使用示例：

代码语言：javascript复制

Order order = new Order()
{
    Name = "Order1",
    TotalPrice = 100,
};

// Handle payment for the order
order.HandlePayment();

// Ship the order
order.ShipOrder();

// Cancel the order
order.State = new CancelledOrderState(order);

输出结果：

代码语言：javascript复制

Order shipped.
Cannot receive payment as the order has been cancelled.

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