用多态替换条件 - 当你的类型改变时如何处理?

Replace Conditional with Polymorphism - How to handle when your type changes?(用多态替换条件 - 当你的类型改变时如何处理?)

问题描述

For a personal project, I'm working on a small web-based game.

I have a Card class that has a Status property, and there are case statements all over the place. I thought, hey, this is a great oppurtunity for Replace Conditional with Polymorphism!

The problem is, I have a couple methods that do stuff like this:

public class Card
{
    public void ChangeStatus()
    {
      switch (Status)
      {
        case MyStatusEnum.Normal:
          Status = MyStatusEnum.Underwater;
          break;
        case MyStatusEnum.Underwater:
          Status = MyStatusEnum.Dead;
          break;
        // etc...
      }
    }
}

When refactoring it in the new NormalCard class, I'm overriding the ChangeStatus method like this:

public override void ChangeStatus()
{
    base.Status = MyStatusEnum.Underwater;
}

The problem is this object of NormalCard has a status of Underwater. I can't reassign the type of this, and I don't really want to change the return of the methods from void to CardBase. What options do I have? Is there a standard way of doing this?

Edit Tormod set me straight. I want the State Pattern. Thanks all!

In your case, I'd have a Card object that contains a CardStatus property. The subtypes of CardStatus correspond to the previous enum values. Refactor behaviour that depends on the current status except the state transition to be inside the CardStatus subtypes.

The state transition that's in your first example should, IMO, remain inside the Card object. The state changing feels more like a behaviour of the containing card than of the state object. What you can do is have the CardStatus objects tell you what state to transition to after an event.

A rough example: (obviously there's many more variations on this that could be used.)

API

interface ICardStatus {
    ICardStatus NextStatus(Card card);

    void DoStuff(Card card);
}

class Card {
    ICardStatus Status = new NormalCardStatus();

    void DoStuff() {
        Status.DoStuff(this);
    }

    void ChangeStatus() {
        Status = Status.NextStatus(this);
    }
}

Status implementations

class NormalCardStatus : ICardStatus {
    ICardStatus NextStatus(Card card) {
        return new UnderwaterCardStatus();
    }

    void DoStuff(Card card) {
        // ...
    }
}

class UnderwaterCardStatus : ICardStatus {
    ICardStatus NextStatus(Card card) {
        return new DeathStatus();
    }

    void DoStuff(Card card) {
        // ...
    }
}

class DeathCardStatus : ICardStatus {
    ICardStatus NextStatus(Card card) {
        // ...
    }

    void DoStuff(Card card) {
        throw new Exception("Cannot do anything while dead");
    }
}

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