游戏编程模式03-设计模式-享元模式

设计模式-享元模式

参考章节：https://gpp.tkchu.me/flyweight.html

脑内画面

享元模式把"大量对象中相同的部分"抽出来共享，让每个对象只保存自己的差异。想象一片森林：一万棵树不需要各自保存一份模型和贴图，它们只需要保存位置、旋转、缩放；树种资料可以大家共用。
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位置/旋转
橡树类型

模型/贴图/碰撞
树实例 B

位置/旋转
树实例 C

位置/旋转
松树类型

模型/贴图/碰撞

它解决的问题

游戏世界里经常有海量相似对象：地形格子、树、子弹、粒子、道具配置。把每个对象都做成"完整对象"会浪费内存，也会让缓存命中率变差。享元模式把稳定、重复、可共享的数据变成一个共享对象。

C# 示例

csharp 复制代码

public sealed class TerrainType
{
    public string Name { get; }
    public int MoveCost { get; }
    public bool BlocksMovement { get; }

    public TerrainType(string name, int moveCost, bool blocksMovement)
    {
        Name = name;
        MoveCost = moveCost;
        BlocksMovement = blocksMovement;
    }
}

public sealed class WorldMap
{
    private readonly TerrainType[,] _tiles;

    public TerrainType Grass { get; } = new("Grass", moveCost: 1, blocksMovement: false);
    public TerrainType Water { get; } = new("Water", moveCost: 4, blocksMovement: false);
    public TerrainType Wall { get; } = new("Wall", moveCost: 999, blocksMovement: true);

    public WorldMap(int width, int height)
    {
        _tiles = new TerrainType[width, height];

        for (var x = 0; x < width; x++)
        for (var y = 0; y < height; y++)
        {
            _tiles[x, y] = Grass;
        }
    }

    public void SetTile(int x, int y, TerrainType terrain)
    {
        _tiles[x, y] = terrain;
    }

    public TerrainType GetTile(int x, int y)
    {
        return _tiles[x, y];
    }
}

模式长什么样

享元通常拆成两部分：

内在状态：可共享，不随实例变化，例如树的模型、材质、基础属性。
外在状态：每个实例独有，例如位置、血量、当前动画时间。

什么时候用

同类对象数量巨大。
每个对象内部有大量重复数据。
共享数据基本不可变，或者变更要影响所有实例。

使用时的锋利边

享元对象最好设计成不可变。共享对象一旦被某个实例偷偷修改，所有引用它的实例都会被影响，这种 bug 很隐蔽。需要个体差异时，把差异放回实例，不要塞进共享类型里。