游戏编程模式18-优化模式-脏标识模式

优化模式-脏标识模式

参考章节：https://gpp.tkchu.me/dirty-flag.html

脑内画面

脏标识模式给"派生数据"贴一张小纸条：源数据改了，派生数据先标记为脏，不立刻重算；等真的有人需要它时，再一次性更新。它像说"这份报表过期了，下次打开时再刷新"。
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标记 worldPosition 脏
暂不计算
有人读取世界坐标
重新计算并清除脏标记

它解决的问题

有些数据由其他数据推导而来，例如世界变换、导航缓存、合批结果。源数据可能一帧内改很多次，如果每次都立刻重算，会浪费。脏标识把重算延迟到"确实需要"的时候。

C# 示例

public readonly record struct Vec2(float X, float Y)
{
    public static Vec2 operator +(Vec2 a, Vec2 b) => new(a.X + b.X, a.Y + b.Y);
}

public sealed class SceneNode
{
    private readonly List<SceneNode> _children = new();
    private Vec2 _localPosition;
    private Vec2 _worldPosition;
    private bool _dirty = true;

    public SceneNode? Parent { get; private set; }

    public Vec2 LocalPosition
    {
        get => _localPosition;
        set
        {
            _localPosition = value;
            MarkDirty();
        }
    }

    public Vec2 WorldPosition
    {
        get
        {
            if (_dirty)
            {
                RecalculateWorldPosition();
            }

            return _worldPosition;
        }
    }

    public void AddChild(SceneNode child)
    {
        child.Parent = this;
        _children.Add(child);
        child.MarkDirty();
    }

    private void MarkDirty()
    {
        if (_dirty)
        {
            return;
        }

        _dirty = true;

        foreach (var child in _children)
        {
            child.MarkDirty();
        }
    }

    private void RecalculateWorldPosition()
    {
        _worldPosition = Parent == null
            ? _localPosition
            : Parent.WorldPosition + _localPosition;

        _dirty = false;
    }
}

什么时候用

• 派生数据计算昂贵。
• 源数据变化频繁，但派生数据不一定马上被读取。
• 多次修改可以合并成一次重算。

使用时的锋利边

脏标记必须沿依赖链传播。父节点移动时，子节点世界坐标也脏了。如果遗漏传播，读到的就是过期数据。另一方面，过度传播会让延迟计算失去意义。