4. Derive Max Health and Attack Speed

Current health is stored because it is mutable combat state. Max health is derived because buffs can change the limit without rewriting the current-health channel.

The example derives max health from base vitality plus active max-health effects:

let max_health = DerivedAttribute::new(move |id| {
    let base = base_vitality.borrow().get(id)?;
    let mut bonus = 0.0;

    effects.borrow().visit_target(id, |effect| {
        if let EffectPayload::MaxHealthBonus { amount } = effect.payload {
            bonus += amount;
        }
    });

    Some(base + bonus)
});

Attack speed uses the same pattern, reading base attack speed and active attack-speed buffs.

After effects are applied or expire, the runtime refreshes the derived attribute:

DerivedAttributeRefresh::new(player).run(&mut max_health);

Next: Grant Player Abilities.