Work on allowing more than one spell to be attached

src/main/java/com/minelittlepony/unicopia/ability/PegasusRainboomAbility.java

@@ -68,7 +68,7 @@ public class PegasusRainboomAbility implements Ability<Hit> {
         if (player.getPhysics().isFlying() && !player.getSpellSlot().isPresent()) {
             player.getMagicalReserves().getMana().multiply(0.1F);
             player.addParticle(new OrientedBillboardParticleEffect(UParticles.RAINBOOM_RING, player.getPhysics().getMotionAngle()), player.getOriginVector(), Vec3d.ZERO);
-            player.setSpell(SpellType.RAINBOOM.create(SpellTraits.EMPTY));
+            player.getSpellSlot().put(SpellType.RAINBOOM.create(SpellTraits.EMPTY));
         }
     }
 

src/main/java/com/minelittlepony/unicopia/ability/magic/Caster.java

@@ -8,7 +8,6 @@ import org.jetbrains.annotations.Nullable;
 
 import com.minelittlepony.unicopia.EquinePredicates;
 import com.minelittlepony.unicopia.Owned;
-import com.minelittlepony.unicopia.ability.magic.spell.Spell;
 import com.minelittlepony.unicopia.entity.Physics;
 import com.minelittlepony.unicopia.entity.PonyContainer;
 import com.minelittlepony.unicopia.particle.ParticleSource;
@@ -29,10 +28,6 @@ public interface Caster<E extends LivingEntity> extends Owned<E>, Levelled, Affi
 
     SpellContainer getSpellSlot();
 
-    default void setSpell(@Nullable Spell spell) {
-        getSpellSlot().put(spell);
-    }
-
     /**
      * Gets the entity directly responsible for casting.
      */

src/main/java/com/minelittlepony/unicopia/ability/magic/SpellContainer.java

@@ -1,6 +1,7 @@
 package com.minelittlepony.unicopia.ability.magic;
 
 import java.util.Optional;
+import java.util.function.Predicate;
 
 import org.jetbrains.annotations.Nullable;
 
@@ -15,6 +16,12 @@ public interface SpellContainer {
 
         @Override
         public void put(Spell effect) { }
+
+        @Override
+        public void clear() { }
+
+        @Override
+        public void removeIf(Predicate<Spell> effect, boolean update) { }
     };
 
     /**
@@ -40,4 +47,39 @@ public interface SpellContainer {
      * Sets the active effect.
      */
     void put(@Nullable Spell effect);
+
+    /**
+     * Removes all matching active effects.
+     */
+    void removeIf(Predicate<Spell> effect, boolean update);
+
+    /**
+     * Removes all effects currently active in this slot.
+     */
+    void clear();
+
+    interface Delegate extends SpellContainer {
+
+        SpellContainer delegate();
+
+        @Override
+        default <T extends Spell> Optional<T> get(@Nullable SpellPredicate<T> type, boolean update) {
+            return delegate().get(type, update);
+        }
+
+        @Override
+        default void put(@Nullable Spell effect) {
+            delegate().put(effect);
+        }
+
+        @Override
+        default void removeIf(Predicate<Spell> effect, boolean update) {
+            delegate().removeIf(effect, update);
+        }
+
+        @Override
+        default void clear() {
+            delegate().clear();
+        }
+    }
 }

src/main/java/com/minelittlepony/unicopia/ability/magic/spell/AbstractDelegatingSpell.java

@@ -63,7 +63,7 @@ public abstract class AbstractDelegatingSpell implements ProjectileSpell {
 
     @Override
     public boolean apply(Caster<?> caster) {
-        caster.setSpell(this);
+        caster.getSpellSlot().put(this);
         return true;
     }
 

src/main/java/com/minelittlepony/unicopia/ability/magic/spell/PlaceableSpell.java

@@ -71,7 +71,7 @@ public class PlaceableSpell extends AbstractDelegatingSpell {
                     CastSpellEntity entity = UEntities.CAST_SPELL.create(source.getWorld());
                     Vec3d pos = castEntity.getPosition().orElse(source.getOriginVector());
                     entity.updatePositionAndAngles(pos.x, pos.y, pos.z, 0, 0);
-                    entity.setSpell(this);
+                    entity.getSpellSlot().put(this);
                     entity.setMaster(source.getMaster());
                     entity.world.spawnEntity(entity);
 

src/main/java/com/minelittlepony/unicopia/ability/magic/spell/ThrowableSpell.java

@@ -50,7 +50,7 @@ public final class ThrowableSpell extends AbstractDelegatingSpell {
             MagicProjectileEntity projectile = new MagicProjectileEntity(world, entity);
 
             projectile.setItem(GemstoneItem.enchanted(UItems.GEMSTONE.getDefaultStack(), spell.getType()));
-            projectile.setSpell(this);
+            projectile.getSpellSlot().put(this);
             projectile.setProperties(entity, entity.getPitch(), entity.getYaw(), 0, 1.5F, 1);
             projectile.setHydrophobic();
             configureProjectile(projectile, caster);

src/main/java/com/minelittlepony/unicopia/ability/magic/spell/effect/AbstractSpell.java

@@ -67,7 +67,7 @@ public abstract class AbstractSpell implements Spell {
 
     @Override
     public final boolean apply(Caster<?> caster) {
-        caster.setSpell(this);
+        caster.getSpellSlot().put(this);
         return true;
     }
 

src/main/java/com/minelittlepony/unicopia/ability/magic/spell/effect/SpellType.java

@@ -141,7 +141,7 @@ public final class SpellType<T extends Spell> implements Affine, SpellPredicate<
     @Nullable
     public T apply(Caster<?> caster, SpellTraits traits) {
         if (isEmpty()) {
-            caster.setSpell(null);
+            caster.getSpellSlot().clear();
             return null;
         }
 

src/main/java/com/minelittlepony/unicopia/entity/CastSpellEntity.java

@@ -37,6 +37,19 @@ public class CastSpellEntity extends Entity implements Caster<LivingEntity> {
 
     private final EntityReference<LivingEntity> owner = new EntityReference<>();
 
+    private final SpellContainer spell = new SpellContainer.Delegate() {
+        @Override
+        public SpellContainer delegate() {
+            return Caster.of(getMaster()).map(Caster::getSpellSlot).orElse(SpellContainer.EMPTY);
+        }
+
+        @Override
+        public void put(Spell spell) {
+            getDataTracker().set(SPELL, Optional.ofNullable(spell).map(Spell::getUuid));
+            SpellContainer.Delegate.super.put(spell);
+        }
+    };
+
     private BlockPos lastPos;
 
     private int orphanedTicks;
@@ -94,7 +107,7 @@ public class CastSpellEntity extends Entity implements Caster<LivingEntity> {
             UUID spellId = dataTracker.get(SPELL).orElse(null);
 
             if (!c.getSpellSlot().get(true).filter(s -> s.getUuid().equals(spellId) && s.tick(this, Situation.GROUND_ENTITY)).isPresent()) {
-                c.setSpell(null);
+                c.getSpellSlot().clear();
 
                 return false;
             }
@@ -118,11 +131,6 @@ public class CastSpellEntity extends Entity implements Caster<LivingEntity> {
         this.owner.set(owner);
     }
 
-    @Override
-    public void setSpell(Spell spell) {
-        getDataTracker().set(SPELL, Optional.ofNullable(spell).map(Spell::getUuid));
-    }
-
     @Override
     public Entity getEntity() {
         return this;
@@ -150,7 +158,7 @@ public class CastSpellEntity extends Entity implements Caster<LivingEntity> {
 
     @Override
     public SpellContainer getSpellSlot() {
-        return Caster.of(getMaster()).map(Caster::getSpellSlot).orElse(SpellContainer.EMPTY);
+        return spell;
     }
 
     @Override

src/main/java/com/minelittlepony/unicopia/entity/Creature.java

@@ -106,7 +106,7 @@ public class Creature extends Living<LivingEntity> {
     public void fromNBT(NbtCompound compound) {
         super.fromNBT(compound);
         if (compound.contains("effect")) {
-            setSpell(SpellType.fromNBT(compound.getCompound("effect")));
+            getSpellSlot().put(SpellType.fromNBT(compound.getCompound("effect")));
         }
         physics.fromNBT(compound);
     }

src/main/java/com/minelittlepony/unicopia/entity/Living.java

@@ -10,7 +10,7 @@ import com.minelittlepony.unicopia.ability.magic.SpellContainer;
 import com.minelittlepony.unicopia.ability.magic.spell.Situation;
 import com.minelittlepony.unicopia.ability.magic.spell.effect.SpellType;
 import com.minelittlepony.unicopia.item.UItems;
-import com.minelittlepony.unicopia.network.EffectSync;
+import com.minelittlepony.unicopia.network.datasync.EffectSync;
 import com.minelittlepony.unicopia.projectile.ProjectileImpactListener;
 import com.minelittlepony.unicopia.util.MagicalDamageSource;
 
@@ -85,11 +85,7 @@ public abstract class Living<T extends LivingEntity> implements Equine<T>, Caste
 
     @Override
     public void tick() {
-        getSpellSlot().get(true).ifPresent(effect -> {
+        getSpellSlot().removeIf(effect -> !effect.tick(this, Situation.BODY), true);
-            if (!effect.tick(this, Situation.BODY)) {
-                setSpell(null);
-            }
-        });
 
         if (invinsibilityTicks > 0) {
             invinsibilityTicks--;

src/main/java/com/minelittlepony/unicopia/entity/behaviour/Disguise.java

@@ -196,7 +196,7 @@ public class Disguise implements NbtSerialisable, PlayerDimensions.Provider, Fli
             return;
         }
 
-        Caster.of(entity).ifPresent(c -> c.setSpell(null));
+        Caster.of(entity).ifPresent(c -> c.getSpellSlot().clear());
 
         if (entity instanceof LivingEntity) {
             ((LivingEntity) entity).getAttributeInstance(PlayerAttributes.ENTITY_GRAVTY_MODIFIER).clearModifiers();

src/main/java/com/minelittlepony/unicopia/entity/effect/RaceChangeStatusEffect.java

@@ -104,7 +104,7 @@ public class RaceChangeStatusEffect extends StatusEffect {
 
             eq.setSpecies(species);
             if (eq instanceof Caster) {
-                ((Caster<?>)eq).setSpell(null);
+                ((Caster<?>)eq).getSpellSlot().clear();
             }
 
             if (eq instanceof Pony) {

src/main/java/com/minelittlepony/unicopia/entity/player/Pony.java

@@ -30,7 +30,8 @@ import com.minelittlepony.unicopia.item.toxin.Toxin;
 import com.minelittlepony.unicopia.network.Channel;
 import com.minelittlepony.unicopia.network.MsgOtherPlayerCapabilities;
 import com.minelittlepony.unicopia.network.MsgRequestSpeciesChange;
-import com.minelittlepony.unicopia.network.Transmittable;
+import com.minelittlepony.unicopia.network.datasync.Transmittable;
+import com.minelittlepony.unicopia.network.datasync.EffectSync.UpdateCallback;
 import com.minelittlepony.unicopia.util.Copieable;
 import com.minelittlepony.unicopia.util.MagicalDamageSource;
 import com.minelittlepony.unicopia.util.Tickable;
@@ -61,7 +62,7 @@ import net.minecraft.text.TranslatableText;
 import net.minecraft.util.math.BlockPos;
 import net.minecraft.util.math.Direction;
 
-public class Pony extends Living<PlayerEntity> implements Transmittable, Copieable<Pony> {
+public class Pony extends Living<PlayerEntity> implements Transmittable, Copieable<Pony>, UpdateCallback {
 
     private static final TrackedData<Integer> RACE = DataTracker.registerData(PlayerEntity.class, TrackedDataHandlerRegistry.INTEGER);
 
@@ -505,17 +506,16 @@ public class Pony extends Living<PlayerEntity> implements Transmittable, Copieab
     public void copyFrom(Pony oldPlayer) {
         speciesPersisted = oldPlayer.speciesPersisted;
         if (!oldPlayer.getEntity().isRemoved()) {
-            setSpell(oldPlayer.getSpellSlot().get(true).orElse(null));
+            getSpellSlot().put(oldPlayer.getSpellSlot().get(true).orElse(null));
         }
-        oldPlayer.setSpell(null);
+        oldPlayer.getSpellSlot().put(null);
         setSpecies(oldPlayer.getSpecies());
         getDiscoveries().copyFrom(oldPlayer.getDiscoveries());
         setDirty();
     }
 
     @Override
-    public void setSpell(@Nullable Spell effect) {
+    public void onSpellSet(@Nullable Spell spell) {
-        super.setSpell(effect);
         setDirty();
     }
 

src/main/java/com/minelittlepony/unicopia/mixin/MixinMilkBucketItem.java

@@ -18,6 +18,6 @@ abstract class MixinMilkBucketItem extends Item {
 
     @Inject(method = "finishUsing", at = @At("HEAD"), cancellable = true)
     private void finishUsing(ItemStack stack, World world, LivingEntity entity, CallbackInfoReturnable<ItemStack> info) {
-        Caster.of(entity).ifPresent(c -> c.setSpell(null));
+        Caster.of(entity).ifPresent(c -> c.getSpellSlot().clear());
     }
 }

src/main/java/com/minelittlepony/unicopia/network/EffectSync.java

@@ -1,106 +0,0 @@
-package com.minelittlepony.unicopia.network;
-
-import java.util.Objects;
-import java.util.Optional;
-
-import org.jetbrains.annotations.Nullable;
-
-import com.minelittlepony.unicopia.ability.magic.Caster;
-import com.minelittlepony.unicopia.ability.magic.SpellContainer;
-import com.minelittlepony.unicopia.ability.magic.SpellPredicate;
-import com.minelittlepony.unicopia.ability.magic.spell.Spell;
-import com.minelittlepony.unicopia.ability.magic.spell.effect.SpellType;
-
-import net.minecraft.entity.data.TrackedData;
-import net.minecraft.nbt.NbtCompound;
-
-/**
- * Synchronisation class for spells.
- * Since we can't have our own serializers, we have to intelligently
- * determine whether to update it from an nbt tag.
- *
- * @param <T> The owning entity
- */
-public class EffectSync implements SpellContainer {
-
-    private Optional<Spell> spell = Optional.empty();
-
-    private final Caster<?> owner;
-
-    private final TrackedData<NbtCompound> param;
-
-    @Nullable
-    private NbtCompound lastValue;
-
-    public EffectSync(Caster<?> owner, TrackedData<NbtCompound> param) {
-        this.owner = owner;
-        this.param = param;
-    }
-
-    @Override
-    @SuppressWarnings("unchecked")
-    public <T extends Spell> Optional<T> get(@Nullable SpellPredicate<T> type, boolean update) {
-        if (update) {
-            sync(true);
-        }
-
-        if (checkReference() && (type == null || type.test(spell.get()))) {
-            return (Optional<T>)spell;
-        }
-
-        return Optional.empty();
-    }
-
-    @Override
-    public boolean isPresent() {
-        sync(false);
-        return checkReference();
-    }
-
-    private boolean checkReference() {
-        return spell.isPresent() && !spell.get().isDead();
-    }
-
-    private void sync(boolean force) {
-        @Nullable
-        NbtCompound comp = owner.getEntity().getDataTracker().get(param);
-
-        @Nullable
-        Spell effect = spell.orElse(null);
-
-        if (comp == null || !comp.contains("effect_id") || !comp.contains("uuid")) {
-            if (effect != null) {
-                updateReference(null);
-            }
-        } else if (effect == null || !effect.getUuid().equals(comp.getUuid("uuid"))) {
-            updateReference(SpellType.fromNBT(comp));
-        } else if (owner.isClient()) {
-            if (!Objects.equals(lastValue, comp)) {
-                lastValue = comp;
-                effect.fromNBT(comp);
-            }
-        } else if (force && effect.isDirty()) {
-            put(effect);
-        }
-    }
-
-    @Override
-    public void put(@Nullable Spell effect) {
-        effect = effect == null || effect.isDead() ? null : effect;
-        updateReference(effect);
-        owner.getEntity().getDataTracker().set(param, effect == null ? new NbtCompound() : SpellType.toNBT(effect));
-    }
-
-    private void updateReference(@Nullable Spell effect) {
-        @Nullable
-        Spell old = spell.orElse(null);
-        if (old != effect) {
-            spell = Optional.ofNullable(effect);
-
-            if (old != null && (effect == null || !old.getUuid().equals(effect.getUuid()))) {
-                old.setDead();
-                old.onDestroyed(owner);
-            }
-        }
-    }
-}

src/main/java/com/minelittlepony/unicopia/network/datasync/EffectSync.java

@@ -0,0 +1,99 @@
+package com.minelittlepony.unicopia.network.datasync;
+
+import java.util.Optional;
+import java.util.function.Consumer;
+import java.util.function.Predicate;
+import java.util.stream.Stream;
+
+import org.jetbrains.annotations.Nullable;
+
+import com.minelittlepony.unicopia.ability.magic.Caster;
+import com.minelittlepony.unicopia.ability.magic.SpellContainer;
+import com.minelittlepony.unicopia.ability.magic.SpellPredicate;
+import com.minelittlepony.unicopia.ability.magic.spell.Spell;
+
+import net.minecraft.entity.data.TrackedData;
+import net.minecraft.nbt.NbtCompound;
+
+/**
+ * Synchronisation class for spells.
+ * Since we can't have our own serializers, we have to intelligently
+ * determine whether to update it from an nbt tag.
+ *
+ * @param <T> The owning entity
+ */
+public class EffectSync implements SpellContainer {
+
+    private final NetworkedReferenceSet<Spell> spells;
+
+    private final Caster<?> owner;
+
+    private final TrackedData<NbtCompound> param;
+
+    @Nullable
+    private NbtCompound lastValue;
+
+    public EffectSync(Caster<?> owner, TrackedData<NbtCompound> param) {
+        spells = new NetworkedReferenceSet<>(Spell::getUuid, () -> new SpellNetworkedReference<>(owner));
+        this.owner = owner;
+        this.param = param;
+    }
+
+    @Override
+    @SuppressWarnings("unchecked")
+    public <T extends Spell> Optional<T> get(@Nullable SpellPredicate<T> type, boolean update) {
+        return (Optional<T>)(type == null ? read(update, true).findFirst() : read(update, true).filter(type).findFirst());
+    }
+
+    @Override
+    public boolean isPresent() {
+        return read(true, false).findFirst().isPresent();
+    }
+
+    @Override
+    public void put(@Nullable Spell effect) {
+        spells.addReference(effect);
+        write();
+        if (owner instanceof UpdateCallback) {
+            ((UpdateCallback)owner).onSpellSet(effect);
+        }
+    }
+
+    @Override
+    public void removeIf(Predicate<Spell> test, boolean update) {
+        read(effect -> {
+            if (test.test(effect)) {
+                spells.removeReference(effect);
+            }
+        });
+    }
+
+    @Override
+    public void clear() {
+        put(null);
+    }
+
+    private Stream<Spell> read(boolean synchronize, boolean sendUpdate) {
+        if (synchronize && spells.fromNbt(owner.getEntity().getDataTracker().get(param)) && sendUpdate) {
+            write();
+        }
+
+        return spells.getReferences();
+    }
+
+    private void read(Consumer<Spell> consumer) {
+        spells.fromNbt(owner.getEntity().getDataTracker().get(param));
+        spells.getReferences().toList().forEach(consumer);
+        write();
+    }
+
+    private void write() {
+        if (spells.isDirty()) {
+            owner.getEntity().getDataTracker().set(param, spells.toNbt());
+        }
+    }
+
+    public interface UpdateCallback {
+        void onSpellSet(@Nullable Spell spell);
+    }
+}

src/main/java/com/minelittlepony/unicopia/network/datasync/NetworkedReference.java

@@ -0,0 +1,19 @@
+package com.minelittlepony.unicopia.network.datasync;
+
+import java.util.Optional;
+
+import org.jetbrains.annotations.Nullable;
+
+import net.minecraft.nbt.NbtCompound;
+
+public interface NetworkedReference<T> {
+    Optional<T> getReference();
+
+    Optional<T> updateReference(@Nullable T newValue);
+
+    boolean fromNbt(NbtCompound comp);
+
+    NbtCompound toNbt();
+
+    boolean isDirty();
+}

src/main/java/com/minelittlepony/unicopia/network/datasync/NetworkedReferenceSet.java

@@ -0,0 +1,110 @@
+package com.minelittlepony.unicopia.network.datasync;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.UUID;
+import java.util.function.Function;
+import java.util.function.Supplier;
+import java.util.stream.Stream;
+
+import org.jetbrains.annotations.Nullable;
+
+import net.minecraft.nbt.NbtCompound;
+import net.minecraft.nbt.NbtElement;
+import net.minecraft.nbt.NbtList;
+import net.minecraft.nbt.NbtString;
+
+public class NetworkedReferenceSet<T> {
+
+    private final List<UUID> ids = new ArrayList<>();
+
+    private final Map<UUID, NetworkedReference<T>> values = new HashMap<>();
+
+    private final Function<T, UUID> uuidConverter;
+    private final Supplier<NetworkedReference<T>> factory;
+
+    private boolean dirty;
+
+    public NetworkedReferenceSet(Function<T, UUID> uuidConverter, Supplier<NetworkedReference<T>> factory) {
+        this.uuidConverter = uuidConverter;
+        this.factory = factory;
+    }
+
+    public Stream<T> getReferences() {
+        return ids.stream().map(id -> values.get(id))
+                .map(a -> a.getReference())
+                .filter(Optional::isPresent)
+                .map(Optional::get);
+    }
+
+    public void addReference(@Nullable T newValue) {
+        if (newValue != null) {
+            addReference(uuidConverter.apply(newValue));
+        }
+    }
+
+    private NetworkedReference<T> addReference(UUID newValue) {
+        return values.computeIfAbsent(newValue, id -> {
+            dirty = true;
+            ids.remove(id);
+            ids.add(0, id);
+            return factory.get();
+        });
+    }
+
+    public void removeReference(@Nullable T oldValue) {
+        if (oldValue != null) {
+            removeReference(uuidConverter.apply(oldValue));
+        }
+    }
+
+    private void removeReference(UUID id) {
+        dirty |= ids.remove(id);
+        NetworkedReference<T> i = values.remove(id);
+        if (i != null) {
+            dirty = true;
+            i.updateReference(null);
+        }
+    }
+
+    public boolean fromNbt(NbtCompound comp) {
+
+        List<UUID> incoming = new ArrayList<>();
+        comp.getList("keys", NbtElement.STRING_TYPE).forEach(key -> {
+            incoming.add(UUID.fromString(key.asString()));
+        });
+
+        ids.stream().filter(id -> !incoming.contains(id)).forEach(this::removeReference);
+
+        boolean[] send = new boolean[0];
+        incoming.forEach(kept -> {
+            NetworkedReference<T> i = addReference(kept);
+            send[0] |= i.fromNbt(comp.getCompound(kept.toString()));
+            if (i.getReference().isEmpty()) {
+                removeReference(kept);
+            }
+        });
+        dirty = false;
+        return send[0];
+    }
+
+    public NbtCompound toNbt() {
+        NbtCompound tag = new NbtCompound();
+        NbtList ids = new NbtList();
+        this.ids.forEach(id -> {
+            String sid = id.toString();
+            ids.add(NbtString.of(sid));
+            tag.put(sid, values.get(id).toNbt());
+        });
+        tag.put("key", ids);
+        dirty = false;
+        return tag;
+    }
+
+    public boolean isDirty() {
+        return dirty || values.values().stream().anyMatch(NetworkedReference::isDirty);
+    }
+}

src/main/java/com/minelittlepony/unicopia/network/datasync/SpellNetworkedReference.java

@@ -0,0 +1,110 @@
+package com.minelittlepony.unicopia.network.datasync;
+
+import java.util.Objects;
+import java.util.Optional;
+
+import org.jetbrains.annotations.Nullable;
+
+import com.minelittlepony.unicopia.ability.magic.Caster;
+import com.minelittlepony.unicopia.ability.magic.spell.Spell;
+import com.minelittlepony.unicopia.ability.magic.spell.effect.SpellType;
+
+import net.minecraft.nbt.NbtCompound;
+
+public class SpellNetworkedReference<T extends Spell> implements NetworkedReference<T> {
+    private Optional<T> currentValue = Optional.empty();
+
+    @Nullable
+    private NbtCompound lastValue;
+
+    private final Caster<?> owner;
+
+    private boolean dirty;
+
+    public SpellNetworkedReference(Caster<?> owner) {
+        this.owner = owner;
+    }
+
+    @Override
+    public Optional<T> getReference() {
+        return currentValue.filter(s -> !s.isDead());
+    }
+
+    private boolean mustDelete(@Nullable NbtCompound comp) {
+        return (comp == null || !comp.contains("effect_id") || !comp.contains("uuid")) && currentValue.isPresent();
+    }
+
+    private boolean mustReplace(NbtCompound comp) {
+        return currentValue.isEmpty() || !currentValue.get().getUuid().equals(comp.getUuid("uuid"));
+    }
+
+    private boolean mustUpdate(NbtCompound comp) {
+        if (owner.isClient() && !Objects.equals(lastValue, comp)) {
+            lastValue = comp;
+            return true;
+        }
+        return false;
+    }
+
+    private boolean mustSend() {
+        return currentValue.filter(Spell::isDirty).isPresent();
+    }
+
+    @Override
+    public Optional<T> updateReference(@Nullable T newValue) {
+        newValue = newValue == null || newValue.isDead() ? null : newValue;
+
+        @Nullable
+        T oldValue = currentValue.orElse(null);
+        if (oldValue != newValue) {
+            dirty = true;
+            currentValue = Optional.ofNullable(newValue);
+
+            if (oldValue != null && (newValue == null || !oldValue.getUuid().equals(newValue.getUuid()))) {
+                oldValue.setDead();
+                oldValue.onDestroyed(owner);
+            }
+        }
+
+        return currentValue;
+    }
+
+    @Override
+    @SuppressWarnings("unchecked")
+    public boolean fromNbt(NbtCompound comp) {
+        dirty = false;
+
+        if (mustDelete(comp)) {
+            updateReference(null);
+            return false;
+        }
+
+        if (mustReplace(comp)) {
+            updateReference((T)SpellType.fromNBT(comp));
+            return false;
+        }
+
+        if (mustUpdate(comp)) {
+            currentValue.ifPresent(s -> s.fromNBT(comp));
+            return false;
+        }
+
+        if (mustSend()) {
+            updateReference(getReference().orElse(null));
+            return true;
+        }
+
+        return false;
+    }
+
+    @Override
+    public NbtCompound toNbt() {
+        dirty = false;
+        return getReference().map(SpellType::toNBT).orElseGet(NbtCompound::new);
+    }
+
+    @Override
+    public boolean isDirty() {
+        return dirty || mustSend();
+    }
+}

src/main/java/com/minelittlepony/unicopia/network/datasync/Transmittable.java

@@ -1,4 +1,4 @@
-package com.minelittlepony.unicopia.network;
+package com.minelittlepony.unicopia.network.datasync;
 
 public interface Transmittable {
     void sendCapabilities(boolean full);

src/main/java/com/minelittlepony/unicopia/projectile/MagicProjectileEntity.java

@@ -16,8 +16,8 @@ import com.minelittlepony.unicopia.entity.Physics;
 import com.minelittlepony.unicopia.entity.UEntities;
 import com.minelittlepony.unicopia.item.UItems;
 import com.minelittlepony.unicopia.network.Channel;
-import com.minelittlepony.unicopia.network.EffectSync;
 import com.minelittlepony.unicopia.network.MsgSpawnProjectile;
+import com.minelittlepony.unicopia.network.datasync.EffectSync;
 
 import net.minecraft.entity.Entity;
 import net.minecraft.entity.EntityType;
@@ -220,7 +220,7 @@ public class MagicProjectileEntity extends ThrownItemEntity implements Caster<Li
         super.readCustomDataFromNbt(compound);
         physics.fromNBT(compound);
         if (compound.contains("effect")) {
-            setSpell(SpellType.fromNBT(compound.getCompound("effect")));
+            getSpellSlot().put(SpellType.fromNBT(compound.getCompound("effect")));
         }
     }