fix(hooks): trigger on_set callback when optional component is removed

Previously, on_set was only triggered when components were added. Now it
also triggers when an optional component is removed, notifying hooks that
the entity state changed (optional component becomes undefined).

Author: codehz

src/__tests__/world-multi-component-hooks.test.ts

@@ -219,6 +219,33 @@ describe("World - Multi-Component Hooks", () => {
     expect(removeCalls.length).toBe(0);
   });
 
+  it("should trigger on_set when optional component is removed", () => {
+    const world = new World();
+    const A = component<number>();
+    const B = component<string>();
+    const setCalls: { entityId: EntityId; components: readonly [number, { value: string } | undefined] }[] = [];
+
+    world.hook([A, { optional: B }], {
+      on_set: (entityId, ...components) => {
+        setCalls.push({ entityId, components });
+      },
+    });
+
+    const entity = world.spawn().with(A, 42).with(B, "hello").build();
+    world.sync();
+
+    expect(setCalls.length).toBe(1);
+    expect(setCalls[0]!.components).toEqual([42, { value: "hello" }]);
+
+    // Remove optional component B - should trigger on_set with undefined for B
+    world.remove(entity, B);
+    world.sync();
+
+    expect(setCalls.length).toBe(2);
+    expect(setCalls[1]!.entityId).toBe(entity);
+    expect(setCalls[1]!.components).toEqual([42, undefined]);
+  });
+
   it("should trigger on_remove with complete snapshot when required component is removed", () => {
     const world = new World();
     const A = component<number>();

src/core/world-hooks.ts

@@ -115,18 +115,20 @@ function triggerMultiComponentHooks(
   oldArchetype: Archetype,
   newArchetype: Archetype,
 ): void {
-  // Handle on_set: triggers if any required or optional component was added and entity matches now
-  if (addedComponents.size > 0) {
-    for (const entry of newArchetype.matchingMultiHooks) {
-      const { hook, requiredComponents, optionalComponents, componentTypes } = entry;
-      if (!hook.on_set) continue;
+  // Handle on_set: triggers if any required or optional component was added/removed and entity still matches
+  for (const entry of newArchetype.matchingMultiHooks) {
+    const { hook, requiredComponents, optionalComponents, componentTypes } = entry;
+    if (!hook.on_set) continue;
 
-      const anyRequiredAdded = requiredComponents.some((c) => anyComponentMatches(addedComponents, c));
-      const anyOptionalAdded = optionalComponents.some((c) => anyComponentMatches(addedComponents, c));
+    const anyRequiredAdded = requiredComponents.some((c) => anyComponentMatches(addedComponents, c));
+    const anyOptionalAdded = optionalComponents.some((c) => anyComponentMatches(addedComponents, c));
+    const anyOptionalRemoved = optionalComponents.some((c) => anyComponentMatches(removedComponents, c));
 
-      if ((anyRequiredAdded || anyOptionalAdded) && entityHasAllComponents(ctx, entityId, requiredComponents)) {
-        hook.on_set(entityId, ...collectMultiHookComponents(ctx, entityId, componentTypes));
-      }
+    if (
+      (anyRequiredAdded || anyOptionalAdded || anyOptionalRemoved) &&
+      entityHasAllComponents(ctx, entityId, requiredComponents)
+    ) {
+      hook.on_set(entityId, ...collectMultiHookComponents(ctx, entityId, componentTypes));
     }
   }