OnRemove and OnCreate callbacks (#130)

* OnRemove and OnCreate callbacks

* Update world_test.go

* Adds world parameter

* Update world_test.go

world.go

@@ -32,6 +32,13 @@ type World interface {
 	StorageAccessor() StorageAccessor
 	// ArcheTypes returns the archetypes in the world.
 	Archetypes() []*storage.Archetype
+
+	// OnCreate registers a callback function that gets triggered when an entity is created.
+	OnCreate(callback func(world World, entity Entity))
+
+	// OnRemove registers a callback function that gets triggered when an entity is removed.
+	// Note that it is called before the entity is removed from the ECS.
+	OnRemove(callback func(world World, entity Entity))
 }
 
 // StorageAccessor is an accessor for the world's storage.
@@ -55,6 +62,9 @@ type world struct {
 	destroyed    []Entity
 	entries      []*Entry
 	nextEntityId storage.EntityId
+
+	removeCallbacks []func(world World, entity Entity)
+	createCallbacks []func(world World, entity Entity)
 }
 
 var nextWorldId WorldId = 0
@@ -111,6 +121,11 @@ func (w *world) createEntity(archetypeIndex storage.ArchetypeIndex) Entity {
 	w.entities.Insert(entity.Id(), archetypeIndex, componentIndex)
 	archetype.PushEntity(entity)
 	w.createEntry(entity)
+
+	for _, callback := range w.createCallbacks {
+		callback(w, entity)
+	}
+
 	return entity
 }
 
@@ -158,6 +173,11 @@ func (w *world) Len() int {
 
 func (w *world) Remove(ent Entity) {
 	if w.Valid(ent) {
+		// Called before any operations so that user code can access all the data it might need
+		for _, callback := range w.removeCallbacks {
+			callback(w, ent)
+		}
+
 		loc := w.entities.LocationMap[ent.Id()]
 		w.entities.Remove(ent.Id())
 		w.removeAtLocation(ent, loc)
@@ -226,6 +246,14 @@ func (w *world) StorageAccessor() StorageAccessor {
 	}
 }
 
+func (w *world) OnCreate(callback func(world World, entity Entity)) {
+	w.createCallbacks = append(w.createCallbacks, callback)
+}
+
+func (w *world) OnRemove(callback func(world World, entity Entity)) {
+	w.removeCallbacks = append(w.removeCallbacks, callback)
+}
+
 func (w *world) Archetypes() []*storage.Archetype {
 	return w.archetypes
 }

world_test.go

@@ -214,9 +214,32 @@ func TestArchetypeStorageExpands(t *testing.T) {
 func TestRemoveAndCreateEntity(t *testing.T) {
 	world := donburi.NewWorld()
 
+	var calledCreate, calledRemove bool
+
+	if calledCreate || calledRemove {
+		t.Fatalf("OnCreate/OnRemove event should not have been called at this point")
+	}
+
+	world.OnCreate(func(world donburi.World, entity donburi.Entity) {
+		calledCreate = true
+	})
+
+	world.OnRemove(func(world donburi.World, entity donburi.Entity) {
+		calledRemove = true
+	})
+
 	entityA := world.Create(tagA)
 
+	if !calledCreate {
+		t.Fatalf("OnCreate event must have been called at this point")
+	}
+
 	world.Remove(entityA)
+
+	if !calledRemove {
+		t.Fatalf("OnCreate event must have been called at this point")
+	}
+
 	if world.Valid(entityA) {
 		t.Errorf("Entity should be invalid")
 	}