added transient entities: when crossed, turns into wall tile

observers/obstacle_observer.go

@@ -0,0 +1,54 @@
+package observers
+
+import (
+	"github.com/mlange-42/arche/ecs"
+	"github.com/mlange-42/arche/ecs/event"
+	"github.com/mlange-42/arche/generic"
+	"github.com/mlange-42/arche/listener"
+)
+
+// will be added as  an ecs.Resource to the world so we  can use it in
+// CollisionSystem to dynamically make it appear or disappear
+type ObstacleObserver struct {
+	// we only have one obstacle so far, if we use multiple ones, turn
+	// this in to a map, see player_observer.go for an example
+	Entities map[ecs.Entity]int
+}
+
+// Create a  new resource of  type PlayerObserver which will  hold all
+// obstacle  entities.   We  also  add  a  Listener  which  looks  for
+// EntityRemoved events and  if a player gets removed,  we also remove
+// it from the Entity map in our observer.
+func NewObstacleObserver(world *ecs.World) {
+	observer := &ObstacleObserver{}
+	observer.Entities = make(map[ecs.Entity]int)
+
+	resmanger := generic.NewResource[ObstacleObserver](world)
+	resmanger.Add(observer)
+
+	listen := listener.NewCallback(
+		func(world *ecs.World, event ecs.EntityEvent) {
+			observerID := ecs.ResourceID[ObstacleObserver](world)
+			observer := world.Resources().Get(observerID).(*ObstacleObserver)
+			observer.RemoveEntity(event.Entity)
+		},
+
+		event.EntityRemoved,
+	)
+
+	world.SetListener(&listen)
+}
+
+func GetObstacleObserver(world *ecs.World) *ObstacleObserver {
+	observerID := ecs.ResourceID[ObstacleObserver](world)
+	observer := world.Resources().Get(observerID).(*ObstacleObserver)
+	return observer
+}
+
+func (observer *ObstacleObserver) AddEntity(entity ecs.Entity) {
+	observer.Entities[entity] = 1
+}
+
+func (observer *ObstacleObserver) RemoveEntity(entity ecs.Entity) {
+	observer.Entities = make(map[ecs.Entity]int)
+}