新增identifierPool用于实体id复用

source/src/ECS/Utils/IdentifierPool.ts

@@ -0,0 +1,22 @@
+module es {
+    export class IdentifierPool {
+        private ids: Bag<number>;
+        private nextAvailableId_ = 0;
+
+        constructor() {
+            this.ids = new Bag<number>();
+        }
+
+        public checkOut(): number {
+            if (this.ids.size() > 0) {
+                return this.ids.removeLast();
+            }
+
+            return this.nextAvailableId_++;
+        }
+
+        public checkIn(id: number): void {
+            this.ids.add(id);
+        }
+    }
+}