Add isOpen to asset manager

src/asset_manager/index.js

@@ -150,6 +150,17 @@ export default () => {
       cmd.stop(assetCmd);
     },
 
+    /**
+     * Checks if the asset manager is open
+     * @returns {Boolean}
+     * @example
+     * assetManager.isOpen(); // true | false
+     */
+    isOpen() {
+      const cmd = this.em.get('Commands');
+      return cmd.isActive(assetCmd);
+    },
+
     /**
      * Add new asset/s to the collection. URLs are supposed to be unique
      * @param {String|Object|Array<String>|Array<Object>} asset URL strings or an objects representing the resource.