Categories can define a load() function with callback

src/CategoryOverpass.js

@@ -109,6 +109,10 @@ function CategoryOverpass (id, data) {
   this.dom.appendChild(this.domStatus)
 }
 
+CategoryOverpass.prototype.load = function (callback) {
+  callback(null)
+}
+
 CategoryOverpass.prototype.setMap = function (map) {
   CategoryBase.prototype.setMap.call(this, map)
 

src/OpenStreetBrowserLoader.js

@@ -52,7 +52,13 @@ OpenStreetBrowserLoader.prototype.getCategoryFromData = function (id, data, call
 
     this.categories[id] = layer
 
-    callback(null, layer)
+    if ('load' in layer) {
+      layer.load(function (err) {
+        callback(err, layer)
+      })
+    } else {
+      callback(null, layer)
+    }
   }
 }