feat: added EngineService onVarsInitialized() to notify services when…

… game vars are ready to use

fxgl-core/src/main/kotlin/com/almasb/fxgl/core/EngineService.kt

@@ -30,6 +30,12 @@ abstract class EngineService : Updatable, SerializableType {
      */
     open fun onMainLoopStarting() { }
 
+    /**
+     * Called after initGameVars() is completed.
+     * This is called on a background thread.
+     */
+    open fun onVarsInitialized(vars: PropertyMap) { }
+
     /**
      * Called when initGame(), initPhysics(), initUI() all completed and
      * the game is ready to be played.

fxgl/src/main/kotlin/com/almasb/fxgl/app/Engine.kt

@@ -138,6 +138,10 @@ internal class Engine(val settings: ReadOnlyGameSettings) {
         services.forEach { it.onGameUpdate(tpf) }
     }
 
+    fun onVarsInitialized(vars: PropertyMap) {
+        services.forEach { it.onVarsInitialized(vars) }
+    }
+
     fun onGameReady(vars: PropertyMap) {
         services.forEach { it.onGameReady(vars) }
     }

fxgl/src/main/kotlin/com/almasb/fxgl/app/FXGLApplication.kt

@@ -338,6 +338,8 @@ class FXGLApplication : Application() {
                 FXGL.getWorldProperties().setValue(name, value)
             }
 
+            engine.onVarsInitialized(FXGL.getWorldProperties())
+
             app.initGame();
         }