class RenderLoop {

 Juggler _juggler = new Juggler();
 List<Stage> _stages = new List<Stage>();
 num _renderTime = -1;
 int _requestId = null;
 bool _invalidate = false;

 EnterFrameEvent _enterFrameEvent = new EnterFrameEvent(0);
 ExitFrameEvent _exitFrameEvent = new ExitFrameEvent();
 RenderEvent _renderEvent = new RenderEvent();

 RenderLoop() {
   start();
 }

 Juggler get juggler => _juggler;

 //-------------------------------------------------------------------------------------------------
 //-------------------------------------------------------------------------------------------------

 void start() {
   _renderTime = -1;
   _requestAnimationFrame();
 }

 void stop() {
   _cancelAnimationFrame();
 }

 void invalidate() {
   _invalidate = true;
 }

 void addStage(Stage stage) {

   if (stage.renderLoop != null) {
     stage.renderLoop.removeStage(stage);
   }

   _stages.add(stage);
   stage._renderLoop = this;
 }

 void removeStage(Stage stage) {

   if (stage.renderLoop == this) {
     _stages.remove(stage);
     stage._renderLoop = null;
   }
 }

 //-------------------------------------------------------------------------------------------------
 //-------------------------------------------------------------------------------------------------

 _requestAnimationFrame() {
   if (_requestId == null) {
     _requestId = html.window.requestAnimationFrame(_onAnimationFrame);
   }
 }

 _cancelAnimationFrame() {
   if (_requestId != null) {
     html.window.cancelAnimationFrame(_requestId);
     _requestId = null;
   }
 }

 _onAnimationFrame(num currentTime) {

   _requestId = null;
   _requestAnimationFrame();

   currentTime = currentTime.toDouble();

   if (_renderTime == -1) _renderTime = currentTime;
   if (_renderTime > currentTime) _renderTime = currentTime;

   var deltaTime = currentTime - _renderTime;
   var deltaTimeSec = deltaTime / 1000.0;
   var currentTimeSec = currentTime / 1000.0;
   var invalidate = false;

   if (deltaTime >= 1) {
     _renderTime = currentTime;
     _enterFrameEvent._passedTime = deltaTimeSec;
     _dispatchBroadcastEvent(_enterFrameEvent, _enterFrameSubscriptions);

     _juggler.advanceTime(deltaTimeSec);

     for(int i = 0; i < _stages.length; i++) {
       _stages[i].juggler.advanceTime(deltaTimeSec);
     }

     if (_invalidate) {
       _invalidate = false;
       _dispatchBroadcastEvent(_renderEvent, _renderSubscriptions);
     }

     for(int i = 0; i < _stages.length; i++) {
       _stages[i].materialize(currentTimeSec, deltaTimeSec);
     }

     _dispatchBroadcastEvent(_exitFrameEvent, _exitFrameSubscriptions);
   }
 }
}