Time for action – creating transitions with a singleton Layer class

We first need to make sure the layer in question can only be instantiated once.

1. The scene static method in GameLayer looks like this:

   Scene* GameLayer::scene(int level, int levelsCompleted)
   {
       // 'scene' is an autorelease object
       auto scene = Scene::create();
      // add layer as a child to scene
       scene->addChild(GameLayer::create(level, levelsCompleted));
      return scene;
   }

   This layer receives two parameters when created: the game level it should load and the number of levels completed by the player. We create a new Scene object and add GameLayer as its child.

2. But take a look at the static create method in GameLayer:

   GameLayer * GameLayer::create (int level, int levelsCompleted) {
       if (!_instance) {
           _instance = new GameLayer();
       } else {
           _instance->clearLayer();
       }
       _instance->setLevelsCompleted(levelsCompleted);
       _instance->loadLevel(level);
       _instance-...