Scenes in Cocos2D

https://www.makeschool.com/docs/?source=mgwu#!/cocos2d/1.0/scenes

Scenes in Cocos2D

In Cocos2D you have to have a CCScene as the root node of your scene graph. CCScene is a subclass of CCNode. Any CCNode subclass can be added to a scene by using the addChild: method. By default any CCScene is initialized as a full screen node. Only one CCScene can be active at a time. The currenty active scene is considered on the stage.

Scene lifecycle

Cocos2D provides multiple lifecyle events that can be relevant for your scenes and other nodes in your scene. These events are trigerred when a scene gets initialized and when it becomes active/inactive. Each of the scene transition events is represented by one method in CCNode that gets called by the CCDirector. You can override these methods in any CCNode. The Objective-C default initializer method init gets called when scene/node is initialized from Cocos2D code. When you created your scene/node in SpriteBuilder didLoadFromCCB is the designated initializer method. Here is an overview of all the lifecycle methods:

• init: The default Obj-C initializer. If the scene is not created in SpriteBuilder you mostly use this method to build the scene by adding different nodes as children.

• didLoadFromCCB: If you created your scene in SpriteBuilder this method is called when the complete scene is loaded and all code connections are set up. You implement this method to access and manipulate the content of the scene. You cannot access child nodes of the scene or code connection variables before this method is called.

• onEnter: This method gets called as soon as the replacement of the currently active scene with this scene begins. If you are using an animated scene transition this method will be called at the beginning of the transition. You can add any code that shall be executed when the scene starts to enter the stage.

• onEnterTransitionDidFinish: If you are using an animated scene transition this method will be called when the transition finishes. If you are not using transitions this method is called at the same time as onEnter. Implement this method to execute code once the scene has fully entered the stage.

• onExit: Is called when the scene leaves the stage. If the scene leaves the stage with a transition, this event is called when the transition finishes.

• onExitTransitionDidStart: Is called when the scene leaves the stage. If the scene leaves the stage with a transition, this event is called when the transition starts.

Note: you have to call the super implementation when you override onEnter, onEnterTransitionDidFinish, onExit or onExitTransitionDidStart.

Scene Graph and Scene Management

Each CCScene stores a hierarchy of different CCNode instances. CCScene itself is an invisible node that is only used as a container. Most CCNodesubclasses are used to present an object on the screen (a single color surface or a texture). The hierarchical structure of CCNode instances is called the scene graph.

The following diagram shows multiple different scenes and scene graphs of a game: 

The diagram represents three different scenes of which each contains multiple nodes. There are a couple of takeaways from this diagram:

• CCDirector is the instance that controls which scene gets presented on the stage. One can tell CCDirector to present the main menu, the gameplay scene or the highscore scene. CCDirector will only present one scene at a time.
• CCScenes are used to build a logical group of objects. In Cocos2D this always means one scene represents one screen. One will have scenes for menus, leaderboards, gameplay, etc. Scenes itself don't have a representation, their sole goal is to group CCNode instances.
• CCNodes Basically anything that is visible in Cocos2D is a subclass of CCNode.

Games in Cocos2D consist of different scenes. The flow of the game is defined by telling the CCDirector when which scene shall be presented (menu first, then gameplay, then leaderboard). The content of the scenes is a compilation of different nodes. A CCNode can again contain other CCNode instances. The hierarchy of CCNode instances in a scene is called the scene graph.

Creating a Scene

The default SpriteBuilder and Cocos2D projects come with one scene. For most games you will have to create additional scenes. You can either create scenes in SpriteBuilder or in code.

• Create a Scene CCB-File in SpriteBuilder and load the scene in code utilizing CCBReader: 

    CCScene *gameplayScene = [CCBReader loadAsScene:@"Gameplay"];
  

• Create a subclass of CCScene in Xcode and intialize: 

    CCScene *gameplayScene = * [GameplayScene scene];
  

When to create the Scene content

For most games you will create the content for your scenes in SpriteBuilder or in the init method of your CCScene subclass. However, when certain scenes need to allocate a lot of memory it can be helpful to create the actual scene content in the onEnter method and removing it when onExit is called. This way the memory is only used when the scene is active on the stage.

Switching between Scenes

The CCDirector is responsible for presenting and hiding scenes. The easiest way to replace a scene, without a transition is using the replaceScene: method:

[[CCDirector sharedDirector] replaceScene:gameplayScene];

Using this method the CCDirector will replace the currently active scene with the gameplayScene.

Transitioned Scene Changes

If you want to provide a transition effect for swapping the scene you can use the replaceScene:withTransition:. It takes a CCTransition as parameter. A CCTransition can be easily created using one of the initializer methods that provide pre defined transition effects:

CCTransition *transition = [CCTransition transitionCrossFadeWithDuration:1.f];
[[CCDirector sharedDirector] replaceScene:gameplayScene withTransition:transition];

To get an overview of the different available transitions and the transition options, take a look at the Cocos2D class reference for CCTransition.

Using the Scene Navigation stack

Another option instead of replacing scenes is pushing and popping scenes. When you push a new scene, the old scene gets deactivated but remains in a the stack of scenes. When you pop a scene that scene gets removed from the navigation stack and the top most controller of the navigation stack gets presented.

Cocos2D provides methods to pop and push scenes with or without transitions:

• pushScene:withTransition:
• popScene:withTransition:
• pushScene:
• popScene

Additionally there is a popToRootScene method that empties the complete navigation stack and returns to the first scene.

When building User Interfaces with multiple scenes (e.g. menu structures) you should use the navigation stack instead of manually replacing scenes. The most common pattern is to provide a back button that pops the latest scene on the navigation stack.