Understanding Layer

What is Layer?

• A CCLayer has a size of the whole drawable area, and knows how to draw itself. 
•  It can be semi transparent (having holes and/or partial transparency in some/all places), allowing to see other layers behind it. 
• Layers are the ones defining appearance and behavior, so most of your programming time will be spent coding CCLayer subclasses that do what you need.
• Each Scene contains several Layers
• The CCLayer is where you define event handlers and where user interacts with Cocos2d objects. 
• Events are propagated to layers (from front to back) until some layer catches the event and accepts it. 
• Although some serious apps will require you to define custom CCLayer classes, cocos2d provides a library of useful predefined layers ( a simple menu layer: CCMenu, a color layer: CCColorLayer, a multiplexor between other layers: CCMultiplexLayer, and more ).
• Layers can contain CCSprite objects, CCLabel objects and even other CCLayer objects as children.
• Since layers are subclass of CCNode, they can be transformed manually or by using actions.

• Each layer (CCLayer) can in turn have sprites (CCSprite), labels (CCLabel), and other objects you want to display onscreen. 

Working Example

BackgroundLayer.h 

#import <Foundation/Foundation.h>

#import "cocos2d.h"

@interface BackgroundLayer : CCLayer {

}

@end

BackgroundLayer.m

#import "BackgroundLayer.h"

@implementation BackgroundLayer

-(id)init {

    self = [super init];

    if (self != nil) {

        CCSprite *backgroundImage;

    if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad) {

            // Indicates game is running on iPad

            backgroundImage = [CCSprite

             spriteWithFile:@"background.png"];

            backgroundImage = [CCSprite

             spriteWithFile:@"backgroundiPhone.png"];

}

        CGSize screenSize = [[CCDirector sharedDirector] winSize]; 

        [backgroundImage setPosition:

}

} else {

         CGPointMake(screenSize.width/2, screenSize.height/2)];

        [self addChild:backgroundImage z:0 tag:0];

    }

    return self;

@end