can't offset ccnode to let me center my main character - iphone

Question

I have a standard cocos2d startup layer( HelloWorldLayer). I created another class of type CCNode named "Terrain" for my terrain. Then i add it to my layer in the layer's init:
terrain = [[Terrain alloc] initWithWorld:world AndLevel:0];
[self addChild:terrain z:1];
i add a 'CarObject' class (a CCSprite class), and add a car object to my terrain
car = [[CarObject alloc] initWithWorld:world];
[terrain addChild:car];
-i.e. in both the initWithWorld for terrain and car, i initialize some Box2d code
I then try to center my car object to my screen when i move it, i do this in my update method:
float offsetX = car.position.x;
float offsetY = car.position.y;
[terrain setOffsetX:(int)offsetX andOffsetY:(int)offsetY];
where the setOffsetX.. method is:
- (void) setOffsetX:(int)newOffsetX andOffsetY:(int)newOffsetY {
_offsetX = newOffsetX;
_offsetY = newOffsetY;
CGSize winSize = [CCDirector sharedDirector].winSize;
self.position = CGPointMake(-(_offsetX - winSize.width/2), -(_offsetY - winSize.height/2));
}
When i use a NSLog to see if the terrain position changes, i can see that the position actually chages, but the view does not. What am i doing wrong? am sure it's a dumb mistake!
btw, if i try this in my HelloWorldLayer's update method (instead of [terrain setOffsetX..])
self.position = CGPointMake(self.position.x-1, self.position.y);
the terrain is moving.

Answer 1

Car is a child of Terrain. Car's position is therefore relative to Terrain's position. Since you base Terrain's position on Car's position, which is actually relative to Terrain's position, you may be simply running into the effect that your position updates simply cancel each other out.
If you want to move the Terrain while keeping the Car centered, you shouldn't add the Car as a child of Terrain. Instead add it to the same node as the Terrain (HelloWorldLayer). Then you can move the Car and Terrain independently of each other.