Cocos2d - Orbiting/spinning one sprite around another

Question

I've got to sprites, essentially a nucleus (parent) and an electron (child). I'm trying to find a way to have the electron orbit the nucleus. I've found a few posts here and there on moving anchor points, but that is apparently related to the texture of the sprite, not the sprite itself.

This is my current init method for my parent sprite:

self = [super init];
if (self) {

    CCSprite *orbitAnchor = [CCSprite new];
    [self addChild:orbitAnchor];
    orbitAnchor.position = ccp(32,32);

    CCSprite *orbiter = [CCSprite spriteWithFile:@"line.png" rect:CGRectMake(0, 0, 8, 8)];
    [orbitAnchor addChild:orbiter];
    orbiter.position = ccp(40,40);

    CCAction *orbitAction = [CCRepeatForever actionWithAction:[CCRotateTo actionWithDuration:1 angle:720]];
    [orbitAnchor runAction:orbitAction];

    [self initAnimations];
}

return self;

Those numbers are all totally arbitrary though - I just stuck them in there and got what looked best. I'm sure there's a more programatic way to do what I want.

Basically, I'm looking for a way to set the child's axis point at the center of the parent, and then have it rotate around that point. It seems like a relatively simple thing to do using CCRotate and such, but I think I'm missing what to search for in order to move the anchor point of the child. Any suggestions/other posts?

Thanks

LearnCocos2D LearnCocos2D · Accepted Answer · 2011-10-27T20:24:43

You have [CCSprite new] which is an unusual use, probably not supported. Unless the orbit anchor node should display a texture, you can just use a CCNode as anchor.

Technically you're doing everything correct from what I can see. You might want to try without the rotate actions and instead change the direction manually in a scheduled update method.

  CCNode *orbitAnchor = [CCNode node];
  [self addChild:orbitAnchor z:0 tag:1];
  orbitAnchor.position = ccp(32,32);

  CCSprite *orbiter = [CCSprite spriteWithFile:@"line.png" rect:CGRectMake(0, 0, 8, 8)];
  [orbitAnchor addChild:orbiter z:0 tag:2];
  orbiter.position = ccp(40,40);

  [self scheduleUpdate];

Update method:

-(void) update:(ccTime)delta
{
  CCNode* orbitAnchor = [self getChildByTag:1];
  orbitAnchor.direction += 5;
}

From the image filename it looks like you're trying to draw a line from the orbitAnchor to the orbiter. You can do that with ccDrawLine:

-(void) draw
{
  CCNode* orbitAnchor = [self getChildByTag:1];
  CCNode* orbiter = [self getChildByTag:2];
  ccDrawLine(orbitAnchor.position, orbiter.position);
}

Cocos2d - Orbiting/spinning one sprite around another

2 Answers