i want my falling sprites to dissapear half way down the screen - iphone

Question

im new to programming, i have been experimenting with cocos2d, heres the problem, i have made a simple game, device in portrait, it has falling sprites, and i want the sprite to disappear when the position of the top of the sprite < screen.height/2 how can this be done?
heres some code you may be interested in:
this is the falling sprite, it falls from the top of the screen to the bottom
-(void)addRock {
CCSprite *rock = [CCSprite spriteWithFile:#"rock.png"
rect:CGRectMake(0, 0, 27, 40)];
// Determine where to spawn the target along the X axis
CGSize winSize = [[CCDirector sharedDirector] winSize];
int minX = rock.contentSize.width/2;
int maxX = winSize.width - rock.contentSize.width/2;
int rangeX = maxX - minX;
int actualX = (arc4random() % rangeX) + minX;
// Create the target slightly off-screen along the right edge,
// and along a random position along the X axis as calculated above
rock.position = ccp(actualX, 500);
[self addChild:rock];
// Determine speed of the target
int actualDuration = spriteDuration;//speed of sprite
// Create the actions
id actionMove = [CCMoveTo actionWithDuration:actualDuration position:ccp(actualX,-winSize.height+ rock.contentSize.height)];
id actionMoveDone = [CCCallFuncN actionWithTarget:self selector:#selector(spriteMoveFinished:)];
[rock runAction:[CCSequence actions:actionMove, nil]];
}
when the sprite move has finished
-(void)spriteMoveFinished:(id)sender {
CCSprite *sprite = (CCSprite *)sender;
[self removeChild:sprite cleanup:YES];
}

Answer 1

To fade your sprite out halfway of the Animation you can use CCSpawn to combine the falling animation with the fadeOut. But as you want it to fade out after a given time, you need to create a Sequence of CCDelayTime and CCFadeOut.
double fadeStart = 0.5;
id wait = [CCDelayTime actionWithDuration: actualDuration * fadeStart];
id fadeOut = [CCFadeOut actionWithDuration: actualDuration * (1.0 - fadeStart)];
id waitThenFade = [CCSequence actions: wait, fadeOut, nil];
id actionMove = ...;
id fullAnimation = [CCSpawn actions:actionMove, waitThenFade, nil];
[rock runAction:[CCSequence actions:fullAnimation, actionMoveDone, nil]];
In case you want the fading to start earlier or later you simply have to move the fadeStart value. 0 will start immediately, 1 will actually not fade, so better pick a value in between.

Answer 2

Maybe something like this?
CGSize size = [[CCDirector sharedDirector] winSize];
CCSprite* img = [[CCSprite alloc] initWithFile:#"yourSprite.png"];
img.position = ccp(size.width/2,size.height + img.boundingBox.size.height/2);
[img runAction:[CCSequence actions:
[CCMoveTo actionWithDuration:3 position:ccp(img.position.x,size.height/2 - img.boundingBox.size.height/2)],
[CCFadeOut actionWithDuration:1],
[CCCallFuncN actionWithTarget:nil selector:#selector(spriteMoveFinished:)],
nil]];