Animation not starting when part of CAAnimationGroup

I have the following code which adds 'flying dots' to a view.

When I return 'pathAnimation', then the animation works as expected. When I add it to a CAAnimationGroup however, then the animation doesn't run. What am I doing wrong here?

- (CAAnimation*)animationForPath:(CGPathRef)thePath 
{
    // THIS CODE WORKS ALONE - BUT NOT AS PART OF A GROUP
    CAKeyframeAnimation 开发者_如何学C*pathAnimation = [CAKeyframeAnimation animation];
    pathAnimation.path = thePath;
    pathAnimation.duration = 3;  // two seconds
    pathAnimation.repeatCount = 10000;  // "forever"
    return pathAnimation;

//    THIS CODE DOESN'T WORK   
//    CAAnimationGroup *animationGroup = [CAAnimationGroup animation];
//    animationGroup.duration = 3.0;
//    animationGroup.repeatCount = 10000;
//    animationGroup.animations = [NSArray arrayWithObjects:pathAnimation, nil];
//    return animationGroup;
}

- (void) animateLayer:(CALayer*)layer fromXOffset:(CGFloat)x_offset timeOffset:(CGFloat)timeOffset
{
    CGMutablePathRef path = CGPathCreateMutable();
    CGPathMoveToPoint(path, NULL, x_offset, 0);
    CGPathAddLineToPoint(path, NULL, x_offset, 1000);

    CAAnimation* animation = [self animationForPath:path];
    animation.timeOffset = timeOffset;

    [layer addAnimation:animation forKey:@"position"];

    CGPathRelease(path);
}

- (void) addDotsToView:(UIView*)view
{
    int numDots = 10;
    CGFloat spacing = view.bounds.size.width / (numDots * 1.0f);

    for(int i = 0; i < 10; ++i) 
    {        
        for(CGFloat offset = 10; offset < view.bounds.size.width; offset += spacing) 
        {
            layer_ = [CALayer layer];
            [layer_ setFrame:CGRectMake(0, 0, 10, 10)];
            [layer_ setBackgroundColor:[[UIColor whiteColor] CGColor]];
            [view.layer addSublayer:layer_];
            [self animateLayer:layer_ fromXOffset:offset timeOffset:i * 0.5f];
        }
    }
}

Answered in another forum:

CAKeyframeAnimation *pathAnimation = [CAKeyframeAnimation animation];

should be

CAKeyframeAnimation *pathAnimation = [CAKeyframeAnimation animationWithKeyPath:@"opacity"];