Strange behavior animating a UIView using [UIView animateKeyframesWithDuration] - ios

Question

I'm trying to create an effect where three arrows go from visible to invisible alternatively.
I made a simple algorithm where every arrow would start from a different alpha value (if there is 3 arrows, the first one would start at alpha=1, the second one at alpha=0.6667, the third one at alpha=0.3337). I then start a key frame animation that:
Change the arrow opacity from its current alpha to 0 (the duration is computed)
Set instantly the arrow opacity to 1
Change the arrow opacity from 1 to the first value set
However it seems that some step are skipped for some reason.
A simple example:
[UIView animateKeyframesWithDuration:2 delay:0 options:UIViewKeyframeAnimationOptionCalculationModeLinear | UIViewKeyframeAnimationOptionRepeat animations:^{
[UIView addKeyframeWithRelativeStartTime:0 relativeDuration:0 animations:^{
_animatedView.alpha = 0.5;
}];
[UIView addKeyframeWithRelativeStartTime:0 relativeDuration:0.5 animations:^{
_animatedView.alpha = 0;
}];
[UIView addKeyframeWithRelativeStartTime:0.5 relativeDuration:0 animations:^{
_animatedView.alpha = 1;
}];
[UIView addKeyframeWithRelativeStartTime:0.5 relativeDuration:0.5 animations:^{
_animatedView.alpha = 0.5;
}];
} completion:nil];
In that case it should go to 0.5 instantly, 0.5 to 0 in 1 second, go to 1 instantly, 1 to 0.5 in 1 second. It should therefore do a seamless transition that looks like the view is appearing and disappearing but it looks like the animation gets stuck on alpha=0.5 for a few time.
Theoretically, the effect should be the same as if using this key frame animation:
[UIView animateKeyframesWithDuration:2 delay:0 options:UIViewKeyframeAnimationOptionCalculationModeLinear | UIViewKeyframeAnimationOptionRepeat animations:^{
[UIView addKeyframeWithRelativeStartTime:0 relativeDuration:0 animations:^{
_animatedView.alpha = 1;
}];
[UIView addKeyframeWithRelativeStartTime:0 relativeDuration:1 animations:^{
_animatedView.alpha = 0;
}];
} completion:nil];

Answer 1

In case you want to animate N views in the same way:
CGFloat count = [self.animatedViews count];
CGFloat period = 1.0f / count;
__weak NSArray *weakViews = self.animatedViews;
[UIView animateKeyframesWithDuration:2.0f delay:0 options:UIViewKeyframeAnimationOptionCalculationModeLinear | UIViewKeyframeAnimationOptionRepeat animations:^{
for (NSUInteger index = 0; index < count; ++index) {
UIView *animatedView = weakViews[index];
CGFloat startDelay = period * index;
[UIView addKeyframeWithRelativeStartTime:0 relativeDuration:0 animations:^{
animatedView.alpha = startDelay;
}];
[UIView addKeyframeWithRelativeStartTime:0.0f relativeDuration:startDelay animations:^{
animatedView.alpha = 0.0f;
}];
[UIView addKeyframeWithRelativeStartTime:startDelay relativeDuration:0.0f animations:^{
animatedView.alpha = 1.0f;
}];
[UIView addKeyframeWithRelativeStartTime:startDelay relativeDuration:(1.0f - startDelay) animations:^{
animatedView.alpha = startDelay;
}];
}
} completion:nil];