iphone - UIView animation takes over CPU time -

i wonder how can have continuos , repeating animations on views w/o freezing rest of app - since current approaches lead 100% animation, 0% user interaction possible - kind of .. bad.

the animation in general simple: want make uilabel act siren, altering alpha-property 1 0 , , forth.

the basic animation block therefore is:

    [uiview animatewithduration:0.4f                           delay:0.0f                         options:uiviewanimationoptionrepeat | uiviewanimationoptionautoreverse | uiviewanimationoptionbeginfromcurrentstate                      animations:^(void){warningmessage.alpha = 0;}                      completion:null];     

i tried call in background, in backgroundthreads, in async gcd block, each time loose control of app - animation works fine. (ok, apple docs must happen on main thread, tried..)

now again, question: how have both of it? possible in way?

thanks in advance!

p.s.: sry if quite repost, did not find proper solution/question here!

p.p.s: yes, know sirens dont alter alpha propery :p

did try adding uiviewanimationoptionallowuserinteraction options flag?