basicly you make your standard tappable overlay - the arrow
And you create any other overlay which is bigger than the arrow - and you will put it on higher layer than the arrow. And you will keep opacity of this overlay on 0 - so it will be transparent.
But because it is on higher layer it will not alow reader to tapp the tappable arrow.
And in the moment you want to alow the tapp - when the arrow points on right number you will shfit the transparent overlay to side so it will no more cover the tappable overlay and it will therefore be possibe to tap on it. And can then again shift it back on the place to cover the arrow when it again shifts to another number