ios - UIPanGestureRecognizer stops working when when panned up, not when panned down -

i'm creating screen dimmer here. i've put black view on screen , using uipangesturerecognizer adjust opacity based on whether user scrolls or down. here's code:

- (ibaction)dimscreen:(uipangesturerecognizer *)sender {     //get translation     cgpoint translation = [sender translationinview:self.view];     cgfloat distance = translation.y;      //add fraction of translation alpha     cgfloat newalpha = self.blackscreen.alpha + distance/self.view.frame.size.height;     //check if alpha more 1 or less 0         if (newalpha>1) {         newalpha = 1;         }else if (newalpha<0.0){         newalpha = 0.0;         }     self.blackscreen.alpha = newalpha;     //reset translation incremental change     [sender settranslation:cgpointzero inview:self.view]; } 

if pan down, opacity goes 1.0, , can still adjust it. if pan till opacity 0, can no longer pan @ all. dimscreen: selector stops getting called. can describe causing issue?

views alpha values of 0 no longer receive touches. in fact, seems below 0.011 alpha view stops receiving touches. so, code can made work changing values in else-if statement:

if (newalpha>1) {         newalpha = 1;     }else if (newalpha <= 0.011){         newalpha = 0.011;     } 

i've seen other people mention value of 0.02 instead of 0.011 found empirically. don't know how "safe" assume either of these numbers fixed , continue work in future, seems work now.