How to display a UIView when it overlaps another iOS? -

i trying re-implement similar apple watch friend picker. have 2 uiviews, both corner radius of half height appear circles. how calculate overlap , show part. ui in first picture , second 1 contains desired behavior. in advance help?

for each of circles initials, can have top view--the unselected state--and bottom view--the selected state. have movable view in shape of circle tracks tip of point. when view intersects of outer circle create mask equal rounded bounds of invisible view.

for example:

if cgrectintersectsrect(self.movableview.frame, self.topview.frame)         {             self.topview.layer.mask = nil             let shapelayer = cashapelayer()             let rect = self.topview.convertrect(self.movableview.bounds, fromview: self.movableview)             let path = uibezierpath(ovalinrect: rect)             let maskpath = cgpathcreatemutable()             cgpathaddrect(maskpath, nil, self.view.frame)             cgpathaddpath(maskpath, nil, path.cgpath)             shapelayer.path = maskpath             shapelayer.fillrule = kcafillruleevenodd             self.topview.layer.mask = shapelayer         } 

to avoid excessive hit detection should keep track of selected view , check intersection view preceding selected view, selected view, , view following selected view. i've created rudimentary example of masking if you're interested.