ios - UIAlertViewController flickers/blinks briefly -

i'm using swift code display alert dialog uiviewcontroller

import uikit  class testviewcontroller: uiviewcontroller {     @ibaction func ontestclick(sender: uibutton) {         let alertcontroller = uialertcontroller(title: "title", message: "message", preferredstyle: .alert)         alertcontroller.addaction(uialertaction(title: "ok", style: .default, handler: nil))          self.presentviewcontroller(alertcontroller, animated: true, completion: nil)     } } 

when alert shown view dark (e.g. black) background, briefly "flickers" (appears white , instantly change color darker white).

this doesn't seem happen on other apps , system os.

what's problem here?

i able reproduce saying , don't think doing wrong. if turn slow animations on simulator (cmd + t) see alert controller being positioned in center of screen in fade animation. when animation in progress alert controller has white background (and apple's code, it's fine) , when animation finishes alert controller has transparent background change according colour of view behind it. don't notice when view white because white + white = white! :)

i hope helps.