Question

Look at this sample code (in AppDelegate):

- (void)showOtherView {
if (self.viewController.view == view1) {
    self.viewController.view = view2;
} else {
    self.viewController.view = view1;
}
CATransition *animation = [CATransition animation];
[animation setType:kCATransitionMoveIn];
[animation setSubtype:kCATransitionFromRight];
[animation setDuration:0.5];
[animation setTimingFunction:[CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseInEaseOut]];
[[self.window layer] addAnimation:animation forKey:@"myKey"];

}

This method simply switches view1 to view2. The thing I don’t understand is, that CATransition is declared and added to window.layer after assigning view2 to self.viewController.view. Why does the animation works?

I did set breakpoints and figured out that switching animates after showOtherView method is finished. Why? Just why view2 doesn’t instantly appear on the screen!? (at least for the first time I call this method)

Answer 1

Drawing to the screen is an expensive operation, and Cocoa only does it once per spin of the run loop, at the end. The effects of setting the controller’s view — it getting put on the screen — don’t actually happen until after this method has returned and control passes to the run loop. So whatever configurations you make to the new view will always be shown.