AppDelegate UIViewController Transition Animation

UIViewController *nextViewController = [[UIViewController alloc] init];
UIView *screenShot = [self.window snapshotViewAfterScreenUpdates:YES];
[nextViewController.view addSubview:screenShot];
self.window.rootViewController = viewController;
[UIView animateWithDuration:0.5 animations:^{
    screenShot.layer.opacity = 0.0;
} completion:^(BOOL finished) {
    [screenShot removeFromSuperview];
}];

for uiviewcontroller transitions in appdelegate