iOS: How to run a function after Device has Rotated (Swift)

The viewWillTransitionToSize delegate method gets called with a UIViewControllerTransitionCoordinator conforming object. A method that protocol declares is animateAlongsideTransition(_:animation, completion:). You can use that to have code execute after the transition is complete.

override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
    super.viewWillTransition(to: size, with: coordinator)
    coordinator.animate(alongsideTransition: nil) { _ in
        // Your code here
    }
}

Although not asked for here Objective C version:

- (void)viewWillTransitionToSize:(CGSize)size withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator {
[coordinator animateAlongsideTransition:^(id<UIViewControllerTransitionCoordinatorContext>  _Nonnull context) {

    // change any properties on your views

} completion:^(id<UIViewControllerTransitionCoordinatorContext>  _Nonnull context) {
    UIDeviceOrientation orientation = [UIDevice currentDevice].orientation;
    if( UIDeviceOrientationIsPortrait(orientation) ) {
        NSLog(@"portrait");
    } else {
        NSLog(@"landscape");
    }  
}];
}