Unbalanced calls when performing a custom segue

问题

I've created a custom segue which is a reversed version of a vertical segue, here is my perform function:

var sourceViewController = self.sourceViewController as UIViewController!
var destinationViewController = self.destinationViewController as UIViewController!

sourceViewController.view.addSubview(destinationViewController.view)
destinationViewController.view.frame = sourceViewController.view.frame
destinationViewController.view.transform = CGAffineTransformMakeTranslation(0, -sourceViewController.view.frame.size.height)
destinationViewController.view.alpha = 1.0

UIView.animateWithDuration(0.5, delay: 0.0, options: UIViewAnimationOptions.CurveEaseOut, animations: { () -> Void in
    destinationViewController.view.transform = CGAffineTransformMakeTranslation(0, 0)
    }) { (finished: Bool) -> Void in
            destinationViewController.view.removeFromSuperview()
            sourceViewController.presentViewController(destinationViewController, animated: false, completion: nil)
    }

When I perform it in my app it works and the animation is exactly what I want but I have this warning in the console:

Unbalanced calls to begin/end appearance transitions for <Custom_Segues.ViewController: 0x7a3f9950>.

I read many posts concerning this problem on Stack Overflow but I didn't find one linked to my situation, does someone know what is the problem? I tried many things on my code and I know the problem is in the two last lines but I don't know what to change.

EDIT/ANSWER:

After reading the answers I found a solution: changing the view then applying the old VC on the new one and do the animation. The code is safe and there is no flash of the old VC at the end of the animation.

var sourceViewController = self.sourceViewController as UIViewController!
var destinationViewController = self.destinationViewController as UIViewController!
var duplicatedSourceView: UIView = sourceViewController.view.snapshotViewAfterScreenUpdates(false) // Screenshot of the old view.

destinationViewController.view.addSubview(duplicatedSourceView) // We add a screenshot of the old view (Bottom) above the new one (Top), it looks like nothing changed.

sourceViewController.presentViewController(destinationViewController, animated: false, completion: {
    destinationViewController.view.addSubview(duplicatedSourceView) // We add the old view (Bottom) above the new one (Top), it looks like nothing changed.

    UIView.animateWithDuration(0.33, delay: 0.0, options: UIViewAnimationOptions.CurveEaseOut, animations: { () -> Void in
        duplicatedSourceView.transform = CGAffineTransformMakeTranslation(0, sourceViewController.view.frame.size.height) // We slide the old view at the bottom of the screen
            }) { (finished: Bool) -> Void in
            duplicatedSourceView.removeFromSuperview()
        }
    })
}

回答1:

This looks runloop timing related.

destinationViewController.view.removeFromSuperview()
sourceViewController.presentViewController(destinationViewController, animated: false, completion: nil)

These two lines should not execute within the same runloop.

destinationViewController.view.removeFromSuperview()

// Force presentViewController() into a different runloop.
let time = dispatch_time(DISPATCH_TIME_NOW, Int64(0.001 * Double(NSEC_PER_SEC)))
dispatch_after(time, dispatch_get_main_queue()) {
    sourceViewController.presentViewController(destinationViewController, animated: false, completion: nil)
}

回答2:

I added this as a comment, but I think it good enough to make a second solution. After looking at How to custom Modal View Controller presenting animation?

The solution converted to swift:

var transition = CATransition()
transition.duration = 1
transition.type = kCATransitionFade
transition.subtype = kCATransitionFromBottom

sourceViewController.view.window?.layer.addAnimation(transition, forKey: kCATransition)
sourceViewController.presentViewController(destinationViewController, animated:false, completion:nil)

You can adjust this to match your needs.

来源：https://stackoverflow.com/questions/25330783/unbalanced-calls-when-performing-a-custom-segue