Custom Segue Class UIView Animation Issue - ios

Question

I am having trouble creating a custom segue using swift 3 and iOS 8. I am trying to transition between view controllers by fading from one VC to a black screen and then fading from black to my second VC. I tried to implement the segue by creating a custom segue using the code below, but it is not working as I would like it to. My goal is to perform an animation when the black square goes from 0.5 alpha to 1.0 alpha, then present my second view controller, then set the black square from 1.0 alpha back to 0.5 alpha and delete it. Right now, it does the first part correctly but after the animation finishes you can see the first VC for a brief instant before the second VC pops up. How should I change my code to make the transition smoother and get the desired result?
override func perform() {
let sourceVC = self.source
let destinationVC = self.destination
let square = UIView(frame: CGRect(x: 0, y: 0, width: UIScreen.main.bounds.width, height: UIScreen.main.bounds.height))
square.alpha = 0.5
square.backgroundColor = UIColor.black
sourceVC.view.addSubview(square)
UIView.animate(withDuration: 0.2, animations: {
square.alpha = 1.0
})
UIView.animate(withDuration: 0.2, delay: 0.2, animations: {
square.alpha = 0.5
}) { (finished) in
sourceVC.present(destinationVC, animated: false, completion: nil)
}
}

Answer 1

Update your code like below and try:
UIView.animate(withDuration: 0.2, animations: {
square.alpha = 1.0
UIView.animate(withDuration: 0.2, delay: 0.2, animations: {
square.alpha = 0.5
}) { (finished) in
DispatchQueue.main.async {
sourceVC.present(destinationVC, animated: false, completion: nil)
}
})
}