Swift: removeFromSubview happens before animation - ios

Question

I have a little setup where I have an MPMoviePlayerController setup as a video background. I'm trying to get it to have a smooth transition when it updates/changes. Here's my current code. BTW, newBackground.view.alpha is initially 0.
let oldBackground = currentBackground
self.view.insertSubview(newBackground.view, aboveSubview: oldBackground.view)
UIView.animateWithDuration(1.5 as NSTimeInterval, animations: {
newBackground.view.alpha = 1
}, completion: {
finished in
oldBackground.view.removeFromSuperview()
println("The subview should be removed now")
})
When this executes, the oldBackground.view is immediately removed before the newBackground starts to fade in. The println, however, happens after the animation is completed. I don't understand why the removeFromSuperview happens immediately, but the println happens when I expect it to. If I remove oldBackground.view.removeFromSuperview(), the animation fades in and looks fine (but the view obviously hasn't been removed, it's just sitting behind newBackground).
EDIT: Bizarrely enough, it seems to work as expected in the simulator. Running on my iPhone 6 Plus gives me the issue every time. I've uninstalled and re-run it from Xcode and the problem persists.
If anyone has any advice for me, I would be very happy. Thank you.

Answer 1

I guess when the newBackground.view.alpha = 1 is executed during animation it brings the background to visibility and overlays the oldBackground.

Answer 2

When you put the new subview above the old subview, old subview goes to the back. This happens before the animation code starts. Make sure your new subview's alpha is 0 before inserting it. And if you want a fade out animation you should set your oldBackground alpha to 0.
Like this :
let oldBackground = currentBackground
newBackground.view.alpha = 0
self.view.insertSubview(newBackground.view, aboveSubview: oldBackground.view)
UIView.animateWithDuration(1.5 as NSTimeInterval, animations: {
newBackground.view.alpha = 1
oldBackground.view.alpha = 0
}, completion: {
finished in
oldBackground.view.removeFromSuperview()
println("The subview should be removed now")
})