UIView.animateWithDuration not executing with semaphore - animation

Question

I'm attempting to write a function that will flash a view's background color to white then back to it's original color. What I have is:
func flashView(view: UIView) {
var sema = dispatch_semaphore_create(0)
var color = view.backgroundColor
println("Animating")
UIView.animateWithDuration(0.5, animations: { view.backgroundColor = UIColor.whiteColor() }) {
(success: Bool) in
println("First block completion")
UIView.animateWithDuration(0.5, animations: { view.backgroundColor = color }) {
(success: Bool) in
println("Nested block completion")
dispatch_semaphore_signal(sema)
}
}
println("Waiting")
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER)
println("Returning")
}
Now, what I'm experiencing is that the Animating message will display followed by the Waiting. The first animateWithDuration block never gets executed (and needless to say the nested one does not either. However, according to Apple's own documentation the UIView.animateWithDuration occurs on a separate thread. So, my understanding, is that the semaphore should wait, the animateWithDuration should complete on a separate thread, and the function should return (after waiting on the semaphore that's signaled in the nested closure). What am I misunderstanding?
Thanks!

Answer 1

Animation block executes in main thread, and your dispatch_semaphore_wait blocks main thread.
So animation block never executes.
This is one solution.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), { () -> Void in
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER)
println("Returning")
})
other solutions are here:
How do I wait for an asynchronously dispatched block to finish?