Images don't update when loaded until end - swift

Question

I am trying to write a routine to roll a die. I want the face to change several times before landing on the final number. The code below does not show the die changing faces. I added the sleep statement in hopes that it would give it time to update but it just stays with the initial face until the end and then shows the final face. Is there a statement to add after changing the texture to force the view to update?
func rollDice() {
var x: Int
for var i = 0; i < 50; i++
{
x = GKRandomSource.sharedRandom().nextIntWithUpperBound(6)
let texture = dice[0].faces[x]
let action:SKAction = SKAction.setTexture(texture)
pieces[3].runAction(action)
pieces[3].texture = dice[0].faces[x]
NSThread.sleepForTimeInterval(0.1)
print(x)
}
}

Answer 1

As has been pointed out in the comments, the screen only redraws on the main thread. Therefore, you could let the dice rolls take place on a background thread, and redraw the screen on the main thread:
func rollDice() {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0)) {
for i in 0..<50 {
let x = GKRandomSource.sharedRandom().nextIntWithUpperBound(6)
dispatch_async(dispatch_get_main_queue) {
let texture = dice[0].faces[x]
let action:SKAction = SKAction.setTexture(texture)
pieces[3].runAction(action)
pieces[3].texture = dice[0].faces[x]
NSThread.sleepForTimeInterval(0.1)
print(x)
}
}
}
}

Answer 2

Thank you for the help. After reading some about Timers, I came up with the following code:
func rollDice() {
rollCount = 0
timer = NSTimer.scheduledTimerWithTimeInterval(0.05, target:self, selector: "changeDie", userInfo: nil, repeats: true)
}
func changeDie () {
x = GKRandomSource.sharedRandom().nextIntWithUpperBound(6)
print(x)
let texture = dice[0].faces[x]
let action:SKAction = SKAction.setTexture(texture)
pieces[3].runAction(action)
++rollCount
if rollCount == 20 {
timer.invalidate()
}
}