NSTimer time interval not firing correctly - swift

Question

I have an NSTimer that calls a function. The function flashes a different button each time it is called. i have the time interval set to one second and repeats is true, it stops repeating when the variable pcChoice == 10, but for some reason when i run the program it is delaying for a second and then calling the function a number of times right after each other. To the user, it looks like one second passes, then a number of buttons are flashed at once. What I need instead is for each button to flash one after another, with one second in between each flash.
override func viewDidLoad() {
var lit = [b0o,b1o,b2o,b3o,b4o,b5o,b6o,b7o,b8o]
var litIndex = 0
super.viewDidLoad()
for i in 1...10{
print(randomIndex)
print(computerChoices)
var buttonChoice = lit[randomIndex]
randomIndex = Int(arc4random_uniform(UInt32(lit.count)))
computerChoices[i] = randomIndex
print("yoyoyo")
self.view.setNeedsDisplay()
timer = NSTimer.scheduledTimerWithTimeInterval(1, target: self, selector: Selector("flashingButtons"), userInfo: nil, repeats: true)
}
}
func flashingButtons(){
var lit = [b0o,b1o,b2o,b3o,b4o,b5o,b6o,b7o,b8o]
one = computerChoices[pcChoice]
lit[one].setImage(UIImage(named: "redb.png"), forState: UIControlState.Normal)
pcChoice += 1
if pcChoice == 10{
timer.invalidate()
}
}

Answer 1

This line
timer = NSTimer.scheduledTimerWithTimeInterval(1, target: self, selector: Selector("flashingButtons"), userInfo: nil, repeats: true)
is executed 10 times because of your for loop. That gives you 10 different timers, each of which will invoke flashingButtons in about 1 second from now. Move that call out of the loop. You also don't need 10 calls to setNeedsDisplay in viewDidLoad.
Since you don't modify lit, make it a let instead of a var. I notice also that you have 9 elements in lit but you're looping through 10 times. The lit array in viewDidLoad is never used, and is different from the one in flashingButtons.
You could invoke arc4random_uniform inside flashingButtons each time the time fires. Or if you want the lights truly shuffled, each one being hit once, try something like this using GameplayKit:
let lit = [b0o,b1o,b2o,b3o,b4o,b5o,b6o,b7o,b8o]
let shuffledLit : [<lightobjects>] // whatever type of b0o, etc is
override func viewDidLoad() {
timer = NSTimer.scheduledTimerWithTimeInterval(1, target: self, selector: Selector("flashingButtons"), userInfo: nil, repeats: true)
shuffledLit = GKRandomSource.sharedRandom().arrayByShufflingObjectsInArray(lit)
super.viewDidLoad()
self.view.setNeedsDisplay()
}