Why does NSLog show screen width and screen height as 0? - ios

Question

So I wrote the following piece of code, which displays the red square in the middle, as I intended to be. However, when I "NSLog"ed the output of the screen size of the device onto the console, I found that the value is 0. Why is that?
let container = UIView()
let redSquare = UIView()
let blueSquare = UIView()
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
let screenSize : CGSize = UIScreen.mainScreen().applicationFrame.size
let screenWidth = screenSize.width
let screenHeight = screenSize.height
NSLog("screenWidth = %f, screenHeight = %f", screenWidth, screenHeight);
self.container.frame = CGRect(x: 50, y: 50, width: screenWidth-100, height: screenWidth-100)
self.view.addSubview(container)
// set red square frame up
// we want the blue square to have the same position as redSquare
// so lets just reuse blueSquare.frame
self.redSquare.frame = CGRect(x: 0, y: 0, width: screenWidth-100, height: screenWidth-100)
self.blueSquare.frame = redSquare.frame
// set background colors
self.redSquare.backgroundColor = UIColor.redColor()
self.blueSquare.backgroundColor = UIColor.blueColor()
// for now just add the redSquare
// we'll add blueSquare as part of the transition animation
self.container.addSubview(self.redSquare)
}