ChildViewController.view.frame changes during screen rotation

class ViewController:UIViewController {

letchildViewController: UIViewController= UIViewController()

override func viewDidLoad(){
    super.viewDidLoad()

    childViewController.view.backgroundColor=UIColor.greenColor()
    self.addChildViewController (childViewController)
    childViewController.didMoveToParentViewController(self)
    self.view.addSubview(childViewController.view)
    childViewController.view.frame = CGRect (x:0, y:0, width:300, height:300)

    self.addObserver(self, forKeyPath: "childViewController.view.frame", options: .New, context:nil)
}

override func viewWillTransitionToSize (size: CGSize, with TransitionCoordinator: UIViewControllerTransitionCoordinator) {
    super.viewWillTransitionToSize(size, withTransitionCoordinator:coordinator)
    childViewController.view.frame = CGRect (x:0, y:0, width:300, height:300)
}

override funcoveValueForKeyPath(keyPath:String?, ofObject object:AnyObject?,change:[String:AnyObject]?,context:UnsafeMutablePointer<Void>){
    print("\(childViewController.view.frame)")
}
}

 (0.0, 0.0, 300.0, 300.0)
(0.0, 0.0, 0.0, 622.0)

childViewController.view.frame = CGRect (x:0, y:0, width:300, height:300)