Swift: How to refresh UICollectionView layout after rotation of the device

I used UICollectionView (flowlayout) to build a simple layout. the width for each cell is set to the width of screen using self.view.frame.width

but when I rotate the device, the cells don't get updated.

I have found a function, which is called upon orientation change :

override func willRotateToInterfaceOrientation(toInterfaceOrientation: 
  UIInterfaceOrientation, duration: NSTimeInterval) {
    //code
}

but I am unable to find a way to update the UICollectionView layout

The main code is here:

class ViewController: UIViewController , UICollectionViewDelegate , UICollectionViewDataSource , UICollectionViewDelegateFlowLayout{

    @IBOutlet weak var myCollection: UICollectionView!

    var numOfItemsInSecOne: Int!
    override func viewDidLoad() {
        super.viewDidLoad()

        numOfItemsInSecOne = 8
        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    override func willRotateToInterfaceOrientation(toInterfaceOrientation: UIInterfaceOrientation, duration: NSTimeInterval) {

        //print("orientation Changed")
    }

    func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int {
        return 1
    }

    func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return numOfItemsInSecOne
    }

    func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCellWithReuseIdentifier("cellO", forIndexPath: indexPath)

        return cell
    }

    func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize{
    let itemSize = CGSize(width: self.view.frame.width, height: 100)
    return itemSize
    }}

Add this function:

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews() 
    myCollection.collectionViewLayout.invalidateLayout()
}

When you change the orientation, this function would be called.