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

Question

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     }} 

Answer 1

Add this function:

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

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

Answer 2

The better option is to call invalidateLayout() instead of reloadData() because it will not force recreation of the cells, so performance will be slightly better:

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