How do I define the size of a CollectionView on rotate

Heres my 2 cents - because your item sizes are static why don't you set the item size on the collectionViewLayout?

It will be quicker to do this rather than expecting the collection view to call its delegate method for every cell.

viewWillLayoutSubviews can be used for detection of rotation on view controllers.invalidateLayout can be used on a collection view layout to force it to prepare the layout again causing it to position the elements with the new sizes in this case.

- (void)viewWillLayoutSubviews;{    [super viewWillLayoutSubviews];    UICollectionViewFlowLayout *flowLayout = (id)self.firstCollectionView.collectionViewLayout;    if (UIInterfaceOrientationIsLandscape(UIApplication.sharedApplication.statusBarOrientation)) {        flowLayout.itemSize = CGSizeMake(170.f, 170.f);    } else {        flowLayout.itemSize = CGSizeMake(192.f, 192.f);    }    [flowLayout invalidateLayout]; //force the elements to get laid out again with the new size}

edit: updated with Swift2.0 example

override func viewWillLayoutSubviews() {  super.viewWillLayoutSubviews()  guard let flowLayout = collectionView.collectionViewLayout as? UICollectionViewFlowLayout else {    return  }  if UIInterfaceOrientationIsLandscape(UIApplication.sharedApplication().statusBarOrientation) {    flowLayout.itemSize = CGSize(width: 170, height: 170)  } else {    flowLayout.itemSize = CGSize(width: 192, height: 192)  }  flowLayout.invalidateLayout()}

With new API since iOS 8 I'm using:

Swift 3:

override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {    super.viewWillTransition(to: size, with: coordinator)    updateCollectionViewLayout(with: size)}private func updateCollectionViewLayout(with size: CGSize) {    if let layout = collectionView.collectionViewLayout as? UICollectionViewFlowLayout {        layout.itemSize = (size.width < size.height) ? itemSizeForPortraitMode : itemSizeForLandscapeMode        layout.invalidateLayout()    }}

Objective-C:

- (void)viewWillTransitionToSize:(CGSize)size withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator {        [super viewWillTransitionToSize:size withTransitionCoordinator:coordinator];        [self updateCollectionViewLayoutWithSize:size];}- (void)updateCollectionViewLayoutWithSize:(CGSize)size {        UICollectionViewFlowLayout *layout = (UICollectionViewFlowLayout *)self.collectionView.collectionViewLayout;        layout.itemSize = (size.width < size.height) ? itemSizeForPortraitMode : itemSizeForLandscapeMode;        [layout invalidateLayout];}

The Verified Answer Is Not Efficient

The Problem - Invalidating the layout in viewWillLayoutSubviews() is heavy work. viewWillLayoutSubviews() gets called multiple times when a ViewController is instantiated.

My Solution (Swift) - Embed your size manipulation within the UICollectionViewDelegateFlowLayout.

// Keep a local property that we will always update with the latest // view size.var updatedSize: CGSize!// Use the UICollectionViewDelegateFlowLayout to set the size of our        // cells.func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize {    // We will set our updateSize value if it's nil.    if updateSize == nil {        // At this point, the correct ViewController frame is set.        self.updateSize = self.view.frame.size    }    // If your collectionView is full screen, you can use the     // frame size to judge whether you're in landscape.    if self.updateSize.width > self.updateSize.height {        return CGSize(width: 170, 170)    } else {        return CGSize(width: 192, 192)    }}// Finally, update the size of the updateSize property, every time // viewWillTransitionToSize is called.  Then performBatchUpdates to// adjust our layout.override func viewWillTransitionToSize(size: CGSize, withTransitionCoordinator coordinator: UIViewControllerTransitionCoordinator) {    super.viewWillTransitionToSize(size, withTransitionCoordinator: coordinator)    self.updateSize = size    self.collectionView!.performBatchUpdates(nil, completion: nil)}