iOS UICollectionView Photo Gallery Orientation issue

I'm currently working on implementing a horizontally scrollable photo gallery feature. For this, I'm using UICollectionView with the flow layout set to horizontal. The problem that I'm currently facing is with the orientation. Lets say, I launched this view in landscape and i scrolled across couple of photos and now if change orientation to portrait, i'm loosing the position where i was before in the landscape mode. i want to make sure i show the same photo centered vertically that i have before changing the orientation. please find the included source code below.

class ViewController: UIViewController, UICollectionViewDataSource,  UICollectionViewDelegate, UICollectionViewDelegateFlowLayout {

var collectionView: UICollectionView!
private var samplePictures:[UIImage] = []
let CELL_IDENTIFIER = "photo_cell"
var currentIndex = 0
var flowLayout: UICollectionViewFlowLayout!


override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.

    samplePictures = [UIImage(named: "Photo-1.png")!, UIImage(named: "Photo-2.png")!, UIImage(named: "Photo-3.png")!, UIImage(named: "Photo-4.png")!, UIImage(named: "Photo-5.png")!, UIImage(named: "Photo-6.png")!]

    flowLayout = UICollectionViewFlowLayout()
    flowLayout.scrollDirection = UICollectionViewScrollDirection.Horizontal
    flowLayout.minimumInteritemSpacing = 0.0
    flowLayout.minimumLineSpacing = 0.0
    flowLayout.sectionInset = UIEdgeInsetsMake(0.0, 0.0, 0.0, 0.0)

    collectionView = UICollectionView(frame: self.view.bounds, collectionViewLayout: flowLayout)
    collectionView.delegate = self
    collectionView.dataSource = self
    collectionView.showsHorizontalScrollIndicator = false
    collectionView.showsVerticalScrollIndicator = false
    collectionView.autoresizingMask = UIViewAutoresizing.FlexibleHeight | UIViewAutoresizing.FlexibleWidth
    collectionView.pagingEnabled = true
    collectionView.registerClass(SampleCollectionCell.self, forCellWithReuseIdentifier: CELL_IDENTIFIER)

    self.view.addSubview(collectionView)
}

override func viewWillAppear(animated: Bool) {
    super.viewWillAppear(animated)
    collectionView.reloadData()

}

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

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

func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return samplePictures.count
}

func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {

    var cell = collectionView.dequeueReusableCellWithReuseIdentifier(
    CELL_IDENTIFIER, forIndexPath: indexPath) as SampleCollectionCell

    cell.backgroundColor = UIColor.brownColor()
    cell.imageView_sample.image = samplePictures[indexPath.item]

    return cell
}

func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize {
    return self.view.bounds.size
}


override func viewWillTransitionToSize(size: CGSize, withTransitionCoordinator coordinator: UIViewControllerTransitionCoordinator) {
    super.viewWillTransitionToSize(size, withTransitionCoordinator: coordinator)
    collectionView.collectionViewLayout.invalidateLayout()
}

//supports all orientations
override func supportedInterfaceOrientations() -> Int {
    return Int(UIInterfaceOrientationMask.All.rawValue)
}

//view will rotate when the interface orientation happens
override func shouldAutorotate() -> Bool {
    return true
}
}

This is the source code for the CustomCollectionCell that I'm using

class SampleCollectionCell: UICollectionViewCell {
var imageView_sample: UIImageView!

override init(frame: CGRect) {
    super.init(frame: frame)

    imageView_sample = UIImageView(frame: self.bounds)
    imageView_sample.contentMode = UIViewContentMode.ScaleAspectFit
    self.contentView.addSubview(imageView_sample)

}

required init(coder aDecoder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
}

override func layoutSubviews() {
    super.layoutSubviews()
    self.imageView_surveillance.frame = self.bounds
}
}

Any help is truly appreciated. The core idea behind using this is to mimic the Photo app and also update view components when i scroll a picture. Please suggest if you think this can be done in a better way.

Regards.