1-setting-item-size.swift

import Foundation
import UIKit

class ItemSizeCollectionViewcontroller : UICollectionViewController
{
    // MARK: Outlets

    @IBOutlet weak var flowLayout: UICollectionViewFlowLayout!
    
    // MARK: Properties
    
    var dataSource = CommonDataSource()
    
    // MARK: UIViewController 
    
    override func viewDidLoad()
    {
        super.viewDidLoad()
        
        let nib = UINib(nibName: "MyCollectionViewCell", bundle: NSBundle.mainBundle())
        
        // Manually set the size to be the same as the collection view width
        // flowLayout.itemSize = CGSize(width: collectionView!.bounds.width, height: 60)
        
        // For completeness the section insets need to be accommodated
        var width = collectionView!.bounds.width - flowLayout.sectionInset.left - flowLayout.sectionInset.right
        flowLayout.itemSize = CGSize(width: width, height: 60)
        
        collectionView?.registerNib(nib, forCellWithReuseIdentifier: "cell")
        collectionView?.dataSource = dataSource
    }
    
    // MARK: Actions
    
    @IBAction func didTapRefresh(sender: UIBarButtonItem)
    {
        collectionView?.reloadData()
    }
}

2-flowlayout-delegate.swift

import Foundation
import UIKit

class FlowLayoutDelegateCollectionViewcontroller : UICollectionViewController, UICollectionViewDelegateFlowLayout
{
    // MARK: Outlets
    
    @IBOutlet weak var flowLayout: UICollectionViewFlowLayout!
    
    // MARK: Properties
    
    var dataSource = CommonDataSource()
    
    // MARK: UIViewController
    
    override func viewDidLoad()
    {
        super.viewDidLoad()
        
        let nib = UINib(nibName: "MyCollectionViewCell", bundle: NSBundle.mainBundle())
        collectionView?.registerNib(nib, forCellWithReuseIdentifier: "cell")
        collectionView?.dataSource = dataSource
    }
    
    // MARK: Actions
    
    @IBAction func didTapRefresh(sender: UIBarButtonItem)
    {
        collectionView?.reloadData()
    }
    
    // MARK: UICollectionViewDelegateFlowLayout
    
    func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize
    {
        var width = collectionView.bounds.width - flowLayout.sectionInset.left - flowLayout.sectionInset.right
        return CGSize(width: width, height:60)
    }
}

2.1-delegate.swift

 // MARK: UICollectionViewDelegateFlowLayout
 
func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize
{
    var width = collectionView.bounds.width - flowLayout.sectionInset.left - flowLayout.sectionInset.right
    return CGSize(width: width, height:60)
}

3-flowlayout-delegate-with-tweaks.swift

import Foundation
import UIKit

class FlowLayoutWithTweaksCollectionViewController : UICollectionViewController, UICollectionViewDelegateFlowLayout
{
    // MARK: Outlets
    
    @IBOutlet weak var flowLayout: UICollectionViewFlowLayout!
    
    // MARK: Properties
    
    var dataSource = CommonDataSource()
    
    // MARK: UIViewController
    
    override func viewDidLoad()
    {
        super.viewDidLoad()
        
        let nib = UINib(nibName: "MyCollectionViewCell", bundle: NSBundle.mainBundle())
        collectionView?.registerNib(nib, forCellWithReuseIdentifier: "cell")
        collectionView?.dataSource = dataSource
    }
    
    override func viewWillTransitionToSize(size: CGSize, withTransitionCoordinator coordinator: UIViewControllerTransitionCoordinator)
    {
        super.viewWillTransitionToSize(size, withTransitionCoordinator: coordinator)
        
        coordinator.animateAlongsideTransition({ (context) -> Void in
            
        }, completion: { (context) -> Void in
            self.flowLayout.invalidateLayout()
        })
    }
    
    // MARK: Actions
    
    @IBAction func didTapRefresh(sender: UIBarButtonItem)
    {
        collectionView?.reloadData()
    }
    
    // MARK: UICollectionViewDelegateFlowLayout
    
    func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize
    {
        var width = collectionView.bounds.width - flowLayout.sectionInset.left - flowLayout.sectionInset.right
        return CGSize(width: width, height:60)
    }
}

3.1-tweaks.swift

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

3.2.1-tweaks.swift

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

3.2.2-tweaks.swift

override func viewWillTransitionToSize(size: CGSize, withTransitionCoordinator coordinator: UIViewControllerTransitionCoordinator)
{
    super.viewWillTransitionToSize(size, withTransitionCoordinator: coordinator)
    
    coordinator.animateAlongsideTransition({ (context) -> Void in
    }, completion: { (context) -> Void in
        self.flowLayout.invalidateLayout()
    })
}

3.3-tweaks.swift

var widthToUse : CGFloat?

override func viewWillTransitionToSize(size: CGSize, withTransitionCoordinator coordinator: UIViewControllerTransitionCoordinator)
{
    super.viewWillTransitionToSize(size, withTransitionCoordinator: coordinator)
    
    widthToUse = size.width
    flowLayout.invalidateLayout()
}

func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize
{
    var collectionViewWidth = collectionView.bounds.width
    if let w = widthToUse
    {
        collectionViewWidth = w
    }
    var width = collectionViewWidth - flowLayout.sectionInset.left - flowLayout.sectionInset.right
    return CGSize(width: width, height:60)
}

4-flowlayout-subclass.swift

class FullWidthCellFlowLayout : UICollectionViewFlowLayout
{
    // MARK: Overrides
    
    override func layoutAttributesForElementsInRect(rect: CGRect) -> [AnyObject]? {
        var attributes = super.layoutAttributesForElementsInRect(rect) as! [UICollectionViewLayoutAttributes]
        updateAttribute(attributes)
        return attributes
    }
    
    override func layoutAttributesForItemAtIndexPath(indexPath: NSIndexPath) -> UICollectionViewLayoutAttributes! {
        var attributes = super.layoutAttributesForItemAtIndexPath(indexPath)
        updateAttribute([attributes])
        return attributes
    }
    
    override func shouldInvalidateLayoutForBoundsChange(newBounds: CGRect) -> Bool {
        if !CGRectEqualToRect(newBounds, self.collectionView!.bounds)
        {
            return true
        }
        return false
    }
    
    // MARK: Private
    
    private func updateAttribute(attributes: [UICollectionViewLayoutAttributes])
    {
        for attr in attributes
        {
            attr.frame.size.width = fullWidth()
            attr.frame.origin.x = 0
        }
    }
    
    func fullWidth() -> CGFloat
    {
        return self.collectionView!.bounds.width - self.sectionInset.left - self.sectionInset.right
    }
}

common-datasource.swift

import Foundation
import UIKit

class CommonDataSource : NSObject, UICollectionViewDataSource
{
    // MARK: UICollectionViewDataSource
    
    func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int
    {
        return 1
    }
    
    func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int
    {
        return 10
    }
    
    func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell
    {
        var cell = collectionView.dequeueReusableCellWithReuseIdentifier("cell", forIndexPath: indexPath) as! MyCollectionViewCell
        
        cell.textLabel.text = " cell: \(indexPath.section) - \(indexPath.item)"
        return cell
    }
}

http://www.matrixprojects.net/p/uicollectionviewcell-dynamic-width