How to change UICollectionViewCell size programmatically in Swift?

Make sure you conform to UICollectionViewDelegateFlowLayout for your class(for swift4)

eg. class MyClass: UICollectionViewDelegateFlowLayout

Prior to Swift 3

//Use for sizefunc collectionView(collectionView: UICollectionView,        layout collectionViewLayout: UICollectionViewLayout,        sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize {    }//Use for interspacing    func collectionView(collectionView: UICollectionView,        layout collectionViewLayout: UICollectionViewLayout,        minimumInteritemSpacingForSectionAtIndex section: Int) -> CGFloat {            return 1.0    }    func collectionView(collectionView: UICollectionView, layout        collectionViewLayout: UICollectionViewLayout,        minimumLineSpacingForSectionAtIndex section: Int) -> CGFloat {            return 1.0    }

// For swift 3

func collectionView(_ collectionView: UICollectionView,                    layout collectionViewLayout: UICollectionViewLayout,                    sizeForItemAt indexPath: IndexPath) -> CGSize {}func collectionView(_ collectionView: UICollectionView,                    layout collectionViewLayout: UICollectionViewLayout,                    minimumInteritemSpacingForSectionAt section: Int) -> CGFloat {    return 1.0}func collectionView(_ collectionView: UICollectionView, layout    collectionViewLayout: UICollectionViewLayout,                    minimumLineSpacingForSectionAt section: Int) -> CGFloat {    return 1.0}

// For swift 4

extension MyClass: UICollectionViewDelegateFlowLayout {         func collectionView(_ collectionView: UICollectionView,                            layout collectionViewLayout: UICollectionViewLayout,                            sizeForItemAt indexPath: IndexPath) -> CGSize {        }        func collectionView(_ collectionView: UICollectionView,                            layout collectionViewLayout: UICollectionViewLayout,                            minimumInteritemSpacingForSectionAt section: Int) -> CGFloat {            return 1.0        }        func collectionView(_ collectionView: UICollectionView, layout            collectionViewLayout: UICollectionViewLayout,                            minimumLineSpacingForSectionAt section: Int) -> CGFloat {            return 1.0        }    }

use the same code as shown above for swift 3 but just make sureyour class conforms to protocol UICollectionViewDelegateFlowLayout

Swift 3

Methods from @sumit-oberoi answer won't be called if you are using Swift 3. To make it work make this adjustments:

1. Conform to the UICollectionViewDelegateFlowLayout protocol.

2. Implement these methods:

   func collectionView(_ collectionView: UICollectionView,                    layout collectionViewLayout: UICollectionViewLayout,                    sizeForItemAt indexPath: IndexPath) -> CGSize {}func collectionView(_ collectionView: UICollectionView,                    layout collectionViewLayout: UICollectionViewLayout,                    minimumInteritemSpacingForSectionAt section: Int) -> CGFloat {    return 1.0}func collectionView(_ collectionView: UICollectionView, layout    collectionViewLayout: UICollectionViewLayout,                    minimumLineSpacingForSectionAt section: Int) -> CGFloat {    return 1.0}

Notice the changes:

1. add _ before collectionView in the method name
2. NSIndexPath changes to IndexPath

Swift 3

override func viewDidLoad() {    super.viewDidLoad()    let cellSize = CGSize(width:80 , height:80)    let layout = UICollectionViewFlowLayout()    layout.scrollDirection = .vertical //.horizontal    layout.itemSize = cellSize    layout.sectionInset = UIEdgeInsets(top: 1, left: 1, bottom: 1, right: 1)    layout.minimumLineSpacing = 1.0    layout.minimumInteritemSpacing = 1.0    myCollectionView.setCollectionViewLayout(layout, animated: true)    myCollectionView.reloadData()}