Simple UICollectionView example Swift: http://swiftiostutorials.com/tutorial-using-uicollectionview-uicollectionviewflowlayout/

GalleryItem.swift

import Foundation

class GalleryItem {
    
    var itemImage: String
    
    init(dataDictionary:Dictionary<String,String>) {
        itemImage = dataDictionary["itemImage"]!
    }
    
    class func newGalleryItem(_ dataDictionary:Dictionary<String,String>) -> GalleryItem {
        return GalleryItem(dataDictionary: dataDictionary)
    }
    
}

GalleryItemCollectionViewCell1.swift

import UIKit

class GalleryItemCollectionViewCell: UICollectionViewCell {
    
    @IBOutlet var itemImageView: UIImageView!
    
}

GalleryItemCollectionViewCell2.swift

import UIKit

class GalleryItemCollectionViewCell: UICollectionViewCell {
    
    @IBOutlet var itemImageView: UIImageView!
    
    func setGalleryItem(_ item:GalleryItem) {
        itemImageView.image = UIImage(named: item.itemImage)
    }
    
}

GalleryItemCommentView.swift

import UIKit

class GalleryItemCommentView: UICollectionReusableView {
    
    @IBOutlet var commentLabel: UILabel!
    
}

ViewController1.swift

import UIKit

class ViewController: UIViewController {
    
    @IBOutlet var collectionView: UICollectionView!
    
}

ViewController10.swift

import UIKit

class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate, UICollectionViewDelegateFlowLayout {
    
    ...
  
    // MARK: - UICollectionViewDataSource
    
    func numberOfSections(in collectionView: UICollectionView) -> Int {
        return 1
    }
    
}

ViewController11.swift

import UIKit

class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate, UICollectionViewDelegateFlowLayout {
  
    // MARK: - UICollectionViewDataSource
    
    ...
    
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return galleryItems.count
    }
    
}

ViewController12.swift

import UIKit

class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate, UICollectionViewDelegateFlowLayout {
  
    // MARK: - UICollectionViewDataSource
    
    ...
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "GalleryItemCollectionViewCell", for: indexPath) as! GalleryItemCollectionViewCell
        
        cell.setGalleryItem(galleryItems[indexPath.row])
        return cell
        
    }
    
}

ViewController13.swift

import UIKit

class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate, UICollectionViewDelegateFlowLayout {
  
    // MARK: - UICollectionViewDataSource
    
    ...
    
    func collectionView(_ collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, at indexPath: IndexPath) -> UICollectionReusableView {
        
        let commentView = collectionView.dequeueReusableSupplementaryView(ofKind: kind, withReuseIdentifier: "GalleryItemCommentView", for: indexPath) as! GalleryItemCommentView
        
        commentView.commentLabel.text = "Supplementary view of kind \(kind)"
        
        return commentView
    }
    
}

ViewController2.swift

import UIKit

class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate, UICollectionViewDelegateFlowLayout {
    
    @IBOutlet var collectionView: UICollectionView!
}

ViewController20.swift

import UIKit

class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate, UICollectionViewDelegateFlowLayout {
  
    ...
   
    // MARK: - UICollectionViewDelegate
    
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        
        let alert = UIAlertController(title: "didSelectItemAtIndexPath:", message: "Indexpath = \(indexPath)", preferredStyle: .alert)
        
        let alertAction = UIAlertAction(title: "Dismiss", style: .destructive, handler: nil)
        alert.addAction(alertAction)
        
        self.present(alert, animated: true, completion: nil)
    }
}

ViewController3.swift

import UIKit

class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate, UICollectionViewDelegateFlowLayout {
    
    ....
    
    var galleryItems: [GalleryItem] = [] // NEW PROPERTY
    
}

ViewController30.swift

import UIKit

class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate, UICollectionViewDelegateFlowLayout {
  
    ...
   
    // MARK: - UICollectionViewFlowLayout
    
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        let picDimension = self.view.frame.size.width / 4.0
        return CGSize(width: picDimension, height: picDimension)
    }
    
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
        let leftRightInset = self.view.frame.size.width / 14.0
        return UIEdgeInsetsMake(0, leftRightInset, 0, leftRightInset)
    }
}

ViewController4.swift

import UIKit

class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate, UICollectionViewDelegateFlowLayout {
    
    ....
    
    fileprivate func initGalleryItems() {
        
        var items = [GalleryItem]()
        let inputFile = Bundle.main.path(forResource: "items", ofType: "plist")
        
        let inputDataArray = NSArray(contentsOfFile: inputFile!)
        
        for inputItem in inputDataArray as! [Dictionary<String, String>] {
            let galleryItem = GalleryItem(dataDictionary: inputItem)
            items.append(galleryItem)
        }
        
        galleryItems = items
    }
    
}

ViewController5.swift

import UIKit

class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate, UICollectionViewDelegateFlowLayout {
    
    ...
  
    override func viewDidLoad() {
        super.viewDidLoad()
        initGalleryItems()
        collectionView.reloadData()
    }
    
    private func initGalleryItems() {
        ...
    }
    
}