Example of organizing UIViewController class with the help of extensions

ViewController.swift

//
//  ViewController.swift
//
//  Created by Dejan Atanasov on 01/04/2017.
//  Copyright © 2017 Dejan Atanasov. All rights reserved.
//

import UIKit

class ViewController: UIViewController {
    
    //MARK: Private Properties
    @IBOutlet fileprivate weak var titleLbl: UILabel!
    @IBOutlet fileprivate weak var actionBtn: UIButton!
    @IBOutlet fileprivate weak var collectionView: UICollectionView!
        
    //MARK: Internal Properties
    var title: String! 
    
    override func viewDidLoad() {
        super.viewDidLoad()
        setupUI()
    }
}

//MARK: Private extension - used to store private methods 
private extension ViewController{
    /* do any UI related stuff here */
    func setupUI(){
      self.title = "Some Title"
    }

    /* keep your actions in this block as well */        
    @IBAction func someAction(btn: UIButton){
      
    }    
}

//MARK: UICollectionViewDelegate 
extension ViewController: UICollectionViewDelegate{
    
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 10
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath) as! CustomCell
        return cell
    }
}

//MARK: UICollectionViewDataSource 
extension ViewController: UICollectionViewDataSource{
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        
    }
}

//MARK: UICollectionViewDelegateFlowLayout 
extension ViewController: UICollectionViewDelegateFlowLayout{
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        return collectionView.frame.size
    }
}

//MARK: SomeCustomDelegate 
extension ViewController: SomeCustomDelegate{
    func customDelegateMethod() {

    }
}