http://blog.ios-developers.io/view-controller-extensions/

Say you are trying to make an iOS app that uses aUITableView. Along with the code that provides custom functionality,your view controller will need to conform to theUITableViewdata source and delegate in order to access methods that bring your views to life. This means including all of the protocol methods in your view controller.

// ViewController.swift
class ViewController: UIViewController {  
    override func viewDidLoad() {
        super.viewDidLoad()
        // Extra view initializations go here...
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }

    // DataSource
    func numberOfSectionsInTableView(tableview: UITableView) -> Int {
        return 1
    }

    func tableView(tableView: UITableView,numberOfRowsInSection section: Int) -> Int {
        return Data.dataStore.count
    }

    func tableView(tableView: UITableView,cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCellWithIdentifier("Cell",forIndexPath: indexPath)

        let row = indexPath.row
        cell.textLabel?.text = Data.dataStore[row]

        return cell
    }

    // Delegate
    func tableView(tableView: UITableView,didSelectRowAtIndexPath indexPath: NSIndexPath) {
        let row = indexPath.row
        print(Data.dataStore[row])
    }
}

Phew,there's a lot of code cluttering up the view controller,and it's really just there to conform to the protocols! Of course you can always just use some//colorfulcomments orMARKorganization in order to add visual separation from the rest of your code. But what if there was a way to just move all of that code into its own file? Good news everyone,there is! All we need are some handyextensions.

Welcome to the Extension Zone

Extensions allow you to extend the functionality of a class,structure,enumeration,or protocol type. Say you wanted to add a custom method to the NSString class. Extensions let you do that. Or maybe you want to separate out some code,say in a view controller,into a different file. Extensions let you do that! Just take the code from before and bring it out into it's own file. This is demonstrated as follows.

ViewController.swift

class ViewController: UIViewController {  
    override func viewDidLoad() {
        super.viewDidLoad()
        // Extra view initializations go here...
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }
}

ViewController+DataSource.swift

extension ViewController: UITableViewDataSource {  
    func numberOfSectionsInTableView(tableview: UITableView) -> Int {
        return 1
    }

    func tableView(tableView: UITableView,forIndexPath: indexPath)

        let row = indexPath.row
        cell.textLabel?.text = Data.dataStore[row]

        return cell
    }
}

ViewController+Delegate.swift

extension ViewController: UITableViewDelegate {  
    func tableView(tableView: UITableView,sans-serif; font-size:18px; letter-spacing:0.12px; line-height:31.5px"> Voilà. Now all of the code related to the DataSource and Delegate protocols are contained within their own files. These methods are isolated from the rest of your view controller,yet they still provide the essential capabilities to your program. Just look at how clean that view controller is now!
 
 One thing to make sure of though is that the delegate and data source for your tableview is assigned correctly. The easiest way to do this is through Interface Builder with thectrl+dragfunctionality. Select the tableview and connect it to the parent view controller for both data source and delegate,like so...
 
 
 
 Now you're all set. Sporting a sleek,slimmed down VC!