what is the function of NSObject in StoryBoard / Interface Builder?

Question

I am currently following a video tutorial course about test driven development of iOS in Swift, but when testing Table View in View Controller, I get stuck, since I don't understand why we need NSObject in Interface builder like the picture below:

Movie Library Data Service is inheritted NSObject class:

the class of MovieLibraryDataService is like this:

import UIKit

class MovieLibraryDataService: NSObject, UITableViewDataSource, UITableViewDelegate {

    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return 0
    }

    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        return UITableViewCell()
    }


}

and the MovieLibraryDataService class will be used in XCTestCase is like this :

@testable import FilmFest
class LibraryViewControllerTests: XCTestCase {

    var sut: LibraryViewController!

    override func setUp() {
        super.setUp()
        // Put setup code here. This method is called before the invocation of each test method in the class.
        sut = UIStoryboard(name: "Main", bundle: nil).instantiateViewController(withIdentifier: "LibraryViewControllerID") as! LibraryViewController
        _ = sut.view
    }

    override func tearDown() {
        // Put teardown code here. This method is called after the invocation of each test method in the class.
        super.tearDown()
    }

    // MARK: Nil Checks
    func testLibraryVC_TableViewShouldNotBeNil() {
        XCTAssertNotNil(sut.libraryTableView)
    }

    // MARK: Data Source
    func testDataSource_ViewDidLoad_SetsTableViewDataSource() {
        XCTAssertNotNil(sut.libraryTableView.dataSource)
        XCTAssertTrue(sut.libraryTableView.dataSource is MovieLibraryDataService)
    }

    // MARK: Delegate
    func testDelegate_ViewDidLoad_SetsTableViewDelegate() {
        XCTAssertNotNil(sut.libraryTableView.delegate)
        XCTAssertTrue(sut.libraryTableView.delegate is MovieLibraryDataService)
    }

    // MARK: Data Service Assumptions
    func testDataService_ViewDidLoad_SingleDataServiceObject() {
        XCTAssertEqual(sut.libraryTableView.dataSource as! MovieLibraryDataService, sut.libraryTableView.delegate as! MovieLibraryDataService)
    }



}

and the definition of LibraryViewController:

import UIKit

class LibraryViewController: UIViewController {

    @IBOutlet weak var libraryTableView: UITableView!
    @IBOutlet var dataService: MovieLibraryDataService!

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        self.libraryTableView.dataSource = dataService
        self.libraryTableView.delegate = dataService
    }


}

I really don't understand why I need to make that MovieLibraryDataService class

I usually use:

self.libraryTableView.dataSource = self
self.libraryTableView.delegate = self

but why do I need to write :

self.libraryTableView.dataSource = dataService
self.libraryTableView.delegate = dataService

Answer 1

Edit

You can use NSObject on the Storyboard for different purposes, one of them could be also the delegation. Instead of setting it programatically like:

self.libraryTableView.dataSource = self
self.libraryTableView.delegate = self

you can hold control and then set the corresponding delegates like below:

Original Answer

Because MovieLibraryDataService that conforms to UITableViewDataSource and UITableViewDelegate and not your LibraryViewController .

If you want to change it as you are always used to, change your code to:

class LibraryViewController: UIViewController, UITableViewDataSource, UITableViewDelegate {

    @IBOutlet weak var libraryTableView: UITableView!
    var dataService = MovieLibraryDataService()

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        self.libraryTableView.dataSource = self
        self.libraryTableView.delegate = self
    }

    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return 0
    }

    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        return UITableViewCell()
    }
}

Personally I would suggest to keep it as you have it, as View Controllers tend to grow and become like we call it Massive View Controller

Answer 2

MovieLibraryDataService is just another class which implements UITableViewDataSource and UITableViewDelegate , with the difference that the storyboard instantiates it and that storyboard-created instance is bound via the IBOutlet @IBOutlet var dataService: MovieLibraryDataService! . All objects in the storyboard are storyboard-created , that's why one has to bind them to variables to use if they are not bound to other variables you use already.

Naming the variable dataService is just a fancy way of saying that it is supposed to serve , in this case as dataSource and delegate for a tableView, since it implements those delegate-protocols.

Since the dataService is instantiated by the storyboard, you could try if you can bind the dataService inside the storyboard to tableView . This is possible because you have the dataService referencable in the storyboard. This would replace setting dataSource and delegate in the viewController .

Another example

AppDelegate is also an NSObject inside the Storyboard so the UIApplication/NSApplication inside the storyboard is able to reference that to use, avoids having to set up AppDelegate yourself outside a storyboard. (Would otherwise be ugly, maybe that's macOS only though, since mac apps have to show a Menu even if its empty.)