Why do we need to set delegate to self? Why isn't it defaulted by the compiler?

Question

I think I fully understand the concept of delegation, my question is that when we do:

class someViewController : UIViewController, UITableViewDelegate{

}

would it ever be possible that we wouldn't want to set tableView.delegate to self?

If there isn't any chance then why is Xcode forcing us to do some extra work here?

If there is a chance that tableView.delegate is set to something other than self...well what is that? Can you please provide some examples?

Answer 1

When mentioning that tableView.delegate = self or tableView.dataSource = self in the desired ViewController, that's means the ViewController is saying "I am responsible for implementing those delegation/dataSource methods", means that this ViewController (self) is taking care of providing the needed method to let tableView knows how it should looks/behaves.

Referring to your questions:

would it ever be possible that we wouldn't want to set tableView.delegate to self?

Actually it's possible, but this causes to let the tableView appears as an empty tableView (no rows in it), because no one is telling it about how it should looks/behave.

If there is a chance that tableView.delegate is set to something other than self...well what is that? Can you please provide some examples?

Yes you can, tableView.dataSource/delegate not necessary to be assigned to the same Viewcontroller that contains this tableView (but I find it more readable and understandable).

For example:

In the following code snippets, I assigning the dataSource of the tableView to another separated class (which is not even a UIViewController) on a different .swift file and it completely works fine:

import UIKit

// ViewController File
class ViewController: UIViewController {
    var handler: Handler!

    @IBOutlet weak var tableView: UITableView!
    override func viewDidLoad() {
        super.viewDidLoad()

        handler = Handler()
        tableView.dataSource = handler
    }
}

Handler Class:

import UIKit

class Handler:NSObject, UITableViewDataSource {
    func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return 10
    }

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

        cell?.textLabel?.text = "row #\(indexPath.row + 1)"

        return cell!
    }
}

The output works fine as it should.

Answer 2

would it ever be possible that we wouldn't want to set tableView.delegate to self

Yes, if you would implement datasource and delegate in it's own class.

why is Xcode forcing us to do some extra work here?

to allow as much freedom in our architectures as possible.

---

An example with separate classes for datasource and delegate.

class TableViewDatasource: NSObject, UITableViewDataSource {
    // implement datasource
}

class TableViewDelegate : NSObject, UITableViewDelegate {
    // implement delegate
}

class ViewController: UIViewController {

     IBOutlet weak var tableView: UITableView! {
         didSet {
              tableView.dataSource = tableViewDatasource
              tableView.delegate = tableViewDelegate
         }
     }

     let tableViewDatasource = TableViewDatasource()
     let tableViewDelegate = TableViewDelegate()

}

This allows higher reusability and favours composition over inheritance, if you'd allow view controllers to hold different implementations of delegate and datasource.

You deal with smaller classes and those are easier to test and maintain.

It is even possible to design complete apps, that don't need any view controller subclasses.

---

datasource/delegate design can be as sophisticated as you like.

As an example I want to show you a new project of mine: TaCoPopulator. It is a framework to populate table view and collection views transparently by splitting it up in distinct task to follow the SOLID Principles:

import UIKit

class CollectionViewController: UIViewController {

    @IBOutlet weak var collectionView: UICollectionView!
    private var datasource: ViewControllerDataSource<UICollectionView>?

    override func viewDidLoad() {
        super.viewDidLoad()
        self.datasource = ViewControllerDataSource(with: collectionView)
    }
}

---

import UIKit

class TableViewController: UIViewController {

    @IBOutlet weak var tableView: UITableView!
    private var datasource: ViewControllerDataSource<UITableView>?

    override func viewDidLoad() {
        super.viewDidLoad()
        self.datasource = ViewControllerDataSource(with: tableView)
    }
}

---

import TaCoPopulator


class IntDataProvider: SectionDataProvider<Int> {

    override init(reuseIdentifer: @escaping (Int, IndexPath) -> String) {
        super.init(reuseIdentifer: reuseIdentifer)
        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 2) {
            self.provideElements([1,2,3,4,5,6,7,8,9])
            DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 2) {
                self.provideElements(self.elements() + [10, 11, 12, 13, 14, 15])
            }
        }
    }
}

---

import TaCoPopulator


class ViewControllerDataSource<TableOrCollectionView: PopulatorView> {
    init(with populatorView: PopulatorView) {
        self.populatorView = populatorView
        setup()
    }

    var intSelected: ((Int, IndexPath) -> Void)?
    var stringSelected: ((String, IndexPath) -> Void)?

    let dp1 = IntDataProvider {
        _ in return "Cell"
    }

    let dp2 = StringDataProvider {
        _ in return "Cell"
    }


    weak var populatorView: PopulatorView?
    var populator: Populator<MyViewPopulator>?

    func setup(){
        dp1.selected = {
            [weak self] element, indexPath in
            self?.intSelected?(element, indexPath)
        }

        dp2.selected = {
            [weak self] element, indexPath in
            self?.stringSelected?(element, indexPath)
        }

        let collectionViewCellConfig: (Any, TextCollectionViewCell, IndexPath) -> TextCollectionViewCell  = {
            element, cell, _ in
            cell.textLabel?.text = "\(element)"
            return cell
        }

        let tableViewViewCellConfig: (Any, UITableViewCell, IndexPath) -> UITableViewCell  = {
            element, cell, _ in
            cell.textLabel?.text = "\(element)"
            return cell
        }

        if  let populatorView  = populatorView as? UICollectionView {
            let  section1factory = SectionCellsFactory<Int, TextCollectionViewCell>(parentView: populatorView, provider: dp1, cellConfigurator: collectionViewCellConfig)
            let  section2factory = SectionCellsFactory<String, TextCollectionViewCell>(parentView: populatorView, provider: dp2, cellConfigurator: collectionViewCellConfig)
            self.populator = Populator(with: populatorView, sectionCellModelsFactories: [section1factory, section2factory])
        }

        if  let populatorView  = populatorView as? UITableView {
            let section1factory = SectionCellsFactory<Int, UITableViewCell>(parentView: populatorView, provider: dp1, cellConfigurator: tableViewViewCellConfig)
            let section2factory = SectionCellsFactory<String, UITableViewCell>(parentView: populatorView, provider: dp2, cellConfigurator: tableViewViewCellConfig)
            self.populator = Populator(with: populatorView, sectionCellModelsFactories: [section1factory, section2factory])
        }
    }

}

Answer 3

would it ever be possible that we wouldn't want to set tableView.delegate to self?

What's tableView.delegate? Your class someViewController is not a subclass of UITableViewController (and does not have a tableView property). It does not even know that you handle a table view.

The fact that your type declaration conforms to UITableViewDelegate does not make it obvious on which actual table view instance it should be set.

If there is a chance that tableView.delegate is set to something other than self...well what is that?

Often times it's wise to split up functionality into multiple types to reduce the complexity of the view controller. Your someViewController might have a viewModel property that handles all thing regarding the table view.