UICollectionView not getting rendered

Question

This code has UICollectionView getting initiated and datasource being assigned. Cells are supposed to be rendered however the cells are not getting rendered. The DataSource is extracted out in this case.

import UIKit
class ViewController: UIViewController {

private let cellReuseIdentifier = "collectionCell"

    override func viewDidLoad() {
        super.viewDidLoad()
        let flowLayout = UICollectionViewFlowLayout()
        flowLayout.scrollDirection = .horizontal

        let collectionView = UICollectionView(frame: self.view.bounds, collectionViewLayout: flowLayout)
        collectionView.register(UICollectionViewCell.self, forCellWithReuseIdentifier: cellReuseIdentifier)

        let dataSource = DateSource()

        collectionView.dataSource = dataSource
        collectionView.backgroundColor = UIColor.cyan

        self.view.addSubview(collectionView)
    }
}

class DateSource:NSObject, UICollectionViewDataSource {
    let leftAndRightPaddings: CGFloat = 80.0
    let numberOfItemsPerRow: CGFloat = 7.0
    let screenSize: CGRect = UIScreen.main.bounds
    private let cellReuseIdentifier = "collectionCell"
    var items = ["1", "2", "3", "4", "5", "6", "7", "8", "9", "10"]

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int
    {
        return self.items.count
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell
    {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: cellReuseIdentifier, for: indexPath)
        cell.backgroundColor = UIColor.green
        return cell
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize
    {
        let width = (screenSize.width-leftAndRightPaddings)/numberOfItemsPerRow
        return CGSize(width: width, height: width)
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets
    {
        return UIEdgeInsets(top: 20, left: 8, bottom: 5, right: 8)
    }
}

But this code renders the cells, when the datasource is sort of merged in the ViewController

import UIKit
class ViewController: UIViewController, UICollectionViewDataSource {

private let cellReuseIdentifier = "collectionCell"
let leftAndRightPaddings: CGFloat = 80.0
let numberOfItemsPerRow: CGFloat = 7.0
let screenSize: CGRect = UIScreen.main.bounds
var items = ["1", "2", "3", "4", "5", "6", "7", "8", "9", "10"]

    override func viewDidLoad() {
        super.viewDidLoad()
        let flowLayout = UICollectionViewFlowLayout()
        flowLayout.scrollDirection = .horizontal

        let collectionView = UICollectionView(frame: self.view.bounds, collectionViewLayout: flowLayout)
        collectionView.register(UICollectionViewCell.self, forCellWithReuseIdentifier: cellReuseIdentifier)

        let dataSource = self

        collectionView.dataSource = dataSource
        collectionView.backgroundColor = UIColor.cyan

        self.view.addSubview(collectionView)
    }

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int
    {
        return self.items.count
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell
    {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: cellReuseIdentifier, for: indexPath)
        cell.backgroundColor = UIColor.green
        return cell
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize
    {
        let width = (screenSize.width-leftAndRightPaddings)/numberOfItemsPerRow
        return CGSize(width: width, height: width)
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets
    {
        return UIEdgeInsets(top: 20, left: 8, bottom: 5, right: 8)
    }
}

I am a little lost here, was something to be instantiated inside DataSource, which was missed ?

Answer 1

In the first snippet, you've instantiated datasource in viewDidLoad(),

let dataSource = DateSource()

This restricts the lifespan of your datasource object within the scope of that method. You need to declare your datasource as a member of your View Controller, as such;

import UIKit

class ViewController: UIViewController {

    let dataSource = DateSource()
    private let cellReuseIdentifier = "collectionCell"

    override func viewDidLoad() {
        super.viewDidLoad()
        let flowLayout = UICollectionViewFlowLayout()
        flowLayout.scrollDirection = .horizontal

        let collectionView = UICollectionView(frame: self.view.bounds, collectionViewLayout: flowLayout)
        collectionView.register(UICollectionViewCell.self, forCellWithReuseIdentifier: cellReuseIdentifier)

        collectionView.dataSource = dataSource
        collectionView.backgroundColor = UIColor.cyan

        self.view.addSubview(collectionView)
    }
...
..
}

Second snippet works since the datasource is self and it's scope will outlive the collectionView's.