在UIView中获取时，UICollectionView不会被渲染

Question

以某种方式未呈现集合视图。 我之前有一个类似的问题UICollectionView not getting rendered 但是这个与弱引用有关，下面的代码没有弱引用，但仍然没有任何东西被渲染。

import UIKit

class ViewController: UIViewController {

    private let cellReuseIdentifier = "collectionCell"

    let scrollerView = ScrollerView()
    override func viewDidLoad() {
        super.viewDidLoad()
        self.view.addSubview(scrollerView)
    }
}

class ScrollerView:UIView, 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"]
    let flowLayout:UICollectionViewFlowLayout
    let collectionView:UICollectionView
    init() {

        flowLayout = UICollectionViewFlowLayout()
        collectionView = UICollectionView(frame: .zero, collectionViewLayout: flowLayout)

        super.init(frame:.zero)
        collectionView.register(UICollectionViewCell.self, forCellWithReuseIdentifier: cellReuseIdentifier)


        collectionView.dataSource = self
        collectionView.backgroundColor = UIColor.cyan
        self.addSubview(collectionView)
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }


    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)
    }
}