Adding a collectionview to a collection view cell leads to memory growing

Question

Ive created a CollectionView called and on each CollectionViewCell I have created a CollectionView. Below I have included the code for the collectionView which goes inside the cell of the MainCollectionView. Every time that I swipe through the cells from one collectionView to another the memory grows and after a cell is deleted the memory is not released. I've read that collectionView holds a strong reference to its layout Im not sure if that could be the cause. Am I implementing this wrong. thank you for you help.

MainViewController

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    guard let cell = collectionView.dequeueReusableCell(withReuseIdentifier: FullPageCell.reuseIdentifier, for: indexPath) as? FullPageCell else{
        assertionFailure("Fatal Error FullPageCell not dequed")
        return UICollectionViewCell()
    }
    cell.backgroundColor = UIColor.green
    cell.cellTappedDelegate = self

    return cell
}

func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return numberOfPages
}

func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
    if let cell = cell as? FullPageCell{
            cell.setUpCollectionView()
    }
}

func collectionView(_ collectionView: UICollectionView, didEndDisplaying cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
    if let cell = cell as? FullPageCell{
        cell.collectionView = nil
    }
}

SubCell CollectionView

    static var reuseIdentifier: String = "FullPageCell"

   var collectionView:UICollectionView? = nil
    weak var cellTappedDelegate:CellTappedDelegate?
    var image:UIImage  = #imageLiteral(resourceName: "Image")

    override init(frame: CGRect) {
        super.init(frame:frame)


        self.setUpCollectionView()
    }

    func setUpCollectionView(){
        let view = self.contentView

        let layout = UICollectionViewFlowLayout()
        layout.minimumLineSpacing = 1
        layout.minimumInteritemSpacing = 1
        layout.scrollDirection = .vertical
        layout.itemSize = CGSize(width: (view.bounds.width - 2)/3, height: (view.bounds.width - 2)/3)

        self.collectionView = UICollectionView(frame:view.bounds, collectionViewLayout: layout)

        guard let collectionView = collectionView else{
            return
        }
        collectionView.dataSource = self
        collectionView.delegate = self
        collectionView.register(SmallCell.self, forCellWithReuseIdentifier: SmallCell.reuseIdentifier)
        collectionView.backgroundColor = UIColor.white

        self.collectionView?.isPagingEnabled = true

        view.addSubview(collectionView)

        collectionView.translatesAutoresizingMaskIntoConstraints = false
        collectionView.topAnchor.constraint(equalTo: view.topAnchor, constant: 0).isActive = true
        collectionView.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 0).isActive = true
        collectionView.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: 0).isActive = true
        collectionView.bottomAnchor.constraint(equalTo: view.bottomAnchor, constant: 0).isActive = true
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        guard let cell = collectionView.dequeueReusableCell(withReuseIdentifier: SmallCell.reuseIdentifier, for: indexPath) as? SmallCell else{
            assertionFailure("Fatal Error FullPageCell not dequed")
            return UICollectionViewCell()
        }
        cell.backgroundColor = UIColor.yellow
        cell.imageView.image = self.image
        return cell
    }

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 1000
    }

You're setting to nil the reference of the collectionView in the cell but you're never removing it from the contentView (you only call `view.addSubview(collectionView)`) so it is never going to be released because the retain count will always be at least 1. — Fabio Felici, Mar 26 '19 at 15:11
should I call `.removeFromSuperView()` on the collectionView before making the collectionView = nil in `didEndDisplaying`. Is it a good practice to set the collectionView to nil in `didEndDisplaying`? — TheRedCamaro3.0 3.0, Mar 26 '19 at 15:17
I don't see why you're doing it in this way. The collectionView should always be part of the view hierarchy IMO. You should only change its data. — Fabio Felici, Mar 26 '19 at 15:21
@FabioFelici I tried originally just placing the collectionView on the collectionViewCell and doing everything inside of the cells `init` but then this retain cycle was occurring so I started trying to make the collectionView nil and rearranging the call order. Do you think a better approach would be to create view controller for the cell? — TheRedCamaro3.0 3.0, Mar 26 '19 at 15:26

score 0 · Answer 1 · answered Mar 26 '19 at 15:56

Not sure if this playground reflects what you're trying to do but it will show you how to avoid creating the collectionView each time:

import UIKit
import PlaygroundSupport

class InnerCell: UICollectionViewCell {
    let textLabel = UILabel()

    override init(frame: CGRect) {
        super.init(frame: frame)
        contentView.backgroundColor = .white
        textLabel.layer.borderColor = UIColor.gray.cgColor
        textLabel.layer.borderWidth = 1
        textLabel.textAlignment = .center
        textLabel.translatesAutoresizingMaskIntoConstraints = false
        contentView.addSubview(textLabel)
        NSLayoutConstraint.activate([
            textLabel.leadingAnchor.constraint(equalTo: contentView.leadingAnchor),
            textLabel.trailingAnchor.constraint(equalTo: contentView.trailingAnchor),
            textLabel.topAnchor.constraint(equalTo: contentView.topAnchor),
            textLabel.bottomAnchor.constraint(equalTo: contentView.bottomAnchor)
        ])
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError()
    }
}

class Cell: UICollectionViewCell, UICollectionViewDataSource {

    var data: [Int]! = [] {
        didSet {
            collectionView.reloadData()
        }
    }
    private let collectionView: UICollectionView

    override init(frame: CGRect) {
        let layout = UICollectionViewFlowLayout()
        layout.minimumLineSpacing = 1
        layout.minimumInteritemSpacing = 1
        layout.itemSize = .init(width: 220, height: 220)
        collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
        super.init(frame: frame)
        collectionView.isPagingEnabled = true
        collectionView.backgroundColor = .white
        collectionView.register(InnerCell.self, forCellWithReuseIdentifier: "Cell")
        collectionView.dataSource = self
        collectionView.translatesAutoresizingMaskIntoConstraints = false
        contentView.addSubview(collectionView)
        NSLayoutConstraint.activate([
            collectionView.leadingAnchor.constraint(equalTo: contentView.leadingAnchor),
            collectionView.trailingAnchor.constraint(equalTo: contentView.trailingAnchor),
            collectionView.topAnchor.constraint(equalTo: contentView.topAnchor),
            collectionView.bottomAnchor.constraint(equalTo: contentView.bottomAnchor)
        ])
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError()
    }

    func numberOfSections(in collectionView: UICollectionView) -> Int {
        return 1
    }

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

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath) as! InnerCell
        cell.textLabel.text = String(data[indexPath.item])
        return cell
    }
}

class VC: UICollectionViewController, UICollectionViewDelegateFlowLayout {

    let data = [
        0: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14],
        1: [6, 4],
        2: [5, 5, 5, 5, 6],
        3: [1, 2, 3, 4, 5, 6, 7]
    ]

    override init(collectionViewLayout layout: UICollectionViewLayout) {
        super.init(collectionViewLayout: layout)
        collectionView.isPagingEnabled = true
        collectionView.register(Cell.self, forCellWithReuseIdentifier: "Cell")
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError()
    }

    override func numberOfSections(in collectionView: UICollectionView) -> Int {
        return 1
    }

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

    override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath) as! Cell
        cell.data = data[indexPath.item]
        return cell
    }

    public func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        return collectionView.bounds.size
    }

}

let layout = UICollectionViewFlowLayout()
layout.scrollDirection = .horizontal
layout.minimumLineSpacing = 0
PlaygroundPage.current.liveView = VC(collectionViewLayout: layout)

Thank you so much for taking the time to make such a detailed answer. The memory holds and doesn't grow like mine did. I tried experimenting with adding in a button to delete cells from the collectionView in the VC class and I noticed that even after `.deleteItemsAt` is called the memory being used remains the same, is there a way to deallocate this memory when a page is deleted? — TheRedCamaro3.0 3.0, Mar 26 '19 at 16:39
You shouldn't worry about this IMO. The collectionView internally handles a cache of cells to support reuse. I don't think you're going to immediately see the cell being deallocated when you remove one because it might be used again. UIKit will most likely clear the cache by responding to a memory warning. — Fabio Felici, Mar 26 '19 at 17:21

Adding a collectionview to a collection view cell leads to memory growing

1 Answers1