Keep Collection View Cells Overlapped When They Are Redrawn

Question

I've gotten my cells to overlap by setting the minimumLineSpacing property of the collection views layout to negative. But, when I scroll and the cells are redrawn, they now overlap in the opposite direction. I've put pictures below.

How do I keep the cells overlapping as seen in the first picture when the collection view is scrolled and cells are redraw?

import UIKit

class PopularView: UIView {

let cellID = "cellID"

// MARK: - Views
let collectionView: UICollectionView = {
    let layout = UICollectionViewFlowLayout()
    layout.scrollDirection = .vertical
    layout.minimumLineSpacing = -55     // -------Allows Overlap-----
    layout.itemSize = CGSize(width: SCREEN_WIDTH, height: 185)
    layout.minimumInteritemSpacing = 17
    let view = UICollectionView(frame: CGRect.zero, collectionViewLayout: layout)
    view.backgroundColor = .white
    return view
}()



// MARK: - Initializers
override init(frame: CGRect) {
    super.init(frame: frame)
    collectionView.dataSource = self
    collectionView.register(PopularCell.self, forCellWithReuseIdentifier: cellID)

    backgroundColor = .white
    setupCollectionView()
}

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



// MARK: - Setup
fileprivate func setupCollectionView() {
    self.addSubview(collectionView)
    collectionView.anchors(top: self.topAnchor, topPad: 0, bottom: self.bottomAnchor, bottomPad: 0, left: self.leftAnchor, leftPad: 0, right: self.rightAnchor, rightPad: 0, height: nil, width: nil)
    collectionView.contentSize = CGSize(width: 700, height: 700)
    }
}

extension PopularView: UICollectionViewDataSource {
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return 500
}

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: cellID, for: indexPath) as! PopularCell
    cell.background.backgroundColor = .random
    return cell
    }
}

Answer 1

Try this, that might help you:

1- Inside your Cells, you could define innerView inside your cell and set the frame to

let innerView:UIView = CGRect(x: 0,y: -overlapHeight,width: screenWidth, height:cell.height + overlapHeight)
cell?.contentView.addSubview(innerView)

2- Configure your cell during initialisation with this:

cell?.contentView.clipsToBounds = false

3- When loading cell, set the z-order:

override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath)
        cell.layer.zPosition = CGFloat(indexPath.row)
// configure your cell after here
}

You should be able to see the nested views inside your content view to have overlapping.

I have drafted a sample code, not looking perfectly, but will help you get started:

private let reuseIdentifier = "Cell"
private let overlapHeight:CGFloat = 100
class CustomCollectionCell:UICollectionViewCell {
    var innerView:UIView?
    override init(frame: CGRect) {
        super.init(frame: CGRect(x: 0, y: 0, width: UIScreen.main.bounds.width, height: 300))
        self.backgroundColor = .darkGray
        let innerView = UIView(frame: CGRect(x: 0,y: -overlapHeight,width: UIScreen.main.bounds.width,height: overlapHeight + self.contentView.frame.height))
        self.innerView = innerView
        innerView.layer.cornerRadius = 20
        self.contentView.addSubview(innerView)
        self.contentView.clipsToBounds = false
    }

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

    func configure(color:UIColor?) {
        innerView?.backgroundColor = color
    }
}

import UIKit

private let reuseIdentifier = "Cell"
private let overlapHeight:CGFloat = 100
class CustomCollectionCell:UICollectionViewCell {
    var innerView:UIView?
    override init(frame: CGRect) {
        super.init(frame: frame)
        self.backgroundColor = .darkGray
        let innerView = UIView(frame: CGRect(x: 0,y: -overlapHeight,width: UIScreen.main.bounds.width,height: overlapHeight + self.contentView.frame.height))
        self.innerView = innerView
        innerView.layer.cornerRadius = 20
        self.contentView.addSubview(innerView)
        self.contentView.clipsToBounds = false
    }

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

    func configure(color:UIColor?) {
        innerView?.backgroundColor = color
    }
}

class CollectionViewController: UICollectionViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        let flowLayout = UICollectionViewFlowLayout()
        flowLayout.itemSize = CGSize(width: UIScreen.main.bounds.width, height: 190)
        flowLayout.sectionInset = UIEdgeInsets(top: 0, left: 5, bottom: 0, right: 5)
        flowLayout.scrollDirection = .vertical
        flowLayout.minimumInteritemSpacing = 0.0
        collectionView.collectionViewLayout = flowLayout


        // Register cell classes
        self.collectionView!.register(CustomCollectionCell.self, forCellWithReuseIdentifier: reuseIdentifier)


    }

    // MARK: UICollectionViewDataSource

    override func numberOfSections(in collectionView: UICollectionView) -> Int {
        // #warning Incomplete implementation, return the number of sections
        return 1
    }


    override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        // #warning Incomplete implementation, return the number of items
        return 30
    }

    override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath)
        cell.layer.zPosition = CGFloat(indexPath.row)

        var color:UIColor?
        switch indexPath.row % 4 {
        case 0:
            color = .purple
        case 1:
            color = .yellow
        case 2:
            color = .green
        default:
            color = .red
        }
        if let cell = cell as? CustomCollectionCell {
            cell.configure(color: color)
        }


        return cell
    }



}

Result: