在Swift collectionView中仅调用一次cellForItemAt

Question

If I use flow layout with collectionView, then all my cells are visible with the data. If I use a custom layout, then cellForItemAt is only accessed for index (0,0), and correspondingly only a single cell is displayed.

I'm baffled why - please help!

Minimal example below:

ViewController:

import UIKit
private let reuseIdentifier = "customCell"

class customCollectionViewController: UICollectionViewController {

@IBOutlet var customCollectionView: UICollectionView!


let dwarfArray = ["dopey", "sneezy", "bashful", "grumpy", "doc", "happy", "sleepy"]


override func viewDidLoad() {
    super.viewDidLoad()
}

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


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

override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath) as! customCollectionViewCell

    let cellContentsIndex = indexPath.row
    if cellContentsIndex <= dwarfArray.count
    {
        cell.displayContent(name: dwarfArray[cellContentsIndex])
    }

    return cell
}

}

Custom Cell

import UIKit
class customCollectionViewCell: UICollectionViewCell {
@IBOutlet weak var nameLabel: UILabel!

required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        setup()
    }

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

    public func displayContent(name: String){
       nameLabel.text = name

    }

func setup(){
    self.layer.borderWidth = 1.0
    self.layer.borderColor = UIColor.black.cgColor
}}

Custom Layout If this is not here - I can see all the cells I expect (albeit without my preferred layout). When I use this, I only see one cell.

import UIKit
class customCollectionViewLayout: UICollectionViewLayout {
let CELL_SIZE = 100.0

    var cellAttrsDictionary = Dictionary<NSIndexPath, UICollectionViewLayoutAttributes>()
    //define the size of the area the user can move around in within the collection view
    var contentSize = CGSize.zero

    var dataSourceDidUpdate = true

    func collectionViewContentSize() -> CGSize{
        return self.contentSize
    }

    override func prepare() {
        if (collectionView?.numberOfItems(inSection: 0))! > 0 {
            /// cycle through each item of the section
            for item in 0...(collectionView?.numberOfItems(inSection: 0))!-1{
                /// build the collection attributes
                let cellIndex = NSIndexPath(item: item, section: 0)
                let xPos = Double(item)*CELL_SIZE
                let yPos = 40.0

                let cellAttributes = UICollectionViewLayoutAttributes(forCellWith: cellIndex as IndexPath)
                cellAttributes.frame = CGRect(x: xPos, y:yPos, width: CELL_SIZE, height: CELL_SIZE)
                // cellAttributes.frame = CGRect(x: xPos, y:yPos, width: CELL_WIDTH + 2*CELL_SPACING, height: CELL_HEIGHT)
                cellAttributes.zIndex = 1

                //save
                cellAttrsDictionary[cellIndex] = cellAttributes
            }
        }
    }

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        /// create array to hold all the elements in our current view
        var attributesInRTect = [UICollectionViewLayoutAttributes]()

        /// check each element to see if they should be returned
        for cellAttributes in cellAttrsDictionary.values  {
            if rect.intersects(cellAttributes.frame)
            {
                attributesInRTect.append(cellAttributes)
            }
        }

        return attributesInRTect
    }

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        return cellAttrsDictionary[indexPath as NSIndexPath]!
    }

    override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
        return true
    }}

Output

Answer 1

The problem is with contentSize value

 func collectionViewContentSize() -> CGSize{
        return self.contentSize
 }

Just replace func collectionViewContentSize()... by something like this:

    func lastLayoutAttributes() -> UICollectionViewLayoutAttributes? {
        return cellAttrsDictionary.values.map { $0 }.sorted(by: { $0.frame.maxX < $1.frame.maxX }).last        
    }

    override var collectionViewContentSize: CGSize {
        guard let collectionView = collectionView else { return .zero }
        guard collectionView.frame != .zero else { return .zero }

        let width: CGFloat
        let height: CGFloat = collectionView.frame.height

        if let lastLayoutAttributes = lastLayoutAttributes() {
            width = lastLayoutAttributes.frame.maxX
        } else {
            width = 0
        }

        return CGSize(width: width, height: height)
    }

And you will see more than one cell.