'UICollectionView必须使用非nil布局参数初始化'-Swift 4

Question

(Don't mark this question as duplicated. I read a lot of questions but I don't find the answer to my issue.)

I have created a IUCollectionView class and a UICollectionViewCell class. Here is the collection view one:

class NearbyPlacesUICollectionView: UICollectionView {


    required init(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)!
        commonInit()
        fatalError("init(coder:) has not been implemented")
    }

    override init(frame: CGRect, collectionViewLayout layout: UICollectionViewLayout) {
        super.init(frame: frame, collectionViewLayout: layout)
        commonInit()
    }

    init(collectionView: UICollectionView, frame: CGRect) {
        let layout = UICollectionViewLayout()
        super.init(frame: frame, collectionViewLayout: layout)
    }

    func commonInit() {

        allowsMultipleSelection = false
        backgroundColor = .white

        let layout = UICollectionViewFlowLayout()
        layout.sectionInset = UIEdgeInsets(top: 20, left: 10, bottom: 10, right: 10)
        layout.itemSize = CGSize(width: UIScreen.main.bounds.width * 0.8875, height: 90*3)
        frame = UIScreen.main.bounds
        collectionViewLayout = layout

        register(NearbyPlacesUICollectionView.self, forCellWithReuseIdentifier: "nearbyPlaceCell")
    }
}

Then I have a UIViewController where I have this (I remove some code to make it shorter):

    class NearbyPlacesViewController: UIViewController, UICollectionViewDataSource {

       var collectionView = NearbyPlacesUICollectionView()

       override func viewDidLoad() {
            super.viewDidLoad()

        collectionView.dataSource = self
        collectionView.delegate = self
        view.addSubview(collectionView)
       }
}

extension NearbyPlacesViewController: UICollectionViewDelegateFlowLayout, UICollectionViewDelegate {

    func numberOfSections(in tableView: UITableView) -> Int {
        return 3
    }

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 2
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {

        let cell: NearbyPlacesUICollectionViewCell? = collectionView.dequeueReusableCell(withReuseIdentifier: "nearbyPlaceCell", for: indexPath) as? NearbyPlacesUICollectionViewCell

        return cell!
    }

    func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
        if let cell = cell as? NearbyPlacesUICollectionViewCell
        {
            _configureCell(cell, atIndexPath: indexPath)
        }
    }

    fileprivate func _configureCell(_ cell: NearbyPlacesUICollectionViewCell, atIndexPath indexPath: IndexPath)
    {
        cell.queue.cancelAllOperations()

        let operation: BlockOperation = BlockOperation()
        operation.addExecutionBlock { [weak operation] () -> Void in

            DispatchQueue.main.sync(execute: { [weak operation] () -> Void in

                if let operation = operation, operation.isCancelled { 
            return }

                ... // cell code here
            })
        }

        cell.queue.addOperation(operation)
    }


    func collectionView(_ collectionView: UICollectionView, viewForHeaderInSection section: Int) -> UIView? {

        let view = UIView()
        view.backgroundColor = .clear
            ... // header code here

        return view
    }
}

I get this error when running: UICollectionView must be initialized with a non-nil layout parameter I override all the init but it seems like they are not called. I read all questions about similar issues but I didn't find any answer. (I'm using Swift 4). Sorry If I paste a lot of code but I think it's necessary. Thanks in advance for your help.

Answer 1

The problem is this line:

var collectionView = NearbyPlacesUICollectionView()

This ends up calling the wrong init method where no layout is set. You've defined 3 init methods in your NearbyPlacesUICollectionView class. Use one of the two that causes the layout to be set.

And this init method:

init(collectionView: UICollectionView, frame: CGRect)

makes no sense. It should be:

init(frame: CGRect)

since you don't use (and definitely don't need) the collectionView property.

It's also odd that this init :

override init(frame: CGRect, collectionViewLayout layout: UICollectionViewLayout)

calls your commonInit which in turn creates and set a layout. But you've already set the layout to the one passed to the init .