问当外部委托没有实现相同的方法时，不触发带有内部委托的UICollectionView子类

EN

class ViewController: UIViewController {
    @IBOutlet weak var collectionView: CustomCollectionView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        collectionView.dataSource = self
        collectionView.delegate = self
    }
}


extension ViewController: UICollectionViewDelegate, UICollectionViewDataSource {
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        1000
    }
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath)
        cell.backgroundColor = .blue
        return cell
    }
    
    // DonMag - comment / un-comment these methods
    //  to see the difference
    
    //func collectionView(_ collectionView: UICollectionView, didEndDisplaying cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
    //  print("view controller - did end displaying: \(indexPath.item)")
    //}
    
    //func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
    //  print("view controller - didSelectItemAt", indexPath)
    //}
}

fileprivate class UICollectionViewDelegateInternal: NSObject, UICollectionViewDelegate {
    var userDelegate: UICollectionViewDelegate?
    
    override func responds(to aSelector: Selector!) -> Bool {
        return super.responds(to: aSelector) || userDelegate?.responds(to: aSelector) == true
    }
    
    override func forwardingTarget(for aSelector: Selector!) -> Any? {
        if userDelegate?.responds(to: aSelector) == true {
            return userDelegate
        }
        return super.forwardingTarget(for: aSelector)
    }
    
    func collectionView(_ collectionView: UICollectionView, didEndDisplaying cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
        let collection = collectionView as! CustomCollectionView
        collection.didEnd(item: indexPath.item)
        userDelegate?.collectionView?(collectionView, didEndDisplaying: cell, forItemAt: indexPath)
    }
    
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        let collection = collectionView as! CustomCollectionView
        collection.didSel(p: indexPath)
        userDelegate?.collectionView?(collectionView, didSelectItemAt: indexPath)
    }
    
}

// DonMag - conform to UICollectionViewDelegate
class CustomCollectionView: UICollectionView, UICollectionViewDelegate {
    private let internalDelegate: UICollectionViewDelegateInternal = UICollectionViewDelegateInternal()
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        super.delegate = internalDelegate
    }
    
    override init(frame: CGRect, collectionViewLayout layout: UICollectionViewLayout) {
        super.init(frame: frame, collectionViewLayout: layout)
        super.delegate = internalDelegate
    }
    
    func didEnd(item: Int) {
        print("internal - didEndDisplaying: \(item)")
    }
    
    func didSel(p: IndexPath) {
        print("internal - didSelectItemAt", p)
    }
    
    // DonMag - these will NEVER be called,
    //  whether or not they're implemented in
    //  UICollectionViewDelegateInternal and/or ViewController
    // but, when implemented here,
    //  it allows (enables?) them to be called in UICollectionViewDelegateInternal
    func collectionView(_ collectionView: UICollectionView, didEndDisplaying cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
        print("CustomCollectionView - didEndDisplaying", indexPath)
    }
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        print("CustomCollectionView - didSelectItemAt", indexPath)
    }
    
    override var delegate: UICollectionViewDelegate? {
        get {
            // DonMag - return self instead of internalDelegate.userDelegate
            return self
            //return internalDelegate.userDelegate
        }
        set {
            self.internalDelegate.userDelegate = newValue
            super.delegate = nil
            super.delegate = self.internalDelegate
        }
    }
}