UIScrollView委托作为协议默认实现

Question

我正在尝试为UIScrollViewDelegate方法实现默认实现：

func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>)

使用protocol。

如果我把这个方法放在类中，就会调用它;但是，如果我尝试通过protocol使用默认实现，它永远不会被调用。

码：

protocol DefaultScrollViewEndDragging {
    func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>)
}

extension DefaultScrollViewEndDragging {
    func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
        print("scrollViewWillEndDragging is called")
        // THIS IS NEVER CALLED
    }
}

class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate, DefaultScrollViewEndDragging {

    @IBOutlet weak var collectionView: UICollectionView!

    override func viewDidLoad() {
        super.viewDidLoad()
        collectionView.dataSource = self
        collectionView.delegate = self
        collectionView!.register(UICollectionViewCell.self, forCellWithReuseIdentifier: reuseIdentifier)
    }

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 5
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath)
        print("Cell \(indexPath.row)")
        cell.contentView.backgroundColor = .orange
        return cell
    }
}

我究竟做错了什么？

Answer 1

我觉得你在找这样的东西

protocol DefaultScrollViewEndDragging : UIScrollViewDelegate {
    func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>)
}

extension DefaultScrollViewEndDragging {
    func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
        print("scrollViewWillEndDragging is called")
        // THIS IS NEVER CALLED
    }
}

遗憾的是，Obj-C无法访问协议扩展。所以它不会被称为scrollViewWillEndDragging

从笔记

同样，唯一的例外是协议扩展。与该语言中的任何其他构造不同，协议扩展方法在虚拟分派会导致不同结果的情况下静态分派。没有编译器错误可以防止这种不匹配。 https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151207/001707.html

Answer 2

正确的代码

class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate, UIScrollViewDelegate {

@IBOutlet weak var collectionView: UICollectionView!

override func viewDidLoad() {
    super.viewDidLoad()
    collectionView.dataSource = self
    collectionView.delegate = self
    collectionView!.register(UICollectionViewCell.self, forCellWithReuseIdentifier: reuseIdentifier)
}

func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return 5
}

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath)
    print("Cell \(indexPath.row)")
    cell.contentView.backgroundColor = .orange
    return cell
}
//MARK: scrollview Delegate Method
func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
    print("scrollViewWillEndDragging is called")
}

}