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。

如果我将此方法放在类中，则称为它；但是，如果我尝试通过protocol用作默认实现，则永远不会调用它。

代码：

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
    }
}

我在做什么错了？

Answer 1

我认为您正在寻找类似的东西

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
    }
}

很遗憾，Obj-C无法访问协议扩展。因此不会被调用 scrollViewWillEndDragging

从笔记

同样，唯一的例外是协议扩展。不像任何语言的其他构造，协议扩展方法是在虚拟分派将要进行的情况下静态分派导致不同的结果。没有编译器错误可防止这种不匹配。 https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151207/001707.html

Answer 2

更正代码

CASE 
    WHEN NetRate_Quote_Insur_Quote_Locat.[Transaction]='ADD' THEN 'Add'
    WHEN NetRate_Quote_Insur_Quote_Locat.[Transaction]='DELETED' THEN 'Delete'
    WHEN NetRate_Quote_Insur_Quote_Locat.[Transaction]='CHANGED' THEN 'Change'
    WHEN NetRate_Quote_Insur_Quote_Locat.[Transaction]='New Quote' THEN 'Add'
    WHEN tblQuotes.OriginalQuoteGuid IS NULL OR NetRate_Quote_Insur_Quote.TypeofBusiness = 'Reinstate' THEN 'Add'
    WHEN tblQuotes.QuoteStatusID = 12 THEN 'Delete' 
    ELSE 'No Change' 
END as TransactionName

}

UIScrollView委托作为协议默认实现

2 个答案: