swift3 - UICollectionViewLayout 未按预期工作

Question

我关注了这个 Custom Collection View Layout tutorial从 raywenderlich.com 使用 xcode 8 和 swift 3。

当我在实现教程中要求的所有方法后运行应用程序时，出现以下错误:

'no UICollectionViewLayoutAttributes instance for -layoutAttributesForItemAtIndexPath: {length = 2, path = 0 - 11}'

所以我在我的 CollectionViewFlowLayout 类中添加了以下方法:

override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {

    return self.cache[indexPath.row]
}

这几乎是有效的，除了一些单元格在向下滚动时覆盖现有单元格然后消失。如果我向上滚动，一切正常。

我还不明白这段代码的完整逻辑，但我已经多次审查和测试它，我无法理解代码的哪一部分触发了这种行为。

任何的想法？

import UIKit

/* The heights are declared as constants outside of the class so they can be easily referenced elsewhere */
struct UltravisualLayoutConstants {
struct Cell {
    /* The height of the non-featured cell */
    static let standardHeight: CGFloat = 100
    /* The height of the first visible cell */
    static let featuredHeight: CGFloat = 280
}
}
class UltravisualLayout: UICollectionViewLayout {
// MARK: Properties and Variables

/* The amount the user needs to scroll before the featured cell changes */
let dragOffset: CGFloat = 180.0

var cache = [UICollectionViewLayoutAttributes]()

/* Returns the item index of the currently featured cell */
var featuredItemIndex: Int {
    get {
        /* Use max to make sure the featureItemIndex is never < 0 */
        return max(0, Int(collectionView!.contentOffset.y / dragOffset))
    }
}

/* Returns a value between 0 and 1 that represents how close the next cell is to becoming the featured cell */
var nextItemPercentageOffset: CGFloat {
    get {
        return (collectionView!.contentOffset.y / dragOffset) - CGFloat(featuredItemIndex)
    }
}

/* Returns the width of the collection view */
var width: CGFloat {
    get {
        return collectionView!.bounds.width
    }
}

/* Returns the height of the collection view */
var height: CGFloat {
    get {
        return collectionView!.bounds.height
    }
}

/* Returns the number of items in the collection view */
var numberOfItems: Int {
    get {
        return collectionView!.numberOfItems(inSection: 0)
    }
}

// MARK: UICollectionViewLayout

/* Return the size of all the content in the collection view */
override var collectionViewContentSize : CGSize {

    let contentHeight = (CGFloat(numberOfItems) * dragOffset) + (height - dragOffset)

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

override func prepare() {


    let standardHeight = UltravisualLayoutConstants.Cell.standardHeight

    let featuredHeight = UltravisualLayoutConstants.Cell.featuredHeight

    var frame = CGRect.zero

    var y:CGFloat = 0

    for item in 0 ... (numberOfItems - 1) {

        let indexPath = NSIndexPath(item: item, section: 0)

        let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath as IndexPath)

        attributes.zIndex = item

        var height = standardHeight

        if indexPath.item == featuredItemIndex {

            let yOffset = standardHeight * nextItemPercentageOffset

            y = self.collectionView!.contentOffset.y - yOffset

            height = featuredHeight


        } else if item == (featuredItemIndex + 1) && item != numberOfItems {

            let maxY = y + standardHeight

            height = standardHeight + max((featuredHeight - standardHeight) * nextItemPercentageOffset, 0)

            y = maxY - height
        }

        frame = CGRect(x: 0, y: y, width: width, height: height)

        attributes.frame = frame

        cache.append(attributes)

        y = frame.maxY
    }
}

/* Return all attributes in the cache whose frame intersects with the rect passed to the method */

override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {

    var layoutAttributes = [UICollectionViewLayoutAttributes]()

    for attributes in cache {

        if attributes.frame.intersects(rect) {

            layoutAttributes.append(attributes)
        }
    }

    return layoutAttributes
}

override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {

    return self.cache[indexPath.row]
}

/* Return true so that the layout is continuously invalidated as the user scrolls */
override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {

    return true
}
}

Answer 1

刚刚在拥有 UICollection View 的 View Controller 的 super.viewDidLoad 中添加了它，它现在按预期工作。

这是一个快速修复，我相信有更合适的方法来解决这个问题，更好地管理来自 IOS10 的预取功能。

if #available(iOS 10.0, *) {
    collectionView!.prefetchingEnabled = false
}