在我的自定义 collectionView 布局中未调用 prepare()

Posted 2023-03-08

【中文标题】在我的自定义 collectionView 布局中未调用 prepare()

【英文标题】：prepare() not called in my custom collectionView layout

【发布时间】：2018-12-13 17:59:26

【问题描述】：

我正在尝试设置自定义 collectionView 布局，感觉可能缺少步骤。

我创建了 collectionView，定义了约束，将布局从流更改为自定义，并选择了我的自定义类。

现在它不工作并且什么都没有显示，所以我正在尝试调试类方法，但我无法让任何打印语句工作，因为似乎没有调用任何函数。我在设置时缺少什么步骤？这是课程：

import UIKit

class CustomCollectionViewLayout: UICollectionViewLayout 

    private var cache = [IndexPath: UICollectionViewLayoutAttributes]()
    private var contentWidth = CGFloat()
    private var visibleLayoutAttributes = [UICollectionViewLayoutAttributes]()
    private var oldBounds = CGRect.zero
    private var collectionViewStartY: CGFloat 
        guard let collectionView = collectionView else 
            return 0
        
        return collectionView.bounds.minY
    
    private var collectionViewHeight: CGFloat 
        return collectionView!.frame.height
    
    override public var collectionViewContentSize: CGSize 
        return CGSize(width: contentWidth, height: collectionViewHeight)
    

    override public func prepare() 
        print("calling prepare") //This is what's not printing
        guard let collectionView = collectionView,
            cache.isEmpty else 
                return
        

        updateInsets()
        collectionView.decelerationRate = .fast
        cache.removeAll(keepingCapacity: true)
        cache = [IndexPath: UICollectionViewLayoutAttributes]()
        oldBounds = collectionView.bounds
        var xOffset: CGFloat = 0
        var cellWidth: CGFloat = 5

        for item in 0 ..< collectionView.numberOfItems(inSection: 0) 
            let cellIndexPath = IndexPath(item: item, section: 0)
            let cellattributes = UICollectionViewLayoutAttributes(forCellWith: cellIndexPath)
            cellattributes.frame = CGRect(x: xOffset, y: 0, width: cellWidth, height: collectionViewHeight)
            xOffset = xOffset + cellWidth
            contentWidth = max(contentWidth,xOffset)
            cache[cellIndexPath] = cellattributes
        
    

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? 
        visibleLayoutAttributes.removeAll(keepingCapacity: true)
        print("layoutattributes called")

        for (_, attributes) in cache 
            visibleLayoutAttributes.append(self.shiftedAttributes(from: attributes))
        
        return visibleLayoutAttributes
    

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? 
        guard let attributes = cache[indexPath] else  fatalError("No attributes cached") 
        print("layoutattributes called")
        return shiftedAttributes(from: attributes)
    
    override public func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool 
        if oldBounds.size != newBounds.size 
            cache.removeAll(keepingCapacity: true)
        
        return true
    

    override func invalidateLayout(with context: UICollectionViewLayoutInvalidationContext) 
        if context.invalidateDataSourceCounts  cache.removeAll(keepingCapacity: true) 
        super.invalidateLayout(with: context)
    

extension CustomCollectionViewLayout 

    func updateInsets() 
        guard let collectionView = collectionView else  return 
        collectionView.contentInset.left = (collectionView.bounds.size.width - 2.5) / 2
        collectionView.contentInset.right = (collectionView.bounds.size.width - 2.5) / 2
    
    override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint 
        guard let collectionView = collectionView else  return super.targetContentOffset(forProposedContentOffset: proposedContentOffset) 
        let midX: CGFloat = collectionView.bounds.size.width / 2
        guard let closestAttribute = findClosestAttributes(toXPosition: proposedContentOffset.x + midX) else  return super.targetContentOffset(forProposedContentOffset: proposedContentOffset) 
        return CGPoint(x: closestAttribute.center.x - midX, y: proposedContentOffset.y)
    

    private func findClosestAttributes(toXPosition xPosition: CGFloat) -> UICollectionViewLayoutAttributes? 
        guard let collectionView = collectionView else  return nil 
        let searchRect = CGRect(
            x: xPosition - collectionView.bounds.width, y: collectionView.bounds.minY,
            width: collectionView.bounds.width * 2, height: collectionView.bounds.height
        )
        let closestAttributes = layoutAttributesForElements(in: searchRect)?.min(by:  abs($0.center.x - xPosition) < abs($1.center.x - xPosition) )
        return closestAttributes
    
    private var continuousFocusedIndex: CGFloat 
        guard let collectionView = collectionView else  return 0 
        let offset = collectionView.bounds.width / 2 + collectionView.contentOffset.x - 2.5 / 2
        return offset / 2.5
    
    private func shiftedAttributes(from attributes: UICollectionViewLayoutAttributes) -> UICollectionViewLayoutAttributes 
        guard let attributes = attributes.copy() as? UICollectionViewLayoutAttributes else  fatalError("Couldn't copy attributes") 
        let roundedFocusedIndex = round(continuousFocusedIndex)
        let focusedItemWidth = CGFloat(20)
        if attributes.indexPath.item == Int(roundedFocusedIndex)
            attributes.transform = CGAffineTransform(scaleX: 10, y: 1)
         else 
            let translationDirection: CGFloat = attributes.indexPath.item < Int(roundedFocusedIndex) ? -1 : 1
            attributes.transform = CGAffineTransform(translationX: translationDirection * 20, y: 0)
        
        return attributes
    

如果类以某种方式无效，是否会阻止调用其方法？在布局上自动执行 prepare() 方法是什么？

这是使用它的视图控制器：

class ViewController: UIViewController, UICollectionViewDelegate,UICollectionViewDataSource 

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int 
        return 10
    

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell 
        let cell = customCollectionView.dequeueReusableCell(withReuseIdentifier: "singleCell", for: indexPath)
        cell.backgroundColor = UIColor.random()
        return cell
    

    @IBOutlet weak var customCollectionView: UICollectionView!

    override func viewDidLoad() 
        super.viewDidLoad()
        customCollectionView.delegate = self

    

【问题讨论】：

为什么不设置数据源委托？还有你在哪里重新加载 collectionView 数据？

【参考方案1】：

您应该提供集合视图的部分数量。否则，假定为 0，因此布局的实现不会开始。

func numberOfSections(in collectionView: UICollectionView) -> Int 
    return 1

同时尝试在视图控制器中调试 collectionView 和 collectionViewLayout 的值

【讨论】：

这并在 viewDidLoad 中将 dataSource 设置为 self 修复了它。谢谢！