使用自定义流程布局时，UICollectionView不显示页脚

编程问答 2022-06-06

问题描述

我有一个UICollectionView，并且启用了页眉和页脚：

enter image description here

在此处设置页脚的高度：

func collectionView(_ collectionView: UICollectionView,layout collectionViewLayout: UICollectionViewLayout,referenceSizeforFooterInSection section: Int) -> CGSize {
    if isLoading {
        return CGSize.zero
    }
    return CGSize(width: collectionView.bounds.size.width,height: 55)
}

...然后在此处设置页眉和页脚：

 func collectionView(_ collectionView: UICollectionView,viewForSupplementaryElementOfKind kind: String,at indexPath: IndexPath) -> UICollectionReusableView {
    
    if kind == UICollectionView.elementKindSectionFooter {
        let aFooterView = collectionView.dequeueReusableSupplementaryView(ofKind: kind,withReuseIdentifier: footerViewReuseIdentifier,for: indexPath) as! CustomFooterView
        self.footerView = aFooterView
        self.footerView?.backgroundColor = UIColor.green
        return aFooterView
    } else {
        let headerView = collectionView.dequeueReusableSupplementaryView(ofKind: UICollectionView.elementKindSectionHeader,withReuseIdentifier: "HeaderView",for: indexPath) as! CollectionViewHeader
          return headerView
    }
}

并在此处注册它们：

collectionView?.register(CollectionViewHeader.self,forSupplementaryViewOfKind: UICollectionView.elementKindSectionHeader,withReuseIdentifier: "HeaderView")
    

collectionView?.register(UINib(nibName: "CustomFooterView",bundle: nil),forSupplementaryViewOfKind: UICollectionView.elementKindSectionFooter,withReuseIdentifier: footerViewReuseIdentifier)

当我将集合视图设置为使用默认布局时，可以看到页眉和页脚。当我将其设置为下面的自定义布局时，我只会看到页眉，而看不到页脚，并且为什么会这样：

class ColumnFlowLayout: UICollectionViewFlowLayout {
    
    private let minColumnWidth: CGFloat = 300.0
    private let cellHeight: CGFloat = 184.0
    
    private var deletingIndexPaths = [IndexPath]()
    private var insertingIndexPaths = [IndexPath]()
    
    // MARK: Layout Overrides
    
    /// - Tag: ColumnFlowExample
    override func prepare() {
        super.prepare()
        
        guard let collectionView = collectionView else { return }
        
        let availableWidth = collectionView.bounds.inset(by: collectionView.layoutMargins).width
        let maxnumColumns = Int(availableWidth / minColumnWidth)
        let cellWidth = (availableWidth / CGFloat(maxnumColumns)).rounded(.down)
        
        self.itemSize = CGSize(width: cellWidth,height: cellHeight)
        self.sectionInset = UIEdgeInsets(top: self.minimumInteritemSpacing,left: 0.0,bottom: 0.0,right: 0.0)
        self.sectionInsetReference = .fromSafeArea
    }
    
    // MARK: Attributes for Updated Items
    
    override func finalLayoutAttributesFordisappearingItem(at itemIndexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        guard let attributes = super.finalLayoutAttributesFordisappearingItem(at: itemIndexPath) else { return nil }
        
        if !deletingIndexPaths.isEmpty {
            if deletingIndexPaths.contains(itemIndexPath) {
                
                attributes.transform = CGAffineTransform(scaleX: 0.5,y: 0.5)
                attributes.alpha = 0.0
                attributes.zIndex = 0
            }
        }
        
        return attributes
    }
    
    override func initialLayoutAttributesForAppearingItem(at itemIndexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        guard let attributes = super.initialLayoutAttributesForAppearingItem(at: itemIndexPath) else { return nil }
        
        if insertingIndexPaths.contains(itemIndexPath) {
            attributes.transform = CGAffineTransform(scaleX: 0.5,y: 0.5)
            attributes.alpha = 0.0
            attributes.zIndex = 0
        }
        
        return attributes
    }
    
    // MARK: Updates
    
    override func prepare(forCollectionViewUpdates updateItems: [UICollectionViewUpdateItem]) {
        super.prepare(forCollectionViewUpdates: updateItems)
        
        for update in updateItems {
            switch update.updateAction {
            case .delete:
                guard let indexPath = update.indexPathBeforeUpdate else { return }
                deletingIndexPaths.append(indexPath)
            case .insert:
                guard let indexPath = update.indexPathAfterUpdate else { return }
                insertingIndexPaths.append(indexPath)
            default:
                break
            }
        }
    }
    
    override func finalizeCollectionViewUpdates() {
        super.finalizeCollectionViewUpdates()
        
        deletingIndexPaths.removeAll()
        insertingIndexPaths.removeAll()
    }
}

EDIT （出于某些原因，如果我设置了 collectionView.collectionViewLayout = ColumnFlowLayout()，页脚为零，但如果我未将布局设置为ColumnFlowLayout，则页脚分配正确。

编辑2 可以在此委托方法中确认这一点：

 func collectionView(_ collectionView: UICollectionView,for: indexPath) as! CollectionViewHeader
          return headerView
    }
}

当我有：

collectionView.collectionViewLayout = ColumnFlowLayout()，

程序永远无法到达 if kind == UICollectionView.elementKindSectionFooter，但确实可以解决collectionView.collectionViewLayout = ColumnFlowLayout()被注释掉的情况。

解决方法

弄清楚了；需要在ColumnFlowLayout实现中手动设置页脚高度，如下所示：

    self.footerReferenceSize = CGSize(width: w,height: h)
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