使用反向流布局创建一个UICollectionView

Question

我希望创建一个类似于聊天视图的视图，最新的消息会出现在屏幕底部。我希望UICollectionViewCells可以'固定'在屏幕底部 - 与默认流布局相反。我该怎么做？

我从另一个stackoverflow答案中得到了这个代码，但它不能处理可变高度的项目（它需要处理）。

import Foundation
import UIKit

class InvertedStackLayout: UICollectionViewLayout {
    let cellHeight: CGFloat = 100 // Your cell height here...

    override func prepare() {
        super.prepare()
    }

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

        if let collectionView = self.collectionView {
            for section in 0 ..< collectionView.numberOfSections {
                if let numberOfSectionItems = numberOfItemsInSection(section) {
                    for item in 0 ..< numberOfSectionItems {
                        let indexPath = IndexPath(item: item, section: section)
                        let layoutAttr = layoutAttributesForItem(at: indexPath)

                        if let layoutAttr = layoutAttr, layoutAttr.frame.intersects(rect) {
                            layoutAttrs.append(layoutAttr)
                        }
                    }
                }
            }
        }

        return layoutAttrs
    }

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        let layoutAttr = UICollectionViewLayoutAttributes(forCellWith: indexPath)
        let contentSize = self.collectionViewContentSize

        layoutAttr.frame = CGRect(
            x: 0, y: contentSize.height - CGFloat(indexPath.item + 1) * cellHeight,
            width: contentSize.width, height: cellHeight)

        return layoutAttr
    }

    func numberOfItemsInSection(_ section: Int) -> Int? {
        if let collectionView = self.collectionView,
            let numSectionItems = collectionView.dataSource?.collectionView(collectionView, numberOfItemsInSection: section)
        {
            return numSectionItems
        }

        return 0
    }

    override var collectionViewContentSize: CGSize {
        get {
            var height: CGFloat = 0
            var bounds: CGRect = .zero

            if let collectionView = self.collectionView {
                for section in 0 ..< collectionView.numberOfSections {
                    if let numItems = numberOfItemsInSection(section) {
                        height += CGFloat(numItems) * cellHeight
                    }
                }

                bounds = collectionView.bounds
            }

            return CGSize(width: bounds.width, height: max(height, bounds.height))
        }
    }

    override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
        if let oldBounds = self.collectionView?.bounds,
            oldBounds.width != newBounds.width || oldBounds.height != newBounds.height
        {
            return true
        }

        return false
    }
}

- Tometoyou

Answer 1

使用CGAffineTransform将您的collectionView翻转，使其从底部开始。

collectionView.transform = CGAffineTransform(scaleX: 1, y: -1)

现在唯一的问题是所有单元格都倒置了。然后你可以对每个单元格应用变换，将其翻转回来。

class InvertedCollectionViewFlowLayout: UICollectionViewFlowLayout {

    // inverting the transform in the layout, rather than directly on the cell, 
    // is the only way I've found to prevent cells from flipping during animated
    // cell updates

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        let attrs = super.layoutAttributesForItem(at: indexPath)
        attrs?.transform = CGAffineTransform(scaleX: 1, y: -1)
        return attrs
    }

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        let attrsList = super.layoutAttributesForElements(in: rect)
        if let list = attrsList {
            for i in 0..<list.count {
                list[i].transform = CGAffineTransform(scaleX: 1, y: -1)
            }
        }
        return attrsList
    }
}

- Devin Pitcher

---

请确保调整您的数据集以确保其按正确顺序排列。对于任何“scrollToIndex”调用和单元格定位指定，同样适用。 - hexley

Answer 2

使用反向流布局的UICollectionView，具有动态单元格和标题高度。

import Foundation
import UIKit

class InvertedFlowLayout: UICollectionViewFlowLayout {

    override func prepare() {
        super.prepare()
    }

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        guard super.layoutAttributesForElements(in: rect) != nil else { return nil }
        var attributesArrayNew = [UICollectionViewLayoutAttributes]()

        if let collectionView = self.collectionView {
            for section in 0 ..< collectionView.numberOfSections {
                for item in 0 ..< collectionView.numberOfItems(inSection: section) {
                    let indexPathCurrent = IndexPath(item: item, section: section)
                    if let attributeCell = layoutAttributesForItem(at: indexPathCurrent) {
                        if attributeCell.frame.intersects(rect) {
                            attributesArrayNew.append(attributeCell)
                        }
                    }
                }
            }

            for section in 0 ..< collectionView.numberOfSections {
                let indexPathCurrent = IndexPath(item: 0, section: section)
                if let attributeKind = layoutAttributesForSupplementaryView(ofKind: UICollectionView.elementKindSectionHeader, at: indexPathCurrent) {
                    attributesArrayNew.append(attributeKind)
                }
            }
        }

        return attributesArrayNew
    }

    override func layoutAttributesForSupplementaryView(ofKind elementKind: String, at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        let attributeKind = UICollectionViewLayoutAttributes(forSupplementaryViewOfKind: elementKind, with: indexPath)

        if let collectionView = self.collectionView {
            var fullHeight: CGFloat = 0.0

            for section in 0 ..< indexPath.section + 1 {
                for item in 0 ..< collectionView.numberOfItems(inSection: section) {
                    let indexPathCurrent = IndexPath(item: item, section: section)
                    fullHeight += cellHeight(indexPathCurrent) + minimumLineSpacing
                }
            }

            attributeKind.frame = CGRect(x: 0, y: collectionViewContentSize.height - fullHeight - CGFloat(indexPath.section + 1) * headerHeight(indexPath.section) - sectionInset.bottom + minimumLineSpacing/2, width: collectionViewContentSize.width, height: headerHeight(indexPath.section))
        }

        return attributeKind
    }

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        let attributeCell = UICollectionViewLayoutAttributes(forCellWith: indexPath)

        if let collectionView = self.collectionView {
            var fullHeight: CGFloat = 0.0

            for section in 0 ..< indexPath.section + 1 {
                for item in 0 ..< collectionView.numberOfItems(inSection: section) {
                    let indexPathCurrent = IndexPath(item: item, section: section)
                    fullHeight += cellHeight(indexPathCurrent) + minimumLineSpacing

                    if section == indexPath.section && item == indexPath.item {
                        break
                    }
                }
            }

            attributeCell.frame = CGRect(x: 0, y: collectionViewContentSize.height - fullHeight + minimumLineSpacing - CGFloat(indexPath.section) * headerHeight(indexPath.section) - sectionInset.bottom, width: collectionViewContentSize.width, height: cellHeight(indexPath) )
        }

        return attributeCell
    }

    override var collectionViewContentSize: CGSize {
        get {
            var height: CGFloat = 0.0
            var bounds = CGRect.zero

            if let collectionView = self.collectionView {
                for section in 0 ..< collectionView.numberOfSections {
                    for item in 0 ..< collectionView.numberOfItems(inSection: section) {
                        let indexPathCurrent = IndexPath(item: item, section: section)
                        height += cellHeight(indexPathCurrent) + minimumLineSpacing
                    }
                }

                height += sectionInset.bottom + CGFloat(collectionView.numberOfSections) * headerHeight(0)
                bounds = collectionView.bounds
            }

            return CGSize(width: bounds.width, height: max(height, bounds.height))
        }
    }

    override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
        if let oldBounds = self.collectionView?.bounds,
            oldBounds.width != newBounds.width || oldBounds.height != newBounds.height {
            return true
        }

        return false
    }

    func cellHeight(_ indexPath: IndexPath) -> CGFloat {
        if let collectionView = self.collectionView, let delegateFlowLayout = collectionView.delegate as? UICollectionViewDelegateFlowLayout {
            let size = delegateFlowLayout.collectionView!(collectionView, layout: self, sizeForItemAt: indexPath)
            return size.height
        }

        return 0
    }

    func headerHeight(_ section: Int) -> CGFloat {
        if let collectionView = self.collectionView, let delegateFlowLayout = collectionView.delegate as? UICollectionViewDelegateFlowLayout {
            let size = delegateFlowLayout.collectionView!(collectionView, layout: self, referenceSizeForHeaderInSection: section)
            return size.height
        }

        return 0
    }
}

- Karen Hovhannisyan

---

之后如何使用？ - Jeff Bootsholz

Answer 3

你可以将collectionView旋转（transform）180度，这样顶部就会在底部，然后将每个单元格旋转（transform）180度，使它们看起来正常。

- Denys