如何通過CollectionView中的Flow Layout將單元格對齊到頂部

Question

在這段代碼中，我試圖改變UICollectionView的第一個單元格的大小和其他具有相同大小的單元格，但是在第一行中，當我想要兩個單元格出來時，只有一個單元格出現：

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout:
    if indexPath.row == 0 {
        return CGSize(width: collectionView.frame.width/1.5-2, height: collectionView.frame.width/1.5-2)
    }
    else {
        return CGSize(width: collectionView.frame.width/3.0-3, height: collectionView.frame.width/3.0-3)
    }
}

我真正想要的是這個：

Answer 1

這不是您問題的准確答案，但也許它會對您有所幫助。

這是一個非常好的教程：

https://www.raywenderlich.com/164608/uicollectionview-custom-layout-tutorial-pinterest-2

項目＃6我自己移交：

// 6. Updates the collection view content height
        contentHeight = max(contentHeight, frame.maxY)
        yOffset[column] = yOffset[column] + height

        //Only for 2 columns
        if yOffset[1] >= yOffset[0] {
            column = 0
        } else {
            column = 1
        }

Answer 2

你需要實現一個UICollectionViewLayout ，我把它叫做FillingLayout ，注意你可以用delegate方法調整列的數量和大單元的大小

說明

你需要添加一個數組來跟蹤列的高度，看看什么是最短的列是private var columsHeights : [CGFloat] = []你需要一個(Int,Float)元組的數組來保持哪些空格可用於為了填充，我還在委托中添加了一個方法來獲取集合視圖中我們想要的列數，以及一個方法來了解是否可以根據大小添加單元格。

然后，如果我們要添加一個單元格，我們檢查是否可以添加，因為第一列已填充，我們在avaiableSpaces數組中添加對應於column2的空格，當我們首先添加下一個單元格時，我們檢查是否可以添加任何單元格如果可以添加可用空間，我們添加和刪除可用空間。

這是完整的代碼

import UIKit

protocol FillingLayoutDelegate: class {
    func collectionView(_ collectionView:UICollectionView, sizeForViewAtIndexPath indexPath:IndexPath) -> Int
    //  Returns the amount of columns that have to display at that moment
    func numberOfColumnsInCollectionView(collectionView:UICollectionView) ->Int
}

class FillingLayout: UICollectionViewLayout {
    weak var delegate: FillingLayoutDelegate!

    fileprivate var cellPadding: CGFloat = 10

    fileprivate var cache = [UICollectionViewLayoutAttributes]()

    fileprivate var contentHeight: CGFloat = 0
    private var columsHeights : [CGFloat] = []
    private var avaiableSpaces : [(Int,CGFloat)] = []

    fileprivate var contentWidth: CGFloat {
        guard let collectionView = collectionView else {
            return 0
        }
        let insets = collectionView.contentInset
        return collectionView.bounds.width - (insets.left + insets.right)
    }

    var columnsQuantity : Int{
        get{
            if(self.delegate != nil)
            {
                return (self.delegate?.numberOfColumnsInCollectionView(collectionView: self.collectionView!))!
            }
            return 0
        }
    }

    //MARK: PRIVATE METHODS
    private func shortestColumnIndex() -> Int{
        var retVal : Int = 0
        var shortestValue = MAXFLOAT

        var i = 0
        for columnHeight in columsHeights {
            //debugPrint("Column Height: \(columnHeight) index: \(i)")
            if(Float(columnHeight) < shortestValue)
            {
                shortestValue = Float(columnHeight)
                retVal = i
            }
            i += 1
        }
        //debugPrint("shortest Column index: \(retVal)")
        return retVal
    }

    //MARK: PRIVATE METHODS
    private func largestColumnIndex() -> Int{
        var retVal : Int = 0
        var largestValue : Float = 0.0

        var i = 0
        for columnHeight in columsHeights {
            //debugPrint("Column Height: \(columnHeight) index: \(i)")
            if(Float(columnHeight) > largestValue)
            {
                largestValue = Float(columnHeight)
                retVal = i
            }
            i += 1
        }
        //debugPrint("shortest Column index: \(retVal)")
        return retVal
    }

    private func canUseBigColumnOnIndex(columnIndex:Int,size:Int) ->Bool
    {
        if(columnIndex < self.columnsQuantity - (size-1))
        {
            let firstColumnHeight = columsHeights[columnIndex]
            for i in columnIndex..<columnIndex + size{
                if(firstColumnHeight != columsHeights[i]) {
                    return false
                }
            }
            return true
        }

        return false
    }

    override var collectionViewContentSize: CGSize {
        return CGSize(width: contentWidth, height: contentHeight)
    }

    override func prepare() {
        // Check if cache is empty
        guard cache.isEmpty == true, let collectionView = collectionView else {
            return
        }

        //  Set all column heights to 0
        self.columsHeights = []
        for _ in 0..<self.columnsQuantity {
            self.columsHeights.append(0)
        }

        for item in 0 ..< collectionView.numberOfItems(inSection: 0) {

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

            let viewSize: Int = delegate.collectionView(collectionView, sizeForViewAtIndexPath: indexPath)
            let blockWidth = (contentWidth/CGFloat(columnsQuantity))
            let width = blockWidth * CGFloat(viewSize)
            let height = width

            var columIndex = self.shortestColumnIndex()
            var xOffset = (contentWidth/CGFloat(columnsQuantity)) * CGFloat(columIndex)
            var yOffset = self.columsHeights[columIndex]

            if(viewSize > 1){//Big Cell
                if(!self.canUseBigColumnOnIndex(columnIndex: columIndex,size: viewSize)){
                    //  Set column height
                    for i in columIndex..<columIndex + viewSize{
                        if(i < columnsQuantity){
                            self.avaiableSpaces.append((i,yOffset))
                            self.columsHeights[i] += blockWidth
                        }
                    }
                    //  Set column height
                    yOffset = columsHeights[largestColumnIndex()]
                    xOffset = 0
                    columIndex = 0
                }

                for i in columIndex..<columIndex + viewSize{
                    if(i < columnsQuantity){
                        //current height
                        let currValue = self.columsHeights[i]
                        //new column height with the update
                        let newValue = yOffset + height
                        //space that will remaing in blank, this must be 0 if its ok
                        let remainder = (newValue - currValue) - CGFloat(viewSize) * blockWidth
                        if(remainder > 0) {
                            debugPrint("Its bigger remainder is \(remainder)")
                            //number of spaces to fill
                            let spacesTofillInColumn = Int(remainder/blockWidth)
                            //we need to add those spaces as avaiableSpaces
                            for j in 0..<spacesTofillInColumn {
                                self.avaiableSpaces.append((i,currValue + (CGFloat(j)*blockWidth)))
                            }
                        }
                        self.columsHeights[i] = yOffset + height
                    }
                }
            }else{
                //if there is not avaiable space
                if(self.avaiableSpaces.count == 0)
                {
                    //  Set column height
                    self.columsHeights[columIndex] += height
                }else{//if there is some avaiable space
                    yOffset = self.avaiableSpaces.first!.1
                    xOffset = CGFloat(self.avaiableSpaces.first!.0) * width
                    self.avaiableSpaces.remove(at: 0)
                }
            }

            print(width)

            let frame = CGRect(x: xOffset, y: yOffset, width: width, height: height)
            let insetFrame = frame.insetBy(dx: cellPadding, dy: cellPadding)

            let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)
            attributes.frame = insetFrame
            cache.append(attributes)

            contentHeight = max(contentHeight, frame.maxY)
        }
    }

    func getNextCellSize(currentCell: Int, collectionView: UICollectionView) -> Int {
        var nextViewSize = 0
        if currentCell < (collectionView.numberOfItems(inSection: 0) - 1) {
            nextViewSize = delegate.collectionView(collectionView, sizeForViewAtIndexPath: IndexPath(item: currentCell + 1, section: 0))
        }
        return nextViewSize
    }

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

        var visibleLayoutAttributes = [UICollectionViewLayoutAttributes]()

        // Loop through the cache and look for items in the rect
        for attributes in cache {
            if attributes.frame.intersects(rect) {
                visibleLayoutAttributes.append(attributes)
            }
        }
        return visibleLayoutAttributes
    }

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        return cache[indexPath.item]
    }
}

更新

您需要將viewController設置為FillingLayoutDelegate

override func viewDidLoad() {
    super.viewDidLoad()

    collectionView.delegate = self
    collectionView.dataSource = self

    // Do any additional setup after loading the view.
    if let layout = self.collectionView.collectionViewLayout as? FillingLayout
    {
        layout.delegate = self
    }

}

ViewController中的FillingLayoutDelegate實現

extension ViewController: FillingLayoutDelegate{
func collectionView(_ collectionView:UICollectionView,sizeForViewAtIndexPath indexPath:IndexPath) ->Int{
        if(indexPath.row == 0 || indexPath.row == 4)
        {
            return 2
        }

        if(indexPath.row == 5)
        {
            return 3
        }

        return 1
    }

    func numberOfColumnsInCollectionView(collectionView:UICollectionView) ->Int{
        return 3
    }
}

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