如何通过CollectionView中的Flow Layout将单元格对齐到顶部
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在这段代码中，我试图改变UICollectionView的第一个单元格的大小和其他具有相同大小的单元格，但是在第一行中，当我想要两个单元格出来时，只有一个单元格出现：
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)
}
}
我真正想要的是这个：
答案
你需要实现一个UICollectionViewLayout，我把它称为FillingLayout，注意你可以使用delegate方法调整列数和大单元格的大小
说明
你需要添加一个数组来跟踪列的高度，看看private var columsHeights : [CGFloat] = []最短的列是什么，你还需要一个(Int,Float)元组数组来保持可填充的空格，我还在委托中添加了一个方法来获取我们在集合View中想要的列数以及知道是否可以根据单元大小添加单元格的方法。
然后，如果我们要添加一个单元格，我们检查是否可以添加，因为第一列已填充，我们在avaiableSpaces数组中添加对应于column2的空格，当我们首先添加下一个单元格时，我们检查是否可以添加任何单元格如果可以添加可用空间，我们添加和删除可用空间。
这是完整的代码
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
}
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