ios 如何在 UICollectionView 中居中行？

Question

提问by deycall

I have a UICollectionViewwith random cells. Is there any method that allows me to center rows?

我有一个UICollectionView随机单元格。有什么方法可以让我居中行吗？

This is how it looks by default:

这是默认情况下的外观：

[ x x x x x x ]  
[ x x x x x x ]  
[ x x         ]

Here is what the desired layout looks like:

这是所需的布局：

[ x x x x x ]  
[ x x x x x ]  
[    x x    ]

Answer 1

采纳答案by lxt

A bit of background first - a UICollectionViewis combined with a UICollectionViewLayoutwhich determines how the cells get placed in the view. This means a collection view is very flexible (you can create almost any layout with it), but also means modifying layouts can be a little confusing.

首先介绍一点背景 - aUICollectionView与 a 结合，UICollectionViewLayout这决定了单元格如何放置在视图中。这意味着集合视图非常灵活（您几乎可以用它创建任何布局），但也意味着修改布局可能会有些混乱。

Creating an entirely new layout class is complex, so instead you want to try and modify the default layout (UICollectionViewFlowLayout) to get your center alignment. To make it even simpler, you probably want to avoid subclassing the flow layout itself.

创建一个全新的布局类很复杂，因此您需要尝试修改默认布局 ( UICollectionViewFlowLayout) 以获得中心对齐。为了使它更简单，您可能希望避免对流布局本身进行子类化。

Here's one approach (it may not be the best approach, but it's the first one I can think of) - split your cells into two sections, as follows:

这是一种方法（它可能不是最好的方法，但它是我能想到的第一种方法） - 将您的单元格分成两部分，如下所示：

[ x x x x x ] <-- Section 1 
[ x x x x x ] <-- Section 1 
[    x x    ] <-- Section 2

This should be fairly straightforward, provided you know the width of your scroll view and the number of cells that can fit in each row.

这应该相当简单，只要您知道滚动视图的宽度和每行可以容纳的单元格数量。

Then, use the collectionView:layout:insetForSectionAtIndex:delegate method to set the margins for your second section so as it appears to be vertically centered. Once you've done this, you just need to ensure you recompute the appropriate section divisions/insets so both portrait and landscape orientations can be supported.

然后，使用collectionView:layout:insetForSectionAtIndex:委托方法设置第二部分的边距，使其看起来垂直居中。完成此操作后，您只需要确保重新计算适当的部分分区/插图，以便支持纵向和横向方向。

There is a somewhat similar question here - How to center align the cells of a UICollectionView?- that goes into more details about the inset methods, although it's not quite trying to do the same thing that you are.

这里有一个有点类似的问题 -如何将 UICollectionView 的单元格居中对齐？- 详细介绍了 inset 方法，尽管它并没有尝试做与您相同的事情。

Answer 2

回答by Kyle Truscott

I had to do something like this, but needed all the cells in the one section. It was pretty simple to extend UICollectionViewFlowLayoutto center cells. I made a pod:

我不得不做这样的事情，但需要一个部分中的所有单元格。扩展UICollectionViewFlowLayout到中心单元非常简单。我做了一个豆荚：

https://github.com/keighl/KTCenterFlowLayout

enter image description here

在此处输入图片说明

Answer 3

回答by Imanou Petit

With Swift 4.1 and iOS 11, according to your needs, you may choose one of the 2 following complete implementationsin order to fix your problem.

对于 Swift 4.1 和 iOS 11，您可以根据您的需要，选择以下 2 个完整实现之一来解决您的问题。

#1. Center `UICollectionViewCell`s with fixed size

#1. `UICollectionViewCell`固定大小的中心

The implementation below shows how to use UICollectionViewLayout's layoutAttributesForElements(in:)and UICollectionViewFlowLayout's itemSizein order to center the cells of a UICollectionView:

下面的实现显示了如何使用UICollectionViewLayout'slayoutAttributesForElements(in:)和UICollectionViewFlowLayout'sitemSize来使 a 的单元格居中UICollectionView：

CollectionViewController.swift

import UIKit

class CollectionViewController: UICollectionViewController {

    let columnLayout = FlowLayout(
        itemSize: CGSize(width: 140, height: 140),
        minimumInteritemSpacing: 10,
        minimumLineSpacing: 10,
        sectionInset: UIEdgeInsets(top: 10, left: 10, bottom: 10, right: 10)
    )

    override func viewDidLoad() {
        super.viewDidLoad()

        title = "Center cells"

        collectionView?.collectionViewLayout = columnLayout
        collectionView?.contentInsetAdjustmentBehavior = .always
        collectionView?.register(CollectionViewCell.self, forCellWithReuseIdentifier: "Cell")
    }

    override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 7
    }

    override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath) as! CollectionViewCell
        return cell
    }

    override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
        collectionView?.collectionViewLayout.invalidateLayout()
        super.viewWillTransition(to: size, with: coordinator)
    }

}

FlowLayout.swift

import UIKit

class FlowLayout: UICollectionViewFlowLayout {

    required init(itemSize: CGSize, minimumInteritemSpacing: CGFloat = 0, minimumLineSpacing: CGFloat = 0, sectionInset: UIEdgeInsets = .zero) {
        super.init()

        self.itemSize = itemSize
        self.minimumInteritemSpacing = minimumInteritemSpacing
        self.minimumLineSpacing = minimumLineSpacing
        self.sectionInset = sectionInset
        sectionInsetReference = .fromSafeArea
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        let layoutAttributes = super.layoutAttributesForElements(in: rect)!.map { import UIKit

class CollectionViewCell: UICollectionViewCell {

    override init(frame: CGRect) {
        super.init(frame: frame)

        contentView.backgroundColor = .cyan
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

}
.copy() as! UICollectionViewLayoutAttributes }
        guard scrollDirection == .vertical else { return layoutAttributes }

        // Filter attributes to compute only cell attributes
        let cellAttributes = layoutAttributes.filter({ import UIKit

class CollectionViewController: UICollectionViewController {

    let array = ["1", "1 2", "1 2 3 4 5 6 7 8", "1 2 3 4 5 6 7 8 9 10 11", "1 2 3", "1 2 3 4", "1 2 3 4 5 6", "1 2 3 4 5 6 7 8 9 10", "1 2 3 4", "1 2 3 4 5 6 7", "1 2 3 4 5 6 7 8 9", "1", "1 2 3 4 5", "1", "1 2 3 4 5 6"]

    let columnLayout = FlowLayout(
        minimumInteritemSpacing: 10,
        minimumLineSpacing: 10,
        sectionInset: UIEdgeInsets(top: 10, left: 10, bottom: 10, right: 10)
    )

    override func viewDidLoad() {
        super.viewDidLoad()

        title = "Center cells"

        collectionView?.collectionViewLayout = columnLayout
        collectionView?.contentInsetAdjustmentBehavior = .always
        collectionView?.register(CollectionViewCell.self, forCellWithReuseIdentifier: "Cell")
    }

    override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return array.count
    }

    override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath) as! CollectionViewCell
        cell.label.text = array[indexPath.row]
        return cell
    }

    override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
        collectionView?.collectionViewLayout.invalidateLayout()
        super.viewWillTransition(to: size, with: coordinator)
    }

}
.representedElementCategory == .cell })

        // Group cell attributes by row (cells with same vertical center) and loop on those groups
        for (_, attributes) in Dictionary(grouping: cellAttributes, by: { (import UIKit

class FlowLayout: UICollectionViewFlowLayout {

    required init(minimumInteritemSpacing: CGFloat = 0, minimumLineSpacing: CGFloat = 0, sectionInset: UIEdgeInsets = .zero) {
        super.init()

        estimatedItemSize = UICollectionViewFlowLayoutAutomaticSize
        self.minimumInteritemSpacing = minimumInteritemSpacing
        self.minimumLineSpacing = minimumLineSpacing
        self.sectionInset = sectionInset
        sectionInsetReference = .fromSafeArea
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        let layoutAttributes = super.layoutAttributesForElements(in: rect)!.map { import UIKit

class CollectionViewCell: UICollectionViewCell {

    let label = UILabel()

    override init(frame: CGRect) {
        super.init(frame: frame)

        contentView.backgroundColor = .orange
        label.preferredMaxLayoutWidth = 120
        label.numberOfLines = 0

        contentView.addSubview(label)
        label.translatesAutoresizingMaskIntoConstraints = false
        contentView.layoutMarginsGuide.topAnchor.constraint(equalTo: label.topAnchor).isActive = true
        contentView.layoutMarginsGuide.leadingAnchor.constraint(equalTo: label.leadingAnchor).isActive = true
        contentView.layoutMarginsGuide.trailingAnchor.constraint(equalTo: label.trailingAnchor).isActive = true
        contentView.layoutMarginsGuide.bottomAnchor.constraint(equalTo: label.bottomAnchor).isActive = true
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

}
.copy() as! UICollectionViewLayoutAttributes }
        guard scrollDirection == .vertical else { return layoutAttributes }

        // Filter attributes to compute only cell attributes
        let cellAttributes = layoutAttributes.filter({ - (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
    NSArray *attributes = [super layoutAttributesForElementsInRect:rect];

    for (UICollectionViewLayoutAttributes *attribute in attributes) {
        NSInteger itemCount = [self.collectionView.dataSource collectionView:self.collectionView
                                                      numberOfItemsInSection:attribute.indexPath.section];
        if (itemCount % 2 == 1 && attribute.indexPath.item == itemCount - 1) {
            CGRect originalFrame = attribute.frame;
            attribute.frame = CGRectMake(self.collectionView.bounds.size.width/2-originalFrame.size.width/2,
                                         originalFrame.origin.y,
                                         originalFrame.size.width,
                                         originalFrame.size.height);
        }
    }

    return attributes;
}
.representedElementCategory == .cell })

        // Group cell attributes by row (cells with same vertical center) and loop on those groups
        for (_, attributes) in Dictionary(grouping: cellAttributes, by: { (/**
 * A simple `UICollectionViewFlowLayout` subclass that would make sure the items are center-aligned in the collection view, when scrolling vertically.
 */
class UICollectionViewFlowCenterLayout: UICollectionViewFlowLayout {

    override func layoutAttributesForElementsInRect(rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        guard let suggestedAttributes = super.layoutAttributesForElementsInRect(rect) else { return nil }

        guard scrollDirection == .Vertical else { return suggestedAttributes }

        var newAttributes: [UICollectionViewLayoutAttributes] = []

        /// We will collect items for each row in this array
        var currentRowAttributes: [UICollectionViewLayoutAttributes] = []
        /// We will use this variable to detect new rows when iterating over items
        var yOffset:CGFloat = sectionInset.top
        for attributes in suggestedAttributes {
            /// If we happen to run into a new row...
            if attributes.frame.origin.y != yOffset {
                /*
                 * Update layout of all items in the previous row and add them to the resulting array
                 */
                centerSingleRowWithItemsAttributes(&currentRowAttributes, rect: rect)
                newAttributes += currentRowAttributes
                /*
                 * Reset the accumulated values for the new row
                 */
                currentRowAttributes = []
                yOffset = attributes.frame.origin.y
            }
            currentRowAttributes += [attributes]
        }
        /*
         * Update the layout of the last row.
         */
        centerSingleRowWithItemsAttributes(&currentRowAttributes, rect: rect)
        newAttributes += currentRowAttributes

        return newAttributes
    }

    /**
     Updates the attributes for items, so that they are center-aligned in the given rect.

     - parameter attributes: Attributes of the items
     - parameter rect:       Bounding rect
     */
    private func centerSingleRowWithItemsAttributes(inout attributes: [UICollectionViewLayoutAttributes], rect: CGRect) {
        guard let item = attributes.last else { return }

        let itemsCount = CGFloat(attributes.count)
        let sideInsets = rect.width - (item.frame.width * itemsCount) - (minimumInteritemSpacing * (itemsCount - 1))
        var leftOffset = sideInsets / 2

        for attribute in attributes {
            attribute.frame.origin.x = leftOffset
            leftOffset += attribute.frame.width + minimumInteritemSpacing
        }
    }
}
.center.y / 10).rounded(.up) * 10 }) {
            // Get the total width of the cells on the same row
            let cellsTotalWidth = attributes.reduce(CGFloat(0)) { (partialWidth, attribute) -> CGFloat in
                partialWidth + attribute.size.width
            }

            // Calculate the initial left inset
            let totalInset = collectionView!.safeAreaLayoutGuide.layoutFrame.width - cellsTotalWidth - sectionInset.left - sectionInset.right - minimumInteritemSpacing * CGFloat(attributes.count - 1)
            var leftInset = (totalInset / 2 * 10).rounded(.down) / 10 + sectionInset.left

            // Loop on cells to adjust each cell's origin and prepare leftInset for the next cell
            for attribute in attributes {
                attribute.frame.origin.x = leftInset
                leftInset = attribute.frame.maxX + minimumInteritemSpacing
            }
        }

        return layoutAttributes
    }

}
.center.y / 10).rounded(.up) * 10 }) {
            // Get the total width of the cells on the same row
            let cellsTotalWidth = attributes.reduce(CGFloat(0)) { (partialWidth, attribute) -> CGFloat in
                partialWidth + attribute.size.width
            }

            // Calculate the initial left inset
            let totalInset = collectionView!.safeAreaLayoutGuide.layoutFrame.width - cellsTotalWidth - sectionInset.left - sectionInset.right - minimumInteritemSpacing * CGFloat(attributes.count - 1)
            var leftInset = (totalInset / 2 * 10).rounded(.down) / 10 + sectionInset.left

            // Loop on cells to adjust each cell's origin and prepare leftInset for the next cell
            for attribute in attributes {
                attribute.frame.origin.x = leftInset
                leftInset = attribute.frame.maxX + minimumInteritemSpacing
            }
        }

        return layoutAttributes
    }

}

CollectionViewCell.swift

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {

    NSArray *attributesForElementsInRect = [super layoutAttributesForElementsInRect:rect];
    NSMutableArray *newAttributesForElementsInRect = [[NSMutableArray alloc] initWithCapacity:attributesForElementsInRect.count];

    CGFloat leftMargin = self.sectionInset.left;
    NSMutableArray *lines = [NSMutableArray array];
    NSMutableArray *currLine = [NSMutableArray array];

    for (UICollectionViewLayoutAttributes *attributes in attributesForElementsInRect) {
        // Handle new line
        BOOL newLine = attributes.frame.origin.x <= leftMargin;
        if (newLine) {
            leftMargin = self.sectionInset.left; //will add outside loop
            currLine = [NSMutableArray arrayWithObject:attributes];
        } else {
            [currLine addObject:attributes];
        }

        if ([lines indexOfObject:currLine] == NSNotFound) {
            [lines addObject:currLine];
        }

        // Align to the left
        CGRect newLeftAlignedFrame = attributes.frame;
        newLeftAlignedFrame.origin.x = leftMargin;
        attributes.frame = newLeftAlignedFrame;

        leftMargin += attributes.frame.size.width + self.minimumInteritemSpacing;
        [newAttributesForElementsInRect addObject:attributes];
    }

    // Center left aligned lines
    for (NSArray *line in lines) {
        UICollectionViewLayoutAttributes *lastAttributes = line.lastObject;
        CGFloat space = CGRectGetWidth(self.collectionView.frame) - CGRectGetMaxX(lastAttributes.frame);

        for (UICollectionViewLayoutAttributes *attributes in line) {
            CGRect newFrame = attributes.frame;
            newFrame.origin.x = newFrame.origin.x + space / 2;
            attributes.frame = newFrame;

        }
    }

    return newAttributesForElementsInRect;
}

Expected result:

预期结果：

#2. Center autoresizing `UICollectionViewCell`s

#2. 中心自动尺寸`UICollectionViewCell`小号

The implementation below shows how to use UICollectionViewLayout's layoutAttributesForElements(in:), UICollectionViewFlowLayout's estimatedItemSizeand UILabel's preferredMaxLayoutWidthin order to center the cells of a UICollectionView:

下面的实现展示了如何使用UICollectionViewLayout's layoutAttributesForElements(in:)、UICollectionViewFlowLayout'sestimatedItemSize和UILabel'spreferredMaxLayoutWidth来使 a 的单元格居中UICollectionView：

CollectionViewController.swift

class UICollectionViewFlowCenterLayout: UICollectionViewFlowLayout {
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        guard let suggestedAttributes = super.layoutAttributesForElements(in: rect) else { return nil }

        guard scrollDirection == .vertical else { return suggestedAttributes }

        var newAttributes: [UICollectionViewLayoutAttributes] = []

        var currentRowAttributes: [UICollectionViewLayoutAttributes] = []
        var yOffset:CGFloat = sectionInset.top
        for attributes in suggestedAttributes {
            if attributes.frame.origin.y != yOffset {
                centerSingleRowWithItemsAttributes(attributes: &currentRowAttributes, rect: rect)
                newAttributes += currentRowAttributes
                currentRowAttributes = []
                yOffset = attributes.frame.origin.y
            }
            currentRowAttributes += [attributes]
        }
        centerSingleRowWithItemsAttributes(attributes: &currentRowAttributes, rect: rect)
        newAttributes += currentRowAttributes

        return newAttributes
    }


    private func centerSingleRowWithItemsAttributes( attributes: inout [UICollectionViewLayoutAttributes], rect: CGRect) {
        guard let item = attributes.last else { return }

        let itemsCount = CGFloat(attributes.count)
        let sideInsets = rect.width - (item.frame.width * itemsCount) - (minimumInteritemSpacing * (itemsCount - 1))
        var leftOffset = sideInsets / 2

        for attribute in attributes {
            attribute.frame.origin.x = leftOffset
            leftOffset += attribute.frame.width + minimumInteritemSpacing
        }
    }
}

FlowLayout.swift

 //
    //  CellAllignmentFlowLayout.swift
    //  UICollectionView
    //
    //  Created by rajeshkumar Lingavel on 8/11/15.
    //  Copyright ? 2015 rajeshkumar Lingavel. All rights reserved.
    //

import UIKit
enum   SZAlignment:Int {
    case Center,
        left,
        Right
}
class CellAllignmentFlowLayout: UICollectionViewFlowLayout {
    var alignment:SZAlignment!
    var padding:CGFloat!
    override func layoutAttributesForElementsInRect(rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
//        NSArray *allAttributesInRect = [super
//            layoutAttributesForElementsInRect:rect];

        let allAttributesInRect:NSArray = super.layoutAttributesForElementsInRect(rect)!
        var changedAttributes:NSArray = NSArray()
        switch(alignment.rawValue){
            case 0:
               changedAttributes  =   alignCenter(allAttributesInRect)
            case 1:
                changedAttributes =  alignLeft(allAttributesInRect)
            case 2:
                changedAttributes =  alignRight(allAttributesInRect)
            default:
                assertionFailure("No Direction")
        }

        return changedAttributes as? [UICollectionViewLayoutAttributes]
    }


   private func alignCenter(allAttributesInRect:NSArray) -> NSArray{


        let numberOfSection:Int = (self.collectionView?.numberOfSections())!

        let redefiendArray = NSMutableArray()

        for  i in 0 ..< numberOfSection {

                let thisSectionObjects = sectionObjects(allAttributesInRect, section: i)
                let totalLines = numberOfLines(thisSectionObjects, section: i)
                let lastrowObjects = lastRow(thisSectionObjects, numberOfRows: totalLines, section: i)
                let lastRowObjectsRow =  setMiddleTheLastRow(lastrowObjects)
                let start = (thisSectionObjects.count - lastrowObjects.count)

                for j in start..<thisSectionObjects.count{
                    thisSectionObjects.replaceObjectAtIndex(j, withObject: lastRowObjectsRow.objectAtIndex(j - start))
                }
            redefiendArray.addObjectsFromArray(thisSectionObjects as [AnyObject])
        }




        return redefiendArray
    }
   private func alignLeft(allAttributesInRect:NSArray) -> NSArray{


        return allAttributesInRect;

    }
   private func alignRight(allAttributesInRect:NSArray) -> NSArray{
        return allAttributesInRect;

    }

   private func getTotalLenthOftheSection(section:Int,allAttributesInRect:NSArray) -> CGFloat{

        var totalLength:CGFloat = 0.0
        totalLength = totalLength + (CGFloat (((self.collectionView?.numberOfItemsInSection(section))! - 1)) * padding)
        for  attributes in allAttributesInRect {

            if(attributes.indexPath.section == section){
                totalLength = totalLength + attributes.frame.width
            }
        }

        return totalLength
    }

   private func numberOfLines(allAttributesInRect:NSArray,section:Int)-> Int{
        var totalLines:Int = 0
        for  attributes in allAttributesInRect {
            if(attributes.indexPath.section == section){
                if (attributes.frame.origin.x == self.sectionInset.left){
                    totalLines = totalLines + 1
                }
            }
        }
        return totalLines
    }
   private func sectionObjects(allAttributesInRect:NSArray,section:Int) -> NSMutableArray{
        let objects:NSMutableArray = NSMutableArray()
        for  attributes in allAttributesInRect {
            if(attributes.indexPath.section == section){
                objects.addObject(attributes)
            }
        }
        return objects
    }

   private func lastRow(allAttributesInRect:NSArray,numberOfRows:Int,section:Int) -> NSMutableArray{
        var totalLines:Int = 0
        let lastRowArrays:NSMutableArray = NSMutableArray()
        for  attributes in allAttributesInRect {
            if(attributes.indexPath.section == section){
                if (attributes.frame.origin.x == self.sectionInset.left){
                    totalLines = totalLines + 1
                    if(totalLines == numberOfRows){
                        lastRowArrays.addObject(attributes)
                    }
                }
                else{
                    if(totalLines == numberOfRows){
                        lastRowArrays.addObject(attributes)
                    }
                }
            }
        }
        return lastRowArrays
    }
   private func setMiddleTheLastRow(lastRowAttrs:NSMutableArray)->NSMutableArray{
        let redefinedValues = NSMutableArray()
        let totalLengthOftheView = self.collectionView?.frame.width
        var totalLenthOftheCells:CGFloat = 0.0
        totalLenthOftheCells = totalLenthOftheCells + (CGFloat (lastRowAttrs.count) - 1) * padding

        for attrs in lastRowAttrs{
            totalLenthOftheCells = totalLenthOftheCells + attrs.frame.width
        }

        var initalValue = (totalLengthOftheView!/2) - (totalLenthOftheCells/2)

        for  i in 0..<lastRowAttrs.count {
            let changeingAttribute:UICollectionViewLayoutAttributes = lastRowAttrs[i] as! UICollectionViewLayoutAttributes
            var frame = changeingAttribute.frame
            frame.origin.x = initalValue
            changeingAttribute.frame = frame
            redefinedValues.addObject(changeingAttribute)
            initalValue = initalValue + changeingAttribute.frame.width + padding
        }

        return redefinedValues;
    }


}

CollectionViewCell.swift

import UIKit

class CenterFlowLayout: UICollectionViewFlowLayout {

    private var attrCache = [IndexPath: UICollectionViewLayoutAttributes]()

    override func prepare() {
        attrCache = [:]
    }

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

        let sections = self.collectionView?.numberOfSections ?? 0
        var section = 0
        while section < sections {
            let items = self.collectionView?.numberOfItems(inSection: section) ?? 0
            var item = 0
            while item < items {
                let indexPath = IndexPath(row: item, section: section)

                if let attributes = layoutAttributesForItem(at: indexPath), attributes.frame.intersects(rect) {
                    updatedAttributes.append(attributes)
                }

                let headerKind = UICollectionElementKindSectionHeader
                if let headerAttributes = layoutAttributesForSupplementaryView(ofKind: headerKind, at: indexPath) {
                    updatedAttributes.append(headerAttributes)
                }

                let footerKind = UICollectionElementKindSectionFooter
                if let footerAttributes = layoutAttributesForSupplementaryView(ofKind: footerKind, at: indexPath) {
                    updatedAttributes.append(footerAttributes)
                }

                item += 1
            }

            section += 1
        }

        return updatedAttributes
    }

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        if let attributes = attrCache[indexPath] {
            return attributes
        }

        // Find the other items in the same "row"
        var rowBuddies = [UICollectionViewLayoutAttributes]()

        // Calculate the available width to center stuff within
        // sectionInset is NOT applicable here because a) we're centering stuff
        // and b) Flow layout has arranged the cells to respect the inset. We're
        // just hiHymaning the X position.
        var collectionViewWidth: CGFloat = 0
        if let collectionView = collectionView {
            collectionViewWidth = collectionView.bounds.width - collectionView.contentInset.left
                    - collectionView.contentInset.right
        }

        // To find other items in the "row", we need a rect to check intersects against.
        // Take the item attributes frame (from vanilla flow layout), and stretch it out
        var rowTestFrame: CGRect = super.layoutAttributesForItem(at: indexPath)?.frame ?? .zero
        rowTestFrame.origin.x = 0
        rowTestFrame.size.width = collectionViewWidth

        let totalRows = self.collectionView?.numberOfItems(inSection: indexPath.section) ?? 0

        // From this item, work backwards to find the first item in the row
        // Decrement the row index until a) we get to 0, b) we reach a previous row
        var rowStartIDX = indexPath.row
        while true {
            let prevIDX = rowStartIDX - 1

            if prevIDX < 0 {
                break
            }

            let prevPath = IndexPath(row: prevIDX, section: indexPath.section)
            let prevFrame: CGRect = super.layoutAttributesForItem(at: prevPath)?.frame ?? .zero

            // If the item intersects the test frame, it's in the same row
            if prevFrame.intersects(rowTestFrame) {
                rowStartIDX = prevIDX
            } else {
                // Found previous row, escape!
                break
            }
        }

        // Now, work back UP to find the last item in the row
        // For each item in the row, add it's attributes to rowBuddies
        var buddyIDX = rowStartIDX
        while true {
            if buddyIDX > totalRows - 1 {
                break
            }

            let buddyPath = IndexPath(row: buddyIDX, section: indexPath.section)

            if let buddyAttributes = super.layoutAttributesForItem(at: buddyPath),
               buddyAttributes.frame.intersects(rowTestFrame),
               let buddyAttributesCopy = buddyAttributes.copy() as? UICollectionViewLayoutAttributes {
                // If the item intersects the test frame, it's in the same row
                rowBuddies.append(buddyAttributesCopy)
                buddyIDX += 1
            } else {
                // Encountered next row
                break
            }
        }

        let flowDelegate = self.collectionView?.delegate as? UICollectionViewDelegateFlowLayout
        let selector = #selector(UICollectionViewDelegateFlowLayout.collectionView(_:layout:minimumInteritemSpacingForSectionAt:))
        let delegateSupportsInteritemSpacing = flowDelegate?.responds(to: selector) ?? false

        // x-x-x-x ... sum up the interim space
        var interitemSpacing = minimumInteritemSpacing

        // Check for minimumInteritemSpacingForSectionAtIndex support
        if let collectionView = collectionView, delegateSupportsInteritemSpacing && rowBuddies.count > 0 {
            interitemSpacing = flowDelegate?.collectionView?(collectionView,
                    layout: self,
                    minimumInteritemSpacingForSectionAt: indexPath.section) ?? 0
        }

        let aggregateInteritemSpacing = interitemSpacing * CGFloat(rowBuddies.count - 1)

        // Sum the width of all elements in the row
        var aggregateItemWidths: CGFloat = 0
        for itemAttributes in rowBuddies {
            aggregateItemWidths += itemAttributes.frame.width
        }

        // Build an alignment rect
        // |  |x-x-x-x|  |
        let alignmentWidth = aggregateItemWidths + aggregateInteritemSpacing
        let alignmentXOffset: CGFloat = (collectionViewWidth - alignmentWidth) / 2

        // Adjust each item's position to be centered
        var previousFrame: CGRect = .zero
        for itemAttributes in rowBuddies {
            var itemFrame = itemAttributes.frame

            if previousFrame.equalTo(.zero) {
                itemFrame.origin.x = alignmentXOffset
            } else {
                itemFrame.origin.x = previousFrame.maxX + interitemSpacing
            }

            itemAttributes.frame = itemFrame
            previousFrame = itemFrame

            // Finally, add it to the cache
            attrCache[itemAttributes.indexPath] = itemAttributes
        }

        return attrCache[indexPath]
    }
}

Expected result:

预期结果：

Answer 4

回答by onmyway133

In case anyone has a CollectionView that has 2 columns, and if number of items is odd, the last item should be center aligned. Then use this

如果有人有一个包含 2 列的 CollectionView，并且如果项目数是奇数，则最后一个项目应该居中对齐。然后用这个

DNLastItemCenteredLayout

##代码##

Answer 5

回答by Alex Staravoitau

This can be achieved with a (relatively) simple custom layout, subclassed from UICollectionViewFlowLayout. Here's an example in Swift:

这可以通过一个（相对）简单的自定义布局来实现，从UICollectionViewFlowLayout. 下面是一个例子Swift：

##代码##

Answer 6

回答by chents

I've subclassed UICollectionViewFlowLayout- altered the code i've found herefor left aligned collection view.

我已经子类化了UICollectionViewFlowLayout- 更改了我在此处找到的用于左对齐集合视图的代码。

Left align the collection view
Group the attributes for line arrays
For each line:
- calculate the space on the right side
- add half of the the space for each attribute of the line

左对齐集合视图
对线阵列的属性进行分组
对于每一行：
- 计算右边的空间
- 为该行的每个属性添加一半的空间

It's looks like this:

它看起来像这样：

##代码##

Hope it helps someone :)

希望它可以帮助某人:)

Answer 7

回答by danielemm

Swift 3.0version of the class:

类的 Swift 3.0版本：

##代码##

Answer 8

回答by Rajesh Kumar

Just link this flow layout. You can align center, left, right also.

只需链接此流程布局。您也可以居中、向左、向右对齐。

##代码##

Answer 9

回答by Nominalista

I made Swift 4version of Kyle Truscott answer:

我制作了Swift 4版本的 Kyle Truscott回答：

##代码##

ios 如何在 UICollectionView 中居中行？

提问by deycall

采纳答案by lxt

回答by Kyle Truscott

回答by Imanou Petit

#1. Center `UICollectionViewCell`s with fixed size

#1. `UICollectionViewCell`固定大小的中心

#2. Center autoresizing `UICollectionViewCell`s

#2. 中心自动尺寸`UICollectionViewCell`小号

回答by onmyway133

回答by Alex Staravoitau

回答by chents

回答by danielemm

回答by Rajesh Kumar

回答by Nominalista

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标签

ios 如何在 UICollectionView 中居中行？

提问by deycall

采纳答案by lxt

回答by Kyle Truscott

回答by Imanou Petit

#1. Center UICollectionViewCells with fixed size

#1. UICollectionViewCell固定大小的中心

#2. Center autoresizing UICollectionViewCells

#2. 中心自动尺寸UICollectionViewCell小号

回答by onmyway133

回答by Alex Staravoitau

回答by chents

回答by danielemm

回答by Rajesh Kumar

回答by Nominalista

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#1. Center `UICollectionViewCell`s with fixed size

#1. `UICollectionViewCell`固定大小的中心

#2. Center autoresizing `UICollectionViewCell`s

#2. 中心自动尺寸`UICollectionViewCell`小号