如何在UICollectionView中居中行？

Question

我有UICollectionView随机细胞.有没有什么方法可以让我居中行？

这是默认情况下的外观:

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

这是所需的布局:

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

Answer 1

我必须做这样的事情,但需要一个部分中的所有细胞.扩展UICollectionViewFlowLayout到中心细胞非常简单.我做了一个吊舱:

https://github.com/keighl/KTCenterFlowLayout

Answer 2

首先UICollectionView是一些背景 - a 与a结合UICollectionViewLayout,决定了单元格在视图中的放置方式.这意味着集合视图非常灵活(您可以使用它创建几乎任何布局),但也意味着修改布局可能会有点混乱.

创建一个全新的布局类很复杂,所以你想尝试修改默认布局(UICollectionViewFlowLayout)以获得中心对齐.为了使其更简单,您可能希望避免对流布局本身进行子类化.

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 

如果你知道滚动视图的宽度和每行中可以容纳的单元格数,这应该是相当简单的.

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,您只需确保重新计算相应的部分/插入部分,以便支持纵向和横向.

这里有一个类似的问题 - 如何居中对齐UICollectionView的单元格？ - 有关插入方法的详细信息,尽管它并不是在尝试做同样的事情.

Answer 3

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

#1. Center UICollectionViewCells with fixed size

The implementation below shows how to use UICollectionViewLayout's layoutAttributesForElements(in:) and UICollectionViewFlowLayout's itemSize in order to center the cells of a 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 { $0.copy() as! UICollectionViewLayoutAttributes }
        guard scrollDirection == .vertical else { return layoutAttributes }

        // Filter attributes to compute only cell attributes
        let cellAttributes = layoutAttributes.filter({ $0.representedElementCategory == .cell })

        // Group cell attributes by row (cells with same vertical center) and loop on those groups
        for (_, attributes) in Dictionary(grouping: cellAttributes, by: { ($0.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

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")
    }

}

Expected result:

#2. Center autoresizing UICollectionViewCells

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

CollectionViewController.swift

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)
    }

}

FlowLayout.swift

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 { $0.copy() as! UICollectionViewLayoutAttributes }
        guard scrollDirection == .vertical else { return layoutAttributes }

        // Filter attributes to compute only cell attributes
        let cellAttributes = layoutAttributes.filter({ $0.representedElementCategory == .cell })

        // Group cell attributes by row (cells with same vertical center) and loop on those groups
        for (_, attributes) in Dictionary(grouping: cellAttributes, by: { ($0.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

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")
    }

}

Expected result: