ios - 在iOS 13之前的设备上返回的单元格的意外高度

Question

我正在制作一个消息传递应用程序，并根据该单元格标签之一的估计帧确定消息单元格的高度。由于某些原因，在运行iOS 13之前的OS的较小设备上，返回的高度比所需的大得多。

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        let message = messageArr[indexPath.item].messageBody 
        let size = CGSize(width: collectionView.frame.width - 120, height: 1000)            
        let options = NSStringDrawingOptions.usesFontLeading.union(.usesLineFragmentOrigin)
        var estimatedFrame = CGRect()
        var cellLbl: UILabel!
        var personLbl: UILabel!

        if let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "CurrentUserMessageCell", for: indexPath) as? CurrentUserMessageCell {
            cellLbl = cell.messageLbl
            personLbl = cell.personLbl
            cell.translatesAutoresizingMaskIntoConstraints = false
        }

        if let font = cellLbl.font {
            estimatedFrame = NSString(string: message).boundingRect(with: size, options: options, attributes: [NSAttributedString.Key.font: font], context: nil)
        }

        let height = estimatedFrame.height + personLbl.frame.height + 16
        return CGSize(width: collectionView.frame.width, height: height)
}

在我更改标签的约束之前，这在所有设备上都效果很好，因此即使有短消息，它也不总是占用大部分可用宽度。这就是 collectionView.frame.width - 120的来源(宽度-插入物，前导/尾随空间)。

我将问题范围缩小到标签的估计宽度(由 size属性确定)-但是为什么不正确的值对iOS 13.1的影响不超过12.2？如何更准确地确定该值？我尝试过用标签的插入物和前导/后缀空间偏移宽度，但是那样的话，单元格的高度总是太短了(字符在iOS 13.1之前被截断，而13.1只是失去了一点填充)。

这是在iOS 13.1上，使用上述代码:



和12.2:

Answer 1

我建议使用自动布局而不是手动计算像元高度。

这里有一个很好的教程(不是我的):https://medium.com/@andrea.toso/uicollectionviewcell-dynamic-height-swift-b099b28ddd23

这是一个简单的示例(全部通过代码-没有@IBOutlet)...创建一个新的UIViewController并将其自定义类分配给AutoSizeCollectionViewController:

//
//  AutoSizeCollectionViewController.swift
//
//  Created by Don Mag on 11/19/19.
//

import UIKit

private let reuseIdentifier = "MyAutoCell"

class MyAutoCell: UICollectionViewCell {

    let personLbl: UILabel = {
        let label = UILabel()
        return label
    }()
    let cellLabel: UILabel = {
        let label = UILabel()
        label.numberOfLines = 0
        return label
    }()

    // used in systemLayoutSizeFitting() for auto-sizing cells
    lazy var width: NSLayoutConstraint = {
        let width = contentView.widthAnchor.constraint(equalToConstant: bounds.size.width)
        width.isActive = true
        return width
    }()

    override init(frame: CGRect) {
        super.init(frame: frame)
        self.backgroundColor = .clear
        self.setupViews()
    }

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

    override func prepareForReuse() {

    }

    private func setupViews() {

        contentView.backgroundColor = .lightGray

        contentView.layer.cornerRadius = 16.0
        contentView.layer.masksToBounds = true

        // we'll be using contentView ... so disable translates...
        contentView.translatesAutoresizingMaskIntoConstraints = false

        contentView.addSubview(personLbl)
        contentView.addSubview(cellLabel)

        [personLbl, cellLabel].forEach {
            $0.translatesAutoresizingMaskIntoConstraints = false
            $0.backgroundColor = .clear
            $0.textColor = .black
            $0.font = UIFont.systemFont(ofSize: 17.0, weight: .bold)
        }

        let g = contentView.layoutMarginsGuide

        // top / bottom padding
        let vPadding: CGFloat = 6.0

        // leading / trailing padding
        let hPadding: CGFloat = 8.0

        // vertical space between labels
        let vSpacing: CGFloat = 6.0

        NSLayoutConstraint.activate([

            // constrain personLbl to top, leading, trailing (with vPadding)
            personLbl.topAnchor.constraint(equalTo: g.topAnchor, constant: vPadding),

            // constrain personLbl to leading and trailing (with hPadding)
            personLbl.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: hPadding),
            personLbl.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -hPadding),

            // constrain cellLabel top to personLbl bottom (with spacing)
            cellLabel.topAnchor.constraint(equalTo: personLbl.bottomAnchor, constant: vSpacing),

            // constrain cellLabel to leading and trailing (with hPadding)
            cellLabel.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: hPadding),
            cellLabel.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -hPadding),

            // constrain cellLabel to bottom (with vPadding)
            cellLabel.bottomAnchor.constraint(equalTo: g.bottomAnchor, constant: -vPadding),

        ])

    }

    // used for auto-sizing collectionView cell
    override func systemLayoutSizeFitting(_ targetSize: CGSize, withHorizontalFittingPriority horizontalFittingPriority: UILayoutPriority, verticalFittingPriority: UILayoutPriority) -> CGSize {
        width.constant = bounds.size.width
        return contentView.systemLayoutSizeFitting(CGSize(width: targetSize.width, height: 1))
    }

}

class AutoSizeCollectionViewController: UIViewController {

    // some sample data
    let theData: [[String]] = [
        ["Joe", "Hi"],
        ["Bob", "Hi back!"],
        ["Joe", "This is a message"],
        ["Bob", "This is a longer message. How does it look?"],
        ["Joe", "Oh yeah? Well, I can type a message that's even longer than yours. See what I mean?"],
        ["Bob", "Well good for you."],
    ]

    var theCollectionView: UICollectionView!
    var theFlowLayout: UICollectionViewFlowLayout!

    override func viewDidLoad() {
        super.viewDidLoad()

        // create vertical flow layout with 16-pt line spacing
        let flowLayout = UICollectionViewFlowLayout()
        flowLayout.scrollDirection = .vertical
        flowLayout.minimumInteritemSpacing = 0.0
        flowLayout.minimumLineSpacing = 16.0

        // this will be modified in viewDidLayoutSubviews()
        // needs to be small enough to fit on initial load
        flowLayout.estimatedItemSize = CGSize(width: 40.0, height: 40.0)

        theFlowLayout = flowLayout

        // create a collection view
        let collectionView = UICollectionView(frame: .zero, collectionViewLayout: flowLayout)
        collectionView.alwaysBounceVertical = true
        collectionView.backgroundColor = .clear

        collectionView.translatesAutoresizingMaskIntoConstraints = false

        view.addSubview(collectionView)

        let g = view.safeAreaLayoutGuide

        NSLayoutConstraint.activate([

            // constraints for the collection view
            // for this example, 100-pts from top, 40-pts from bottom, 80-pts on each side
            collectionView.topAnchor.constraint(equalTo: g.topAnchor, constant: 100.0),
            collectionView.bottomAnchor.constraint(equalTo: g.bottomAnchor, constant: -40.0),
            collectionView.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 80.0),
            collectionView.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -80.0),

        ])

        theCollectionView = collectionView

        // Register cell classes
        theCollectionView.register(MyAutoCell.self, forCellWithReuseIdentifier: reuseIdentifier)

        theCollectionView.dataSource = self
        theCollectionView.delegate = self

    }

    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()

        // collection view frame has now been determined by auto-layout
        // so set estimated item width to collection view frame width
        // height is approximate, as it will be auto-determined by the cell content (the labels)
        theFlowLayout.estimatedItemSize = CGSize(width: theCollectionView.frame.width, height: 100.0)
    }

    override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
        super.viewWillTransition(to: size, with: coordinator)

        // reset estimated item size width to something small
        // to avoid layout issues on size change (such as device rotation)
        // it will be properly set again in viewDidLayoutSubviews()
        theFlowLayout.estimatedItemSize = CGSize(width: 40.0, height: 40.0)
    }

}

// MARK: UICollectionViewDataSource
extension AutoSizeCollectionViewController: UICollectionViewDataSource {

    func numberOfSections(in collectionView: UICollectionView) -> Int {
        return 1
    }

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

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath) as! MyAutoCell

        cell.personLbl.text = theData[indexPath.item][0].uppercased()
        cell.cellLabel.text = theData[indexPath.item][1]

        return cell
    }

}

// MARK: UICollectionViewDelegate

extension AutoSizeCollectionViewController: UICollectionViewDelegate {
    // delegate methods here...
}

结果: