This is an example of using a UICollectionView to create a custom segment control. This example shows a strategy to resize a UILabel's font so that it fits within the frame provided. This sample code is for iOS using Swift and was created in response to this stack overflow question: https://stackoverflow.com/q/71566815/1619193

SegmentCollectionViewVC.swift

//
//  SegmentCollectionViewVC.swift
//  TestApp
//
//  Created by Shawn Frank on 22/03/2022.
//

import UIKit

fileprivate class SegmentCell: UICollectionViewCell {
    
    static let reuseIdentifier = "SegmentCell"
    
    let title = UILabel()
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        configureLabel()
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    private func configureLabel() {
        
        title.translatesAutoresizingMaskIntoConstraints = false
        title.textAlignment = .center
        title.numberOfLines = 1
        title.textColor = .black
        contentView.addSubview(title)
        
        // Auto layout config to pin label to the edges of the content view
        title.leadingAnchor
            .constraint(equalTo: contentView.leadingAnchor)
            .isActive = true
        
        title.topAnchor
            .constraint(equalTo: contentView.topAnchor)
            .isActive = true
        
        title.trailingAnchor
            .constraint(equalTo: contentView.trailingAnchor)
            .isActive = true
        
        title.bottomAnchor
            .constraint(equalTo: contentView.bottomAnchor)
            .isActive = true
    }
}


class SegmentCollectionViewVC: UIViewController {
    
    var collectionView: UICollectionView!
    
    // Different segments
    let segments = ["Active Time",
                    "Reminders",
                    "Vibration Intensify"]
    
    // Padding between segments
    let horizontalPadding: CGFloat = 5
    
    // Height of the collection view and segments
    let collectionViewHeight: CGFloat = 50
    
    // Font size for each segment
    var segmentFontSize = CGFloat.zero
    
    override func viewDidLoad() {
        
        super.viewDidLoad()
        
        title = "Segment CV"
        view.backgroundColor = .white
        
        configureCollectionView()
    }
    
    // Just some normal set up, you can skip over it
    // Added for completeness
    private func configureCollectionView() {
        
        collectionView = UICollectionView(frame: CGRect.zero,
                                          collectionViewLayout: createLayout())
        
        collectionView.backgroundColor = .white
        
        collectionView.register(SegmentCell.self,
                                forCellWithReuseIdentifier: SegmentCell.reuseIdentifier)
        
        collectionView.dataSource = self
        
        collectionView.translatesAutoresizingMaskIntoConstraints = false
        
        view.addSubview(collectionView)
        
        // Auto layout config
        collectionView.leadingAnchor
            .constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor)
            .isActive = true
        
        collectionView.topAnchor
            .constraint(equalTo: view.safeAreaLayoutGuide.topAnchor)
            .isActive = true
        
        collectionView.trailingAnchor
            .constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor)
            .isActive = true
        
        collectionView.heightAnchor
            .constraint(equalToConstant: collectionViewHeight)
            .isActive = true
    }
    
    // Flow layout configuration, mainly to figure out width of the cells
    private func createLayout() -> UICollectionViewFlowLayout {
        
        let flowLayout = UICollectionViewFlowLayout()
        flowLayout.minimumLineSpacing = horizontalPadding
        flowLayout.minimumInteritemSpacing = 0
        flowLayout.scrollDirection = .horizontal
        flowLayout.sectionInset = UIEdgeInsets(top: 0,
                                               left: horizontalPadding,
                                               bottom: 0,
                                               right: horizontalPadding)
        
        // Calculate the available width to divide the segments evenly
        var availableWidth = UIScreen.main.bounds.width
        
        // There will always be segments - 1 gaps, for 3 segments, there will be
        // 2 gaps and for 4 segments there will be 3 gaps etc
        availableWidth -= horizontalPadding * CGFloat(segments.count - 1)
        
        // Remove the insets
        availableWidth -= flowLayout.sectionInset.left + flowLayout.sectionInset.right
        
        let cellWidth = availableWidth / CGFloat(segments.count)
        
        // Add this function
        calculateApproxFontSize(forWidth: cellWidth)
        
        flowLayout.itemSize = CGSize(width: cellWidth,
                                     height: collectionViewHeight)
        
        return flowLayout
    }

    private func calculateApproxFontSize(forWidth width: CGFloat) {
        
        if let longestSegmentTitle = segments.max(by: { $1.count > $0.count }) {
            
            let tempLabel = UILabel()
            tempLabel.numberOfLines = 1
            tempLabel.text = longestSegmentTitle
            tempLabel.sizeToFit()
            
            guard var currentFont = tempLabel.font else { return }
            
            var intrinsicSize
                = (longestSegmentTitle as NSString).size(withAttributes: [.font : currentFont])
            
            // Keep looping and reduce the font size till the text
            // fits into the label
            // This could be optimized further using binary search
            // However this should be ok for small strings
            while intrinsicSize.width > width
            {
                currentFont = currentFont.withSize(currentFont.pointSize - 1)
                tempLabel.font = currentFont
                
                intrinsicSize
                    = (longestSegmentTitle as NSString).size(withAttributes: [.font : currentFont])
            }

            // Set the font of the current label
            // segmentFontSize is a global var in the VC
            segmentFontSize = currentFont.pointSize
        }
    }
}

extension SegmentCollectionViewVC: UICollectionViewDataSource {
    
    func collectionView(_ collectionView: UICollectionView,
                        numberOfItemsInSection section: Int) -> Int {
        
        return segments.count
    }
    
    func collectionView(_ collectionView: UICollectionView,
                        cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        
        let cell = collectionView
            .dequeueReusableCell(withReuseIdentifier: SegmentCell.reuseIdentifier,
                                 for: indexPath) as! SegmentCell
        
        cell.backgroundColor = .orange
        cell.title.text = segments[indexPath.item]
        
        // Adjust the font
        cell.title.font = cell.title.font.withSize(segmentFontSize)
        
        return cell
    }
}