A UILabel that responds to embedded link tap

LinkableLabel.swift

import UIKit

/// A UILabel subclass that responds to links tap.
public class LinkableLabel: UILabel {

    // MARK: - Private
    
    private var touchedLink: URL?
    
    @objc
    private func handleTouch(_ sender: UIGestureRecognizer) {
        if let link = touchedLink, let linkHandler = linkHandler {
            linkHandler(link, sender)
        } else {
            tapHandler?(sender)
        }
    }
    
    private var alignmentOffset: CGFloat {
        switch textAlignment {
        case .left, .natural, .justified:
            return 0.0
        case .center:
            return 0.5
        case .right:
            return 1.0
        default:
            return 0
        }
    }
    
    private var needsLinkColorUpdate: Bool = true
    
    /// Set the text color for links.
    /// - Important: One can't change the color for a `link`. We need to use another attribute type. In our case, we use `attachment`.
    private func setLinkColor(color: UIColor) -> NSAttributedString? {
        guard let attributedText = attributedText else { return nil }
        let mutableCopy = NSMutableAttributedString(attributedString: attributedText)
        let range = NSRange(location: 0, length: mutableCopy.length)
        mutableCopy.enumerateAttributes(in: range, options: []) { (attributes, range, _) in
            var currentAttributes = mutableCopy.attributes(at: range.location, longestEffectiveRange: nil, in: range)
            if let link = currentAttributes[NSAttributedString.Key.link] as? String, let url = URL(string: link) {
                currentAttributes.removeValue(forKey: .link)
                currentAttributes[.attachment] = url
                currentAttributes[.foregroundColor] = color
                mutableCopy.setAttributes(currentAttributes, range: range)
            } else if let _ = currentAttributes[NSAttributedString.Key.attachment] as? URL {
                currentAttributes[.foregroundColor] = color
                mutableCopy.setAttributes(currentAttributes, range: range)
            }
        }
        return mutableCopy
    }
    
    // MARK: - API
    
    /// Set the text color for links in the attributedText property
    public var linkColor: UIColor? {
        didSet {
            guard let linkColor = linkColor else { return }
            needsLinkColorUpdate = false
            attributedText = setLinkColor(color: linkColor)
            needsLinkColorUpdate = true
        }
    }
    
    /// Use tapHandler for a unique gesture accross all text. Does not differentiate between links.
    public var tapHandler: ((UIGestureRecognizer) -> Void)?
    
    /// Use linkHandler for opening links in the attributedText property.
    public var linkHandler: ((URL, UIGestureRecognizer) -> Void)?

    // MARK: - Init
    
    public override init(frame: CGRect) {
        super.init(frame: frame)
        let rec = UITapGestureRecognizer(target: self, action: #selector(handleTouch(_:)))
        addGestureRecognizer(rec)
        isUserInteractionEnabled = true
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    public override var attributedText: NSAttributedString? {
        didSet {
            guard let linkColor = linkColor, needsLinkColorUpdate else { return }
            let mutableCopy = setLinkColor(color: linkColor)
            if mutableCopy != attributedText {
                attributedText = mutableCopy
            }
        }
    }
    
    public override func hitTest(_ point: CGPoint, with event: UIEvent?) -> UIView? {
        guard let attributedText = attributedText, super.hitTest(point, with: event) != nil else { return nil }

        let size = bounds.size
        
        let textContainer = NSTextContainer(size: size)
        textContainer.lineFragmentPadding = 0.0
        textContainer.lineBreakMode = .byWordWrapping
        textContainer.maximumNumberOfLines = numberOfLines
        
        let layoutManager = NSLayoutManager()
        layoutManager.addTextContainer(textContainer)
        layoutManager.usesFontLeading = false
        
        let textStorage = NSTextStorage(attributedString: attributedText)
        textStorage.addLayoutManager(layoutManager)
        
        let textBoundingBox = layoutManager.usedRect(for: textContainer)
        let xOffset = ((size.width - textBoundingBox.size.width) * alignmentOffset) - textBoundingBox.origin.x
        let yOffset = ((size.height - textBoundingBox.size.height) * alignmentOffset) - textBoundingBox.origin.y
        
        let locationOfTouchInTextContainer = CGPoint(x: point.x - xOffset, y: point.y - yOffset)
        let tappedCharacterIndex: Int = layoutManager.characterIndex(for: locationOfTouchInTextContainer, in: textContainer, fractionOfDistanceBetweenInsertionPoints: nil)
        
        if tappedCharacterIndex < 0 || tappedCharacterIndex >= attributedText.string.count {
            touchedLink = nil
            return nil
        }
        let attributesAtTappedIndex = attributedText.attributes(at: tappedCharacterIndex, effectiveRange: nil)
        if let absoluteLink = attributesAtTappedIndex[NSAttributedString.Key.link] as? String, let link = URL(string: absoluteLink) {
            touchedLink = link
            return self
        } else if let link = attributesAtTappedIndex[NSAttributedString.Key.attachment] as? URL {
            touchedLink = link
            return self
        } else if let link = attributesAtTappedIndex[NSAttributedString.Key(rawValue: "href")] as? URL {
            touchedLink = link
            return self
        } else if tapHandler != nil {
            touchedLink = nil
            return self
        } else {
            touchedLink = nil
            return nil
        }
    }
}