Interactive NSAttributedString link highlighting with TextKit

ViewController.swift

import UIKit

public extension CGSize {

    func snapped(scale: CGFloat) -> CGSize {
        var size = self
        size.width = ceil(size.width * scale) / scale
        size.height = ceil(size.height * scale) / scale
        return size
    }

    func resized(inset: UIEdgeInsets) -> CGSize {
        var size = self
        size.width += inset.left + inset.right
        size.height += inset.top + inset.bottom
        return size
    }

}

internal extension NSLayoutManager {

    func size(textContainer: NSTextContainer, width: CGFloat, scale: CGFloat) -> CGSize {
        textContainer.size = CGSize(width: width, height: 0)
        let bounds = usedRect(for: textContainer)
        return bounds.size.snapped(scale: scale)
    }

    func render(
        size: CGSize,
        textContainer: NSTextContainer,
        scale: CGFloat,
        backgroundColor: UIColor? = nil
        ) -> CGImage? {
        textContainer.size = size

        UIGraphicsBeginImageContextWithOptions(size, backgroundColor != nil, scale)
        defer { UIGraphicsEndImageContext() }

        if let backgroundColor = backgroundColor {
            backgroundColor.setFill()
            UIBezierPath(rect: CGRect(origin: .zero, size: size)).fill()
        }

        let range = glyphRange(for: textContainer)
        drawBackground(forGlyphRange: range, at: .zero)
        drawGlyphs(forGlyphRange: range, at: .zero)
        return UIGraphicsGetImageFromCurrentImageContext()?.cgImage
    }

}

class TextKitView: UIView {

    let layoutManager: NSLayoutManager
    let textContainer: NSTextContainer
    let storage: NSTextStorage

    init(text: NSAttributedString) {
        textContainer = NSTextContainer()
        textContainer.exclusionPaths = []
        textContainer.maximumNumberOfLines = 0
        textContainer.lineFragmentPadding = 0

        layoutManager = NSLayoutManager()
        layoutManager.allowsNonContiguousLayout = false
        layoutManager.hyphenationFactor = 0
        layoutManager.showsInvisibleCharacters = false
        layoutManager.showsControlCharacters = false
        layoutManager.usesFontLeading = true
        layoutManager.addTextContainer(textContainer)

        storage = NSTextStorage(attributedString: text)
        storage.addLayoutManager(layoutManager)

        super.init(frame: .zero)

        layer.contentsGravity = kCAGravityTopLeft
        layer.contentsScale = UIScreen.main.scale
    }

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

    func fitAndDraw(width: CGFloat) {
        let scale = UIScreen.main.scale
        let size = layoutManager.size(textContainer: textContainer, width: width, scale: scale)
        layer.contents = layoutManager.render(size: size, textContainer: textContainer, scale: scale)
        frame = CGRect(origin: frame.origin, size: size)
    }

    public func attributes(at point: CGPoint) -> (attrs: [NSAttributedStringKey: Any], index: Int)? {
        var fractionDistance: CGFloat = 1.0
        let index = layoutManager.characterIndex(
            for: point,
            in: textContainer,
            fractionOfDistanceBetweenInsertionPoints: &fractionDistance
        )
        if index != NSNotFound,
            fractionDistance < 1.0,
            let attrs = layoutManager.textStorage?.attributes(at: index, effectiveRange: nil) {
            return (attrs, index)
        }
        return nil
    }

}

class ViewController : UIViewController {

    let highlightLayer = CAShapeLayer()
    let textView: TextKitView = {
        let text = NSMutableAttributedString(string: "This is some text with a ", attributes: [
            .font: UIFont.systemFont(ofSize: 18),
            .foregroundColor: UIColor.black,
            ])
        text.append(NSMutableAttributedString(string: "link that spans multiple lines and opens", attributes: [
            .font: UIFont.boldSystemFont(ofSize: 18),
            .foregroundColor: UIColor.black,
            .link: URL(string: "http://google.com")!
            ]))
        text.append(NSMutableAttributedString(string: " to some website.", attributes: [
            .font: UIFont.systemFont(ofSize: 18),
            .foregroundColor: UIColor.black,
            ]))
        let view = TextKitView(text: text)
        view.layer.borderColor = UIColor.red.withAlphaComponent(0.2).cgColor
        view.layer.borderWidth = 1
        return view
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .white
        view.addSubview(textView)

        let long = UILongPressGestureRecognizer(target: self, action: #selector(onTap(gesture:)))
        long.minimumPressDuration = 0.1
        textView.addGestureRecognizer(long)

        highlightLayer.fillColor = UIColor.black.withAlphaComponent(0.3).cgColor
        textView.layer.addSublayer(highlightLayer)
    }

    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        let width = view.bounds.width - 40
        textView.frame = CGRect(x: 20, y: 100, width: width, height: 0)
        textView.fitAndDraw(width: width)
    }

    @objc func onTap(gesture: UILongPressGestureRecognizer) {

        switch gesture.state {
        case .began, .possible:
            let point = gesture.location(in: textView)
            guard let result = textView.attributes(at: point), result.attrs[.link] != nil else { return }
            print("tapped link")

            let maxLen = textView.storage.length
            var min = result.index
            var max = result.index

            textView.storage.enumerateAttributes(in: NSRange(location: 0, length: result.index), options: .reverse) { (attrs, range, stop) in
                if attrs[.link] != nil && min > 0 {
                    min = range.location
                } else {
                    stop.pointee = true
                }
            }

            textView.storage.enumerateAttributes(in: NSRange(location: result.index, length: maxLen - result.index), options: []) { (attrs, range, stop) in
                if attrs[.link] != nil && max < maxLen {
                    max = range.location + range.length
                } else {
                    stop.pointee = true
                }
            }

            print("tapped \(result.index) with range from \(min) to \(max) with len \(max - min)")

            let range = NSRange(location: min, length: max - min)

            let path = UIBezierPath()
            textView.layoutManager.enumerateEnclosingRects(forGlyphRange: range, withinSelectedGlyphRange: NSRange(location: NSNotFound, length: 0), in: textView.textContainer) { (rect, stop) in
                print("rect: \(rect)")
                path.append(UIBezierPath(roundedRect: rect.insetBy(dx: -2, dy: -2), cornerRadius: 4))
            }
            highlightLayer.path = path.cgPath
        case .changed: break
        case .cancelled, .ended, .failed:
            highlightLayer.path = nil
        }
    }

}