TextKit Framework Text View Extensibility

 import UIKit
 
−// Pre-iOS 27: you had to set yourself as the viewport controller's
−// delegate externally — UITextView did NOT conform to
−// NSTextViewportLayoutControllerDelegate, so subclassing was not enough.
+// MARK: - Line-number aware text view (iOS 27+)
 
−final class LegacyCodeEditorViewController: UIViewController,
−                                             NSTextViewportLayoutControllerDelegate {
−    private var paragraphFrames: [(index: Int, frame: CGRect)] = []
+final class LineNumberTextView: UITextView {
 
−    private lazy var textView: UITextView = {
−        let tv = UITextView()
+    // Paragraph index → frame in the text view's coordinate space
+    private(set) var paragraphFrames: [(index: Int, frame: CGRect)] = []
+
+    // Called once before the viewport layout pass begins
+    override func textViewportLayoutControllerWillLayout(
+        _ controller: NSTextViewportLayoutController
+    ) {
+        paragraphFrames.removeAll(keepingCapacity: true)
+    }
+
+    // Called for every layout fragment entering the viewport
+    override func textViewportLayoutController(
+        _ controller: NSTextViewportLayoutController,
+        configureRenderingSurface surface: any NSTextViewportRenderingSurface,
+        for layoutFragment: NSTextLayoutFragment
+    ) {
+        // Find the paragraph index by walking the content manager
+        guard
+            let contentManager = textLayoutManager?.textContentManager,
+            let range = layoutFragment.textElement?.elementRange
+        else { return }
+
+        var index = 0
+        contentManager.enumerateTextElements(from: nil) { element in
+            if element.elementRange == range { return false } // stop
+            index += 1
+            return true
+        }
+
+        // Store the layout fragment's frame (in text-view coordinates)
+        let frame = CGRect(
+            origin: CGPoint(
+                x: layoutFragment.layoutFragmentFrame.minX,
+                y: layoutFragment.layoutFragmentFrame.minY
+            ),
+            size: layoutFragment.layoutFragmentFrame.size
+        )
+        paragraphFrames.append((index: index, frame: frame))
+    }
+
+    // Called once after the viewport layout pass finishes
+    override func textViewportLayoutControllerDidLayout(
+        _ controller: NSTextViewportLayoutController
+    ) {
+        // Notify any observer (e.g. the gutter view) to redraw
+        NotificationCenter.default.post(
+            name: .lineNumbersDidUpdate,
+            object: self
+        )
+    }
+}
+
+extension Notification.Name {
+    static let lineNumbersDidUpdate = Notification.Name("lineNumbersDidUpdate")
+}
+
+// MARK: - Minimal gutter view
+
+final class LineNumberGutterView: UIView {
+    var paragraphFrames: [(index: Int, frame: CGRect)] = []
+
+    override func draw(_ rect: CGRect) {
+        let attrs: [NSAttributedString.Key: Any] = [
+            .font: UIFont.monospacedSystemFont(ofSize: 12, weight: .regular),
+            .foregroundColor: UIColor.secondaryLabel
+        ]
+        for entry in paragraphFrames {
+            let label = "\(entry.index + 1)"
+            let size = label.size(withAttributes: attrs)
+            let y = entry.frame.minY + (entry.frame.height - size.height) / 2
+            let origin = CGPoint(x: bounds.width - size.width - 4, y: y)
+            label.draw(at: origin, withAttributes: attrs)
+        }
+    }
+}
+
+// MARK: - Container view controller
+
+final class CodeEditorViewController: UIViewController {
+    private let gutterWidth: CGFloat = 44
+    private lazy var textView: LineNumberTextView = {
+        let tv = LineNumberTextView()
         tv.font = .monospacedSystemFont(ofSize: 16, weight: .regular)
−        tv.text = "func greet() {\n    print(\"Hello!\")\n}\n"
+        tv.autocorrectionType = .no
+        tv.autocapitalizationType = .none
+        tv.text = "func greet() {\n    print(\"Hello, iOS 27!\")\n}\n"
         tv.translatesAutoresizingMaskIntoConstraints = false
         return tv
     }()
+    private lazy var gutterView: LineNumberGutterView = {
+        let v = LineNumberGutterView()
+        v.backgroundColor = .systemGroupedBackground
+        v.translatesAutoresizingMaskIntoConstraints = false
+        return v
+    }()
 
     override func viewDidLoad() {
         super.viewDidLoad()
+        view.addSubview(gutterView)
         view.addSubview(textView)
         NSLayoutConstraint.activate([
−            textView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
+            gutterView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
+            gutterView.topAnchor.constraint(equalTo: view.topAnchor),
+            gutterView.bottomAnchor.constraint(equalTo: view.bottomAnchor),
+            gutterView.widthAnchor.constraint(equalToConstant: gutterWidth),
+            textView.leadingAnchor.constraint(equalTo: gutterView.trailingAnchor),
             textView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
             textView.topAnchor.constraint(equalTo: view.topAnchor),
             textView.bottomAnchor.constraint(equalTo: view.bottomAnchor)
         ])
−        // Had to reach into textLayoutManager and steal the delegate —
−        // bypassing UITextView's own internal use of it, which was unsupported.
−        // In practice this caused layout glitches and was not recommended.
−        textView.textLayoutManager?.textViewportLayoutController.delegate = self
+        NotificationCenter.default.addObserver(
+            self,
+            selector: #selector(refreshGutter),
+            name: .lineNumbersDidUpdate,
+            object: textView
+        )
     }
 
−    // MARK: NSTextViewportLayoutControllerDelegate (manual, fragile)
−
−    func viewportBounds(for controller: NSTextViewportLayoutController) -> CGRect {
−        return textView.bounds
+    @objc private func refreshGutter() {
+        gutterView.paragraphFrames = textView.paragraphFrames
+        gutterView.setNeedsDisplay()
     }
−
−    func textViewportLayoutControllerWillLayout(
−        _ controller: NSTextViewportLayoutController
−    ) {
−        paragraphFrames.removeAll()
−    }
−
−    func textViewportLayoutController(
−        _ controller: NSTextViewportLayoutController,
−        configureRenderingSurface surface: Any, // no typed protocol pre-iOS 27
−        for layoutFragment: NSTextLayoutFragment
−    ) {
−        // No NSTextViewportRenderingSurface protocol existed;
−        // surface was untyped and you couldn't safely cast it.
−    }
−
−    func textViewportLayoutControllerDidLayout(
−        _ controller: NSTextViewportLayoutController
−    ) {
−        // Trigger gutter redraw — but layout glitches likely
−    }
 }