TextKit Viewport Rendering Surfaces & Framework Text View Delegate Hooks

import UIKit
import UniformTypeIdentifiers

// MARK: - Line-number-aware UITextView subclass (iOS 27+)
final class CodeTextView: UITextView {

    // Callback fired after every viewport layout cycle with
    // a mapping of line-index -> fragment frame (in text-view coords)
    var onViewportDidLayout: (([Int: CGRect]) -> Void)?

    // Accumulated during configureRenderingSurface overrides
    private var pendingLineFrames: [Int: CGRect] = [:]

    // MARK: NSTextViewportLayoutControllerDelegate overrides (new in iOS 27)

    override func textViewportLayoutControllerWillLayout(
        _ controller: NSTextViewportLayoutController
    ) {
        // Clear state at the start of each layout pass
        pendingLineFrames.removeAll()
    }

    override func textViewportLayoutController(
        _ controller: NSTextViewportLayoutController,
        configureRenderingSurface renderingSurface: any NSTextViewportRenderingSurface,
        for textLayoutFragment: NSTextLayoutFragment
    ) {
        // Resolve which paragraph index this fragment belongs to
        guard let contentManager = textLayoutManager?.textContentManager else { return }

        var paragraphIndex = 0
        contentManager.enumerateTextElements(from: nil) { element in
            if element === textLayoutFragment.textElement { return false }
            paragraphIndex += 1
            return true
        }

        // Convert fragment frame from text-layout coordinates to view coordinates
        let fragmentFrameInView = CGRect(
            origin: CGPoint(
                x: textLayoutFragment.layoutFragmentFrame.minX + textContainerInset.left,
                y: textLayoutFragment.layoutFragmentFrame.minY + textContainerInset.top
            ),
            size: textLayoutFragment.layoutFragmentFrame.size
        )
        pendingLineFrames[paragraphIndex] = fragmentFrameInView
    }

    override func textViewportLayoutControllerDidLayout(
        _ controller: NSTextViewportLayoutController
    ) {
        // Notify the container view so it can redraw the gutter
        onViewportDidLayout?(pendingLineFrames)
    }
}

// MARK: - Container view wiring up the gutter
final class CodeEditorContainerView: UIView {

    private let gutterWidth: CGFloat = 44
    private let gutterView = UIView()
    private var lineNumberLabels: [UILabel] = []
    private let codeTextView = CodeTextView()

    override init(frame: CGRect) {
        super.init(frame: frame)
        setupViews()
    }
    required init?(coder: NSCoder) { fatalError() }

    private func setupViews() {
        gutterView.backgroundColor = UIColor.systemGray6
        addSubview(gutterView)
        addSubview(codeTextView)

        codeTextView.font = UIFont.monospacedSystemFont(ofSize: 14, weight: .regular)
        codeTextView.autocorrectionType = .no
        codeTextView.autocapitalizationType = .none
        codeTextView.textContainerInset = UIEdgeInsets(
            top: 8, left: gutterWidth + 8, bottom: 8, right: 8
        )
        codeTextView.text = "func hello() {\n    print(\"Hello, TextKit!\")\n}\n"

        // iOS 27: react to viewport layout cycles
        codeTextView.onViewportDidLayout = { [weak self] lineFrames in
            self?.updateGutter(with: lineFrames)
        }
    }

    override func layoutSubviews() {
        super.layoutSubviews()
        gutterView.frame = CGRect(x: 0, y: 0, width: gutterWidth, height: bounds.height)
        codeTextView.frame = bounds
    }

    private func updateGutter(with lineFrames: [Int: CGRect]) {
        lineNumberLabels.forEach { $0.removeFromSuperview() }
        lineNumberLabels.removeAll()

        for (index, frame) in lineFrames.sorted(by: { $0.key < $1.key }) {
            let label = UILabel()
            label.font = UIFont.monospacedSystemFont(ofSize: 12, weight: .regular)
            label.textColor = .secondaryLabel
            label.textAlignment = .right
            label.text = "\(index + 1)"
            label.frame = CGRect(
                x: 4,
                y: frame.minY - codeTextView.contentOffset.y,
                width: gutterWidth - 8,
                height: frame.height
            )
            gutterView.addSubview(label)
            lineNumberLabels.append(label)
        }
    }
}