ruby.swift

func rubyAnnotation(text: String, ruby: String) -> NSAttributedString {
    let rubyAttribute: [CFString: Any] = [
        kCTRubyAnnotationSizeFactorAttributeName: 0.5
    ]
    let rubyAnnotation = CTRubyAnnotationCreateWithAttributes(
        .auto,
        .auto,
        .before,
        ruby as CFString,
        rubyAttribute as CFDictionary
    )

    return NSAttributedString(
        string: text,
        attributes:
            [
                kCTRubyAnnotationAttributeName as NSAttributedString.Key: rubyAnnotation
            ]
    )
}

var dummyAttributedString: NSAttributedString {
    let txt = NSMutableAttributedString(string: "だれもが")
    txt.append(rubyAnnotation(text: "創作", ruby: "そうさく"))
    txt.append(.init(string: "をはじめ、\n"))
    txt.append(rubyAnnotation(text: "続", ruby: "つづ"))
    txt.append(.init(string: "けられるようにする。"))
    
    // ルビが範囲外に表示されてしまうので行間を広げる
    let paragraphStyle = NSMutableParagraphStyle()
    paragraphStyle.lineHeightMultiple = 1.5

    txt.addAttributes([
        .font: UIFont.systemFont(ofSize: 16),
        .paragraphStyle: paragraphStyle,
    ], range: .init(location: 0, length: txt.length))
  
    return txt
}

RubyCanvas.swift

public struct RubyCanvas: View {
    public var body: some View {
        Canvas { context, size in
            context.scaleBy(x: 1, y: -1)
            context.translateBy(x: 0, y: -size.height)

            let setter = CTFramesetterCreateWithAttributedString(dummyAttributedString)
            let path = CGPath(rect: CGRect(origin: .zero, size: size), transform: nil)
            let ct = CTFramesetterCreateFrame(setter, CFRange(), path, nil)
            context.withCGContext { cgContext in
                CTFrameDraw(ct, cgContext)
            }
        }
    }
}

#Preview {
    RubyCanvas()
        .padding()
}

RubyTextView.swift

struct RubyTextView: UIViewRepresentable {
    func makeUIView(context _: Context) -> UITextView {
        let view = UITextView()
        view.attributedText = dummyAttributedString
        view.isScrollEnabled = false
        return view
    }

    func updateUIView(_: UITextView, context _: Context) {}
}

#Preview {
    RubyTextView()
}

RubyViewController.swift

final class RubyViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()

        let textView = UITextView()
        textView.attributedText = dummyAttributedString
        textView.isScrollEnabled = true

        textView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(textView)
        NSLayoutConstraint.activate([
            textView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 0),
            textView.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor, constant: 0),
            textView.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor, constant: 0),
            textView.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor, constant: 0),
        ])
    }
}

#Preview(traits: .defaultLayout) {
    RubyViewController()
}