如何用UITextView调整自定义下划线的垂直位置，用非零lineSpacing NSParagraphStyle调整NSLayoutManager？

Question

我想自定义NSSAttributedString的下划线样式，因为patternDot实际上只是短破折号，我想要真正的圆点(例如，参见Customize underline pattern in NSAttributedString (iOS7+))。

我取得了相当远，但我有一个不一致的对齐问题与结果的下划线，显然是因为自定义行距。如果下划线不是在文本的最后一行，那么下划线就太低了，但是如果是在最后一行文本上，下划线就太高了。大概是因为最后一行没有任何行距。我想，如果有一种方法，在我的drawUnderline中找出我开的是哪一行文字，我可以相应地调整y偏移量，但还有更简单的方法吗？

示例：

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class DotUnderlineLayoutManager: NSLayoutManager {
  let color: UIColor

  init(color: UIColor = .black) {
    self.swatch = swatch
    super.init()
  }

  override func drawUnderline(forGlyphRange glyphRange: NSRange, underlineType _: NSUnderlineStyle, baselineOffset: CGFloat, lineFragmentRect: CGRect, lineFragmentGlyphRange _: NSRange, containerOrigin: CGPoint) {
    guard let container = textContainer(
      forGlyphAt: glyphRange.location,
      effectiveRange: nil
    ) else { return }

    let rect = boundingRect(forGlyphRange: glyphRange, in: container)

    let offsetRect = rect.offsetBy(
      dx: containerOrigin.x,
      dy: containerOrigin.y // + baselineOffset <- adding this helps a bit with an underline on the last line but messes up other lines even more
    )

    let path = UIBezierPath()
    path.strokeDottedLine(under: offsetRect, color: color)    
  }
}

private extension UIBezierPath {
  func strokeDottedLine(under rect: CGRect, color: UIColor) {
    lineWidth = 2
    lineCapStyle = .round
    setLineDash([0.1, 5], count: 2, phase: 0)
    move(to: .init(x: rect.minX, y: rect.maxY))
    addLine(to: .init(x: rect.maxX, y: rect.maxY))
    color.setStroke()
    stroke()
  }
}



extension NSUnderlineStyle {
  static var patternCircularDot: NSUnderlineStyle {
    NSUnderlineStyle(rawValue: 0x11)
  }
}

let textView: UITextView = {
    let layout = DotUnderlineLayoutManager(swatch: swatch)
    let storage = NSTextStorage()
    storage.addLayoutManager(layout)
    let initialSize = CGSize(width: 0, height: CGFloat.greatestFiniteMagnitude)
    let container = NSTextContainer(size: initialSize)
    container.widthTracksTextView = true
    layout.addTextContainer(container)

    let textView = UITextView(frame: .zero, textContainer: container)
    textView.isUserInteractionEnabled = false
    textView.isEditable = false
    textView.isScrollEnabled = false
    textView.backgroundColor = .clear
    return textView
}()

let paragraphStyle = NSMutableParagraphStyle()
paragraphStyle.lineBreakMode = .byWordWrapping
paragraphStyle.lineSpacing = 10 // any non-zero value

textView.attributedText = NSAttributedString(
    string: myText,
    attributes: [
        .paragraphStyle: paragraphStyle
        .underlineStyle: NSUnderlineStyle.patternCircularDot.union(.single).rawValue
    ]
)

编辑:自定义字体实际上并不重要，只是段落样式上的行距，相应地简化了问题：

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Answer 1

与其通过NSMutableParagraphStyle设置行距，不如使用NSLayoutManagerDelegate的layoutManager(lineSpacingAfterGlyphAt…)。

func layoutManager(_ layoutManager: NSLayoutManager, lineSpacingAfterGlyphAt glyphIndex: Int, withProposedLineFragmentRect rect: CGRect) -> CGFloat {
    // set line spacing as needed here
}

我甚至将我的NSLayoutManager子类设置为它自己的委托。疯了，我知道。

然后，我调整了我的直角偏移，减去一半的行距，这似乎很好。