如何获取 UIFont 的百分比 lineSpacing？ （计算百分比值）

Question

有没有办法将 lineSpacing property of NSParagraphStyle / NSMutableParagraphStyle attribute of an NSAttributedString 对象设置为字体 base/original lineSpacing 的百分比？

lineSpacing 是 NSParagraphStyle 的 属性 而不是 UIFont，因此不可能知道字体的原始 lineSpacing 是什么。

作为参考，这是我写的一个小扩展，用于将 lineSpacing 分配给 UILabel（累积，不会导致标签的 attributedString 上的信息丢失）：

import UIKit

extension UILabel {
    /// -important: must be called after setting the `text` property so the computed NSRange would be valid
    func setLineSpacing(with lineSpacing: CGFloat) {
        // get safe string
        guard let textString = self.text, !textString.isEmpty
            else { return }

        // get the range
        let entireRange: NSRange = (textString as NSString).range(of: textString)
        guard entireRange.isValidNSRange(within: textString)
            else { assertionFailure() ; return }

        // NSMutableAttributedText
        guard let mutableAttributedText: NSMutableAttributedString = self.attributedText?.mutableCopy() as? NSMutableAttributedString
            else { assertionFailure() ; return }

        // NSParagraphStyle
        var paragraphStyle: NSParagraphStyle? = mutableAttributedText.attribute(NSParagraphStyleAttributeName, at: 0, longestEffectiveRange: nil, in: entireRange) as? NSParagraphStyle
        if paragraphStyle == nil {
            // why TTTAttributedLabel :(
            paragraphStyle = NSParagraphStyle()
        }

        // safe NSParagraphStyle
        guard let safeParagraphStyle: NSParagraphStyle = paragraphStyle
            else { assertionFailure() ; return }

        // NSMutableParagraphStyle
        guard let mutableParagraphStyle: NSMutableParagraphStyle = safeParagraphStyle.mutableCopy() as? NSMutableParagraphStyle
            else { assertionFailure() ; return }

        // this is where the magic happens
        mutableParagraphStyle.lineSpacing = lineSpacing
        mutableAttributedText.addAttribute(NSParagraphStyleAttributeName, value: mutableParagraphStyle, range: entireRange)

        // assign attributed text which has all the existing attributes plus the new line spacing attribute which is inside the paragraphstyle attribute...
        self.attributedText = mutableAttributedText
    }
}

extension NSRange {
    // Is it safe to use range on the string?
    func isValidNSRange(within string: String) -> Bool {
        if self.location != NSNotFound
            && self.length > 0
            && self.location + self.length - 1 < string.length
            && self.location >= 0
        {
            return true
        } else {
            return false
        }
    }
}

我已经研究过这个问题，在 Whosebug 上找不到答案。所以我分享我自己的 FWIW。 （我想包含 CoreText 标签，这样可以很容易地搜索到，但我的名声不允许）

Answer 1

解决方案

extension UIFont {
    func getLineSpacing(with percentage: CGFloat = 1) -> CGFloat {
        let lineSpacing: CGFloat = (self.lineHeight - self.pointSize)// /2

        return lineSpacing * percentage
    }
}

我基于我的解决方案 this article and this interesting Chinese article。更具体地说，我附上了我用来推断答案的部分的屏幕截图：