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

【问题标题】：How to get percentage lineSpacing of UIFont? (to calculate a percentage value)如何获得 UIFont 的百分比 lineSpacing？（计算百分比值）
【发布时间】：2019-06-08 14:43:31
【问题描述】：

有没有办法将NSAttributedString 对象的NSParagraphStyle / NSMutableParagraphStyle 属性的lineSpacing 属性设置为字体基本/原始lineSpacing 的百分比？

lineSpacing 是NSParagraphStyle 而不是UIFont 的属性，因此无法知道字体的原始lineSpacing 是什么。

作为参考，这是我为将lineSpacing 分配给UILabel 编写的一个小扩展（累积，不会导致标签的属性字符串上的信息丢失）：

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 Stackoverflow 上没有找到答案。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，但我的名声不允许我这样做）

【问题讨论】：

标签： ios uikit foundation core-text uifont

【解决方案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
    }
}

我的解决方案基于this article 和this interesting Chinese article。更具体地说，我附上了我用来推断答案的部分截图：

【讨论】：