对 NSAttributedString 属性的表情符号支持（紧缩/段落样式）

Question

我在UILabel 上使用字距调整属性来显示带有一些自定义字母间距的文本。不幸的是，当我显示用户生成的字符串时，我有时会看到如下内容：

即有时某些表情符号字符不显示。

如果我注释掉字距调整但应用了一些段落样式，我会得到相同类型的错误渲染。

我在文档中找不到任何明确拒绝支持特殊 unicode 字符的内容。我做错了什么还是 iOS 错误？

重现该错误的代码可在此处作为游乐场使用：https://github.com/Bootstragram/Playgrounds/tree/master/LabelWithEmoji.playground

并复制到这里：

//: A UIKit based Playground for presenting user interface

import UIKit
import PlaygroundSupport

extension NSAttributedString {
    static func kernedSpacedText(_ text: String,
                                    letterSpacing: CGFloat = 0.0,
                                    lineHeight: CGFloat? = nil) -> NSAttributedString {
        // TODO add the font attribute

        let attributedString = NSMutableAttributedString(string: text)
        attributedString.addAttribute(NSAttributedStringKey.kern,
                                      value: letterSpacing,
                                      range: NSRange(location: 0, length: text.count))

        if let lineHeight = lineHeight {
            let paragraphStyle = NSMutableParagraphStyle()
            paragraphStyle.lineSpacing = lineHeight

            attributedString.addAttribute(NSAttributedStringKey.paragraphStyle,
                                          value: paragraphStyle,
                                          range: NSRange(location: 0, length: text.count))
        }

        return attributedString
    }
}

//for familyName in UIFont.familyNames {
//    for fontName in UIFont.fontNames(forFamilyName: familyName) {
//        print(fontName)
//    }
//}

class MyViewController : UIViewController {
    override func loadView() {
        let view = UIView()
        view.backgroundColor = .white

        let myString = "1⚽????????⚾️????????‍♂️????\n2 ???????????? ????"

        let label = UILabel()
        label.frame = CGRect(x: 150, y: 200, width: 200, height: 100)
        label.attributedText = NSAttributedString.kernedSpacedText(myString)
        label.numberOfLines = 0
        label.textColor = .black

        view.addSubview(label)
        self.view = view
    }
}
// Present the view controller in the Live View window
PlaygroundPage.current.liveView = MyViewController()

谢谢。

Answer 1

TL、DR：

String.count != NSString.length。任何时候看到NSRange，都必须将String 转换为UTF-16：

static func kernedSpacedText(_ text: String,
                                letterSpacing: CGFloat = 0.0,
                                lineHeight: CGFloat? = nil) -> NSAttributedString {
    // TODO add the font attribute

    let attributedString = NSMutableAttributedString(string: text)
    attributedString.addAttribute(NSAttributedStringKey.kern,
                                  value: letterSpacing,
                                  range: NSRange(location: 0, length: text.utf16.count))

    if let lineHeight = lineHeight {
        let paragraphStyle = NSMutableParagraphStyle()
        paragraphStyle.lineSpacing = lineHeight

        attributedString.addAttribute(NSAttributedStringKey.paragraphStyle,
                                      value: paragraphStyle,
                                      range: NSRange(location: 0, length: text.utf16.count))
    }

    return attributedString
}

---

更长的解释

您的问题是在 Swift 的 String 和 ObjC 的 NSString 之间转换的常见问题。 String 的长度是扩展字形簇的数量；在 ObjC 中，它是编码该字符串所需的 UTF-16 代码点数。

以大拇指字符为例：

let str = "?"
let nsStr = str as NSString

print(str.count)    // 1
print(nsStr.length) // 2

当涉及到国旗表情符号时，事情会变得更加奇怪：

let str = "??"
let nsStr = str as NSString

print(str.count)    // 1
print(nsStr.length) // 4    

尽管这篇文章早在 2003 年就已经写好了，但今天仍然是一本好书： The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets.