iOS - Swift - NSAttributedString 显示Emoji乱码问题

最近在开发中，遇到一个奇怪的问题，就是当显示一串文本，当文本最后一个字符是Emoji时，Emoji显示乱码，如图：

错误展示

正确展示应该是这样的

正确展示

而展示逻辑也很简单，一个UILabel控件通过attributedText展示

signLabel.attributedText = user.introductionAttribute

如果用label.text展示的话，则是正常的

signLabel.text= user.introduction

直接说问题点和解决方案：

    var introductionAttribute: NSAttributedString {
        let attributedString = NSMutableAttributedString(string: introduction,
                                                         attributes: [.font: YRUserHeaderLayoutable.introFont])
        let paragraphStyle = NSMutableParagraphStyle()
        paragraphStyle.paragraphSpacing = 0
        paragraphStyle.firstLineHeadIndent = 0
        paragraphStyle.headIndent = 0
        paragraphStyle.paragraphSpacingBefore = 0
        attributedString.addAttribute(NSAttributedString.Key.paragraphStyle, value: paragraphStyle, range: NSRange(location: 0, length: introduction.count))
        return attributedString
    }

问题出在了最后一行代码

 attributedString.addAttribute(NSAttributedString.Key.paragraphStyle, value: paragraphStyle, range: NSRange(location: 0, length: introduction.count))

在计算字符串长度时， Swift计算的是

/// The number of characters in a string.
 public var count: Int { get }

let swiftCount = introduction.count
let ocLength = (introduction as NSString).length
print("introducetion:\(introduction)")
print("swiftCount:\(swiftCount)")
print("ocLength:\(ocLength)")

打印：

introducetion:这是一个emoji
swiftCount:10
ocLength:11

当用Swift.count计算长度时出错，修复也很简单，

attributedString.addAttribute(NSAttributedString.Key.paragraphStyle, value: paragraphStyle, range: NSRange(location: 0, length: (introduction as NSString).length))