为什么设置attributionText的背景会引发NSRangeException错误？

Question

所以我尝试动态设置蓝色背景颜色为UILabel的归属文字，因为用户将手指沿着屏幕平移，但我不能为生活做好准备我弄清楚为什么我收到NSRangeException时出现以下错误消息：Terminating app due to uncaught exception 'NSRangeException', reason: 'NSMutableRLEArray objectAtIndex:effectiveRange:: Out of bounds。

这是我的代码：

var afterContext = ""                    
    if textDocumentProxy.documentContextAfterInput != nil {
       afterContext = textDocumentProxy.documentContextAfterInput!
    } else {
       afterContext = ""
    }

    if beforeContext != nil {              
        if initialContext == nil {                                         
            myRange = NSRange(location: 0, length:afterContext.characters.count)

           let attrString = NSMutableAttributedString(string: selectedTextView.text!)
           attrString.addAttribute(NSBackgroundColorAttributeName, value: UIColor(red: 20/255, green: 111/255, blue: 225/255, alpha: 1), range: myRange)
        }
     }

Answer 1

这个简单的if条件好像已经修好了

if afterContext.characters.count > 0 {
   myRange = NSRange(location: 0, length: afterContext.characters.count-1)
} else {
   myRange = NSRange(location: 0, length: afterContext.characters.count)
}