Swift 5解决方案

简短的回答带有主要的扩展

extension NSRange {

    public init(range: Range<String.Index>, 
                originalText: String) {

        self.init(location: range.lowerBound.utf16Offset(in: originalText),
                  length: range.upperBound.utf16Offset(in: originalText) - range.lowerBound.utf16Offset(in: originalText))
    }
}

详情请看这里

Martin R的回答似乎是正确的，因为它代表了Unicode。

然而，在发布这篇文章(Swift 1)时，他的代码不能在Swift 2.0 (Xcode 7)中编译，因为他们删除了advance()函数。更新版本如下:

斯威夫特2

extension String {
    func rangeFromNSRange(nsRange : NSRange) -> Range<String.Index>? {
        let from16 = utf16.startIndex.advancedBy(nsRange.location, limit: utf16.endIndex)
        let to16 = from16.advancedBy(nsRange.length, limit: utf16.endIndex)
        if let from = String.Index(from16, within: self),
            let to = String.Index(to16, within: self) {
                return from ..< to
        }
        return nil
    }
}

斯威夫特3

extension String {
    func rangeFromNSRange(nsRange : NSRange) -> Range<String.Index>? {
        if let from16 = utf16.index(utf16.startIndex, offsetBy: nsRange.location, limitedBy: utf16.endIndex),
            let to16 = utf16.index(from16, offsetBy: nsRange.length, limitedBy: utf16.endIndex),
            let from = String.Index(from16, within: self),
            let to = String.Index(to16, within: self) {
                return from ..< to
        }
        return nil
    }
}

斯威夫特4

extension String {
    func rangeFromNSRange(nsRange : NSRange) -> Range<String.Index>? {
        return Range(nsRange, in: self)
    }
}

这与Emilie的回答相似，但是，因为你特别问如何将NSRange转换为Range<String。索引>，你会这样做:

func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {

     let start = advance(textField.text.startIndex, range.location) 
     let end = advance(start, range.length) 
     let swiftRange = Range<String.Index>(start: start, end: end) 
     ...

}

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    
    guard let current = textField.text, let r = Range(range, in: current) else {
        return false
    }
    
    let text = current.replacingCharacters(in: r, with: string)
    // ...
    return true
}

extension StringProtocol where Index == String.Index {

    func nsRange(of string: String) -> NSRange? {
        guard let range = self.range(of: string) else {  return nil }
        return NSRange(range, in: self)
    }
}

我发现最干净的swift2唯一的解决方案是在NSRange上创建一个类别:

extension NSRange {
    func stringRangeForText(string: String) -> Range<String.Index> {
        let start = string.startIndex.advancedBy(self.location)
        let end = start.advancedBy(self.length)
        return Range<String.Index>(start: start, end: end)
    }
}

然后调用它from for textfield delegate函数:

func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {
    let range = range.stringRangeForText(textField.text)
    let output = textField.text.stringByReplacingCharactersInRange(range, withString: string)

    // your code goes here....

    return true
}