swift Range <---> objc NSRange 互換

extension String {

    func rangeFromNSRange(nsRange : NSRange) -> Range<String.Index>? {

        if let from = String.Index(self.utf16.startIndex + nsRange.location, within: self),

            let to = String.Index(self.utf16.startIndex + nsRange.location + nsRange.length, within: self) {

                return from ..< to

        }

        return nil

    }

    func NSRangeFromRange(range : Range<String.Index>) -> NSRange {

        let utf16view = self.utf16

        let from = String.UTF16View.Index(range.startIndex, within: utf16view)

        let to = String.UTF16View.Index(range.endIndex, within: utf16view)

        return NSMakeRange(from - utf16view.startIndex, to - from)

    }

}