Swift4 在String中 NSRange转Range

官方文档

         let snowy = "❄️ Let it snow! ☃️"
         let nsrange = NSRange(location: 3, length: 12)
         if let range = Range(nsrange, in: snowy) {
             print(snowy[range])
         }
         // Prints "Let it snow!"

String拓展

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