swift4使用截取字符串的时候提示：string slicing subscript with a 'partial range from'.......，swift4slicing

swift4使用截取字符串的时候提示：string slicing subscript with a 'partial range from'.......，swift4slicing

let newStr = String(str[..<index]) // = str.substring(to: index) In Swift 3
let newStr = String(str[index...]) // = str.substring(from: index) In Swif 3
let newStr = String(str[range]) // = str.substring(with: range) In Swift 3

eg：

 

//截取字符串
    func getValue(fromStart start: String, toEnd end: String) -> String? {
        
        let startRange = range(of: start)
        guard startRange != nil else {
            return nil
        }
        //获取字符串：从startRange后面的字符开始
        let newStr = String(self[startRange!.upperBound...])
        let endRange = newStr.range(of: "&")
        if endRange != nil {
            //截取newStr的字符串，范围0到endRange
            var value = String(newStr[..<endRange!.upperBound])
            //将字符串中『=』替换掉：达到删除的效果
             value = value.replacingOccurrences(of: "=", with: "")
             value = value.replacingOccurrences(of: end, with: "")
            return value
        } else {
           var value = newStr
            value = value.replacingOccurrences(of: "=", with: "")
            value = value.replacingOccurrences(of: end, with: "")
            return value
        }
    }