Swift3 & Xcode8 : ’subscript' is unavailable: cannot subscript String with a CountableClosedRange,

Question

错误 1:当我尝试从元数据中获取 stringValue 时，在 Swift3 中显示上述错误:

let myMetadata: AVMetadataMachineReadableCodeObject = metadataObjects[0] as! AVMetadataMachineReadableCodeObject

// take out the system and check-digits
let myBarcode = myMetadata.stringValue[1...11] //error 

错误 2:在 String 的扩展中，我编写了这些以获取 right(x) 和 left(x) 函数以获取子字符串:

extension String {

    // length of string
    var length: Int {
        return self.characters.count
    }

    //  right(x) and left(x) function to get substring
    func right(_ i: Int) -> String?
    {
        return self[self.length-i ... self.length-1 ] //error  
    }

    func left(_ i: Int) -> String?
    {
        return self[0 ... i-1] //error     
    }
}

Answer 1

将此扩展用于可数闭区间 [0...4] 下标

extension String {

  subscript (r: CountableClosedRange<Int>) -> String {
    get {
      let startIndex =  self.index(self.startIndex, offsetBy: r.lowerBound)
      let endIndex = self.index(startIndex, offsetBy: r.upperBound - r.lowerBound)
      return self[startIndex...endIndex]
    }
  }
}

或更安全的版本，它检查边界并返回 nil 而不是超出范围的异常:

extension String {

    subscript (r: CountableClosedRange<Int>) -> String? {
        get {
            guard r.lowerBound >= 0, let startIndex = self.index(self.startIndex, offsetBy: r.lowerBound, limitedBy: self.endIndex),
                let endIndex = self.index(startIndex, offsetBy: r.upperBound - r.lowerBound, limitedBy: self.endIndex) else { return nil }
            return self[startIndex...endIndex]
        }
    }
}

Swift 4 的变化:你需要从结果中创建一个新的字符串

return String(self[startIndex...endIndex])