Swift截取字符串(String.Index)

extension String {
  func toInt() -> Int? {
      return Int(self)
  }

  func toFloat() -> Float? {
      return Float(self)
  }

  func toDouble() -> Double? {
      return Double(self)
  }

  //MARK:- 去除字符串两端的空白字符
  func trim() -> String {
      return self.trimmingCharacters(in: CharacterSet.whitespaces)
  }   

  func subString(to: Int) -> String {
    var to = to
    if to > self.count {
        to = self.count
    }
    return String(self.prefix(to))
}

  func subString(from: Int) -> String {
    if from >= self.count {
        return ""
    }
    let startIndex = self.index(self.startIndex, offsetBy: from)
    let endIndex = self.endIndex
    return String(self[startIndex..<endIndex])
}

 func subString(start: Int, end: Int) -> String {
    if start < end {
        let startIndex = self.index(self.startIndex, offsetBy: start)
        let endIndex = self.index(self.startIndex, offsetBy: end)
        
        return String(self[startIndex..<endIndex])
    }
    return ""
}
    
}