Swift - 扩展String下标方法

// MARK: - 扩展String下标方法
extension String
{
    /// 根据开始位置和长度截取字符串，索引从0开始
    func subString(start: Int, length: Int = -1) -> String
    {
        var len = length
        if len == -1
        {
            len = self.count - start
        }
        let st = self.index(startIndex, offsetBy: start)
        let en = self.index(st, offsetBy: len)
        return String(self[st ..< en])
    }
}

转自：航歌