Swift4.0以后字符串截取发生了变化，为了使用起来方便，创建了一个String的扩展实现分类效果
extension String{
/// 从某个位置开始截取：
/// - Parameter index: 起始位置
public func substring(from index: Int) -> String {
if(self.count > index){
let startIndex = self.index(self.startIndex,offsetBy: index)
let subString = self[startIndex..<self.endIndex];
return String(subString);
}else{
return ""
}
}
/// 从零开始截取到某个位置：
/// - Parameter index: 达到某个位置
public func substring(to index: Int) -> String {
if(self.count > index){
let endindex = self.index(self.startIndex, offsetBy: index)
let subString = self[self.startIndex..<endindex]
return String(subString)
}else{
return self
}
}
/// 某个范围内截取
/// - Parameter rangs: 范围
public func subString(rang rangs:NSRange) -> String{
var string = String()
if(rangs.location >= 0) && (self.count > (rangs.location + rangs.length)){
let startIndex = self.index(self.startIndex,offsetBy: rangs.location)
let endIndex = self.index(self.startIndex,offsetBy: (rangs.location + rangs.length))
let subString = self[startIndex..<endIndex]
string = String(subString)
}
return string
}
}
