swift 小数精确到小数点几位数

extension Double {
///四舍五入 到小数点后某一位
func roundTo(places: Int) -> Double {
let divisor = pow(10.0, Double(places))
return (self * divisor).rounded() / divisor
}
///截断 到小数点后某一位
func truncate(places: Int) -> Double {
let divisor = pow(10.0, Double(places))
return Double(Int(self * divisor)) / divisor
}
}