• 参数为要保留的小数个数

extension Float {
    /// Rounds the float to decimal places value

    func roundTo(places:Int) -> Float {
        let divisor = pow(10.0, Float(places))

        return (self * divisor).rounded() / divisor

    }

}

extension Double {
    /// Rounds the double to decimal places value

    func roundTo(places:Int) -> Double {
        let divisor = pow(10.0, Double(places))

        return (self * divisor).rounded() / divisor

    }

}

参考博客：
保留X位小数（Double） swift

String保留两位小数

var f = 123.123121313

var s = String(format:"%.2f",f)//123.32

参考博客：
swift 保留两位小数以及前面不0 - CSDN