给所有的double类型扩展一个新方法.

extension Double {
    func roundTo(places: Int) -> Double {
        let divisor = pow(10.0,Double(places))
        return (self * divisor).rounded() / divisor
    }
}

这样就可以像这样使用了: 3.1415.roundTo(places: 2)

其中rounded用法如下:

(5.2).rounded()
// 5.0
(5.5).rounded()
// 6.0
(-5.2).rounded()
// -5.0
(-5.5).rounded()
// -6.0

参考: http://stackoverflow.com/questions/27338573/rounding-a-double-value-to-x-number-of-decimal-places-in-swift