项目中遇见精度问题， 代码给的 down 向下取整，但是当负数时候就会出错，变成向上取整
代码如下：
func number(string: String){
//方式1：不进行四舍五入
let number1 = NSDecimalNumber(string: string)
let number2 = NSDecimalNumber(string: "0")
let num = number1.adding(number2)
print("num =>\(num.stringValue)")
//方式2：进行四舍五入
//scale四舍五入，舍入位数
let scale:Int16 = 4
var round = NSDecimalNumberHandler(roundingMode: .down, scale:scale , raiseOnExactness: false, raiseOnOverflow: false, raiseOnUnderflow: false, raiseOnDivideByZero: true)
let down = number1.adding(number2, withBehavior: round)
print("down =>\(down.stringValue)")
round = NSDecimalNumberHandler(roundingMode: .up, scale: scale, raiseOnExactness: false, raiseOnOverflow: false, raiseOnUnderflow: false, raiseOnDivideByZero: true)
let Up = number1.adding(number2, withBehavior: round)
print("Up =>\(Up.stringValue)")
round = NSDecimalNumberHandler(roundingMode: .plain, scale: scale, raiseOnExactness: false, raiseOnOverflow: false, raiseOnUnderflow: false, raiseOnDivideByZero: true)
let plain = number1.adding(number2, withBehavior: round)
print("plain =>\(plain.stringValue)")
}
number(string: "0.012369")
number(string: "-0.012369")
精度设置的第四位，打印结果如下：
num =>0.012369
down =>0.0123
Up =>0.0124
plain =>0.0124
===============================
num =>-0.012369
down =>-0.0124
Up =>-0.0123
plain =>-0.0124
可以看出来， 当负数时， down 时向上取整，Up 是 向下取整
所以对于负数 需要特殊处理
