NSDecimalNumber是使用货币时最好的格式。
官方文档

NSDecimalNumber 是 NSNumber 的一个子类, 比 NSNumber 的功能更为强大, 四舍五入, 取整, 输入后自动去掉数值前面无用的 0 等等. 由于 NSDecimalNumber 精度较高, 所以会比基本数据类型费时, 所以需要权衡考虑, 苹果官方建议在货币以及要求精度很高的场景下使用.
通常情况下我们会使用 NSDecimalNumberHandler 这个格式化器对其需要约束的格式进行设置

NSDecimalNumberHandler

 let decHandler:NSDecimalNumberHandler = NSDecimalNumberHandler.init(roundingMode: .plain, scale: 2, raiseOnExactness: false, raiseOnOverflow: false, raiseOnUnderflow: false, raiseOnDivideByZero: true)

roundingMode 文档是一个枚举类型

 public enum RoundingMode : UInt {

        
        case plain = 0 //只入不舍

        case down = 1//只舍不入

        case up = 2//四舍五入

        case bankers = 3//四舍六入, 中间值时, 取最近的,保持保留最后一位为偶数
    }

scale: 保留几位小数

raiseOnExactness: 发生精确错误时是否抛出异常，一般为false

raiseOnOverflow: 发生溢出错误时是否抛出异常，一般为false

raiseOnUnderflow: 发生不足错误时是否抛出异常，一般为false

raiseOnDivideByZero: 除数是0时是否抛出异常，一般为true

补零问题

NSDecimalNumber 做精度限制, 如果小数点后面是以0结尾, 则不会显示,如1024.00 保留2位小数 结果为:1024
前端为了显示效果需要补零操作

    let decNumber = NSDecimalNumber.init(string: "1024")
    let decHandler:NSDecimalNumberHandler = NSDecimalNumberHandler.init(roundingMode: .plain, scale: 2,   raiseOnExactness: false, raiseOnOverflow: false, raiseOnUnderflow: false, raiseOnDivideByZero: true)
    let dNumber = decNumber.rounding(accordingToBehavior: decHandler)
    let p = 2
    let count : Int = (p < 0 ? 0 : p)
    var formatterString : String = "0."
    if count == 0 {
        formatterString = "0"
    }
    for _ in 0 ..< count {
        formatterString.append("0")
    }
    let formatter : NumberFormatter = NumberFormatter()
    formatter.positiveFormat = formatterString
    formatter.string(from: dNumber)!

完整方法

func precisionString(with number:String, precision: String , isRound: Bool = false) -> String {
  
   guard isPurnFloat(string: number) && isPurnFloat(string: precision) else {
       return number
   }
   guard conerData(precision, .MyInt).0! >= 0 else {
       return number
   }

   let decNumber = NSDecimalNumber.init(string: number)

   var decHandler:NSDecimalNumberHandler?
   if isRound {
       decHandler = NSDecimalNumberHandler.init(roundingMode: .plain, scale: Int16(precision)!, raiseOnExactness: false, raiseOnOverflow: false, raiseOnUnderflow: false, raiseOnDivideByZero: true)
   }else{
       if conerData(number, .MyDouble).2! > 0 {
           decHandler = NSDecimalNumberHandler.init(roundingMode: .up, scale: Int16(precision)!, raiseOnExactness: false, raiseOnOverflow: false, raiseOnUnderflow: false, raiseOnDivideByZero: true)
       }else{
           decHandler = NSDecimalNumberHandler.init(roundingMode: .up, scale: Int16(precision)!, raiseOnExactness: false, raiseOnOverflow: false, raiseOnUnderflow: false, raiseOnDivideByZero: true)
       }
   }
   let dNumber = decNumber.rounding(accordingToBehavior: decHandler)
   let p = conerData(precision, .MyInt).0!
   let count : Int = (p < 0 ? 0 : p)
   var formatterString : String = "0."
   if count == 0 {
       formatterString = "0"
   }
   for _ in 0 ..< count {
       formatterString.append("0")
   }
   let formatter : NumberFormatter = NumberFormatter()
   formatter.positiveFormat = formatterString
   return formatter.string(from: dNumber)!
}

//MARK:- 是否数字
public func isPurnFloat(string: String) -> Bool {
    let scan: Scanner = Scanner(string: string)
    var val:Double = 0
    return scan.scanDouble(&val) && scan.isAtEnd
}

NSDecimalNumber运算

NSDecimalNumber:是苹果针对浮点型计算时存在精度误差的问题而提供的一个类

let num1 = NSDecimalNumber.init(string: "1024.1024")
let num2 = NSDecimalNumber.init(string: "1024.1024")
//加
String(format: "%@", num1.adding(num2))
//减
String(format: "%@", num1.subtracting(num2))
//乘
String(format: "%@", num1.multiplying(num2))
//除
String(format: "%@", num1.dividing(num2))
//n次方(2次方)
String(format: "%@", num1.raising(toPower: 2))
//指数
String(format: "%@", num1.multiplying(byPowerOf10: 2))

相关文档

    open func adding(_ decimalNumber: NSDecimalNumber) -> NSDecimalNumber

    open func adding(_ decimalNumber: NSDecimalNumber, withBehavior behavior: NSDecimalNumberBehaviors?) -> NSDecimalNumber

    
    open func subtracting(_ decimalNumber: NSDecimalNumber) -> NSDecimalNumber

    open func subtracting(_ decimalNumber: NSDecimalNumber, withBehavior behavior: NSDecimalNumberBehaviors?) -> NSDecimalNumber

    
    open func multiplying(by decimalNumber: NSDecimalNumber) -> NSDecimalNumber

    open func multiplying(by decimalNumber: NSDecimalNumber, withBehavior behavior: NSDecimalNumberBehaviors?) -> NSDecimalNumber

    
    open func dividing(by decimalNumber: NSDecimalNumber) -> NSDecimalNumber

    open func dividing(by decimalNumber: NSDecimalNumber, withBehavior behavior: NSDecimalNumberBehaviors?) -> NSDecimalNumber

    
    open func raising(toPower power: Int) -> NSDecimalNumber

    open func raising(toPower power: Int, withBehavior behavior: NSDecimalNumberBehaviors?) -> NSDecimalNumber

 // compare two NSDecimalNumbers
    
    open class var defaultBehavior: NSDecimalNumberBehaviors

    // One behavior per thread - The default behavior is
    //   rounding mode: NSRoundPlain
    //   scale: No defined scale (full precision)
    //   ignore exactnessException
    //   raise on overflow, underflow and divide by zero.
    
    open var objCType: UnsafePointer<CChar> { get }

    // return 'd' for double
    
    open var doubleValue: Double { get }
    
    open func multiplying(byPowerOf10 power: Int16) -> NSDecimalNumber

    open func multiplying(byPowerOf10 power: Int16, withBehavior behavior: NSDecimalNumberBehaviors?) -> NSDecimalNumber