iOS 精准计算之 NSDecimalNumber

在有些场景下，我们需要精确到每一个小数点，而有些地方则需要显示最大的小数位；

如：[NSString stringWithFormat:@"%.2f",2000/100] 最终显示的是20.00 ，而需求则是显示20；

NSDecimalNumber

NSDecimalNumber 是iOS中处理精准计算的Number类；在一般的货币计算中，苹果推荐使用这个类；
下面将介绍下常用的方法：

• +

- (NSDecimalNumber *)decimalNumberByAdding:(NSDecimalNumber *)decimalNumber;
- (NSDecimalNumber *)decimalNumberByAdding:(NSDecimalNumber *)decimalNumber withBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;

• -

- (NSDecimalNumber *)decimalNumberBySubtracting:(NSDecimalNumber *)decimalNumber;
- (NSDecimalNumber *)decimalNumberBySubtracting:(NSDecimalNumber *)decimalNumber withBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;

• x

- (NSDecimalNumber *)decimalNumberByMultiplyingBy:(NSDecimalNumber *)decimalNumber;
- (NSDecimalNumber *)decimalNumberByMultiplyingBy:(NSDecimalNumber *)decimalNumber withBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;

• /

- (NSDecimalNumber *)decimalNumberByDividingBy:(NSDecimalNumber *)decimalNumber;
- (NSDecimalNumber *)decimalNumberByDividingBy:(NSDecimalNumber *)decimalNumber withBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;

• 幂运算

- (NSDecimalNumber *)decimalNumberByRaisingToPower:(NSUInteger)power;
- (NSDecimalNumber *)decimalNumberByRaisingToPower:(NSUInteger)power withBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;

• 10的次方运算

- (NSDecimalNumber *)decimalNumberByMultiplyingByPowerOf10:(short)power;
- (NSDecimalNumber *)decimalNumberByMultiplyingByPowerOf10:(short)power withBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;

• 比较

- (NSComparisonResult)compare:(NSNumber *)decimalNumber;

NSDecimalNumberHandler

这个类允许设置NSDecimalNumber对象四舍五入和处理错误的方式，而不必创建自定义类。

/*
  roundingMode：NSRoundPlain、NSRoundDown、NSRoundUp、NSRoundBankers
  scale: 小数点后的位数
  raiseOnExactness：如果YES，发生精准错误，处理程序将引发异常；否则忽略错误；
  raiseOnOverflow： 如果YES，发生溢出错误，处理程序将引发异常；否则忽略错误；
  raiseOnUnderflow： 如果YES，发生向下溢出错误，处理程序将引发异常；否则忽略错误；
  raiseOnDivideByZero：如果YES，发生除零错误，处理程序将引发异常；否则忽略错误；
**/
+ (instancetype)decimalNumberHandlerWithRoundingMode:(NSRoundingMode)roundingMode  scale:(short)scale  raiseOnExactness:(BOOL)exact  raiseOnOverflow:(BOOL)overflow  raiseOnUnderflow:(BOOL)underflow  raiseOnDivideByZero:(BOOL)divideByZero;