NSDecimalNumber用于精度准确的计算

在处理金额计算时，往往会涉及到小数，由于Double类型不准确，无法做到产品的要求。为了保证金额计算的准确性，建议使用NSDecimalNumber。

1.创建对象（常用的方法）

// mantissa：长整型数值；exponent：指数（几次方）；flag：正负数。
+ (NSDecimalNumber *)decimalNumberWithMantissa:(unsigned long long)mantissa exponent:(short)exponent isNegative:(BOOL)flag;

NSDecimalNumber *decimalNumber = [NSDecimalNumber decimalNumberWithMantissa:829 exponent:-2 isNegative:NO];   //8.29
decimalNumber = [NSDecimalNumber decimalNumberWithMantissa:829 exponent:2 isNegative:YES]; //-82900

// locale代表一种格式
+ (NSDecimalNumber *)decimalNumberWithString:(nullable NSString *)numberValue locale:(nullable id)locale;
NSDictionary *locale = [NSDictionary dictionaryWithObject:@"," forKey:NSLocaleDecimalSeparator];    //以","当做小数点格式
NSDecimalNumber *discountAmount = [NSDecimalNumber decimalNumberWithString:@"123,40" locale:locale];    //123.4

+ (NSDecimalNumber *)decimalNumberWithString:(nullable NSString *)numberValue;

2.逻辑运算

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

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

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

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

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

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

四舍五入运算：
- (NSDecimalNumber *)decimalNumberByRoundingAccordingToBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;

比较运算：
-(NSComparisonResult)compare:(NSNumber *)decimalNumber;

3.数值格式设置

// roundingMode:舍入模式； scale：保留几位小数；exact：发生精确错误时是否抛出异常，一般为NO；overflow：发生溢出错误时是否抛出异常，一般为NO； underflow：发生不足错误时是否抛出异常，一般为NO； divideByZero：被0除时是否抛出异常，一般为YES
+ (instancetype)decimalNumberHandlerWithRoundingMode:(NSRoundingMode)roundingMode scale:(short)scale raiseOnExactness:(BOOL)exact raiseOnOverflow:(BOOL)overflow raiseOnUnderflow:(BOOL)underflow raiseOnDivideByZero:(BOOL)divideByZero;

    NSDecimalNumberHandler *roundUp = [NSDecimalNumberHandler
                                       decimalNumberHandlerWithRoundingMode:NSRoundDown
                                       scale:2
                                       raiseOnExactness:NO
                                       raiseOnOverflow:NO
                                       raiseOnUnderflow:NO
                                       raiseOnDivideByZero:YES];