NSDecimalNumber简介
苹果针对浮点类型计算精度问题提供出来的计算类，基于十进制的科学计数法来计算，同时可以指定舍入模式，一般用于货币计算。中国货币最小流通单位是分，也就是小数点后两位。
NSDecimalNumberHandler

/**
 初始化方法
 @param roundingMode 舍入方式
 @param scale 小数点后舍入值的位数
 @param exact 精度错误处理
 @param overflow 溢出错误处理
 @param underflow 下溢错误处理
 @param divideByZero 除以0的错误处理
 @return NSDecimalNumberHandler对象
 */
+ (instancetype)decimalNumberHandlerWithRoundingMode:(NSRoundingMode)roundingMode scale:(short)scale raiseOnExactness:(BOOL)exact raiseOnOverflow:(BOOL)overflow raiseOnUnderflow:(BOOL)underflow raiseOnDivideByZero:(BOOL)divideByZero;

NSRoundingMode 的类型

typedef NS_ENUM(NSUInteger, NSRoundingMode) {
    NSRoundPlain,   // Round up on a tie
    NSRoundDown,    // Always down == truncate
    NSRoundUp,      // Always up
    NSRoundBankers  // on a tie round so last digit is even
};

NSRoundPlain 是四舍五入
NSRoundDown 是向下取整
NSRoundUp 是向上取整
NSRoundBankers 是在四舍五入的基础上，加上末尾数为5时，变成偶数的规则

一：四舍五入

// 四舍五入
    NSDecimalNumberHandler *roundPlain = [NSDecimalNumberHandler decimalNumberHandlerWithRoundingMode:NSRoundPlain scale:2 raiseOnExactness:NO raiseOnOverflow:NO raiseOnUnderflow:NO raiseOnDivideByZero:YES];
    {
        NSString *addStr = @"1.0";
        NSString *oneStr = @"1.114";
        NSString *twoStr = @"1.116";
        NSDecimalNumber *oneNum = [NSDecimalNumber decimalNumberWithString:oneStr];
        NSDecimalNumber *twoNum = [NSDecimalNumber decimalNumberWithString:twoStr];
        NSDecimalNumber *addNum = [NSDecimalNumber decimalNumberWithString:addStr];
        
        // 加法
        oneNum = [oneNum decimalNumberByAdding:addNum withBehavior:roundPlain];
        twoNum = [twoNum decimalNumberByAdding:addNum withBehavior:roundPlain];
        NSLog(@"\n1.114 + 1.0 （保留两位小数）四舍五入的结果为:%@\n1.116 + 1.0 （保留两位小数）四舍五入的结果为:%@", oneNum, twoNum);
    }

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二：向下取整

    // 向下取整
    NSDecimalNumberHandler *roundDown = [NSDecimalNumberHandler decimalNumberHandlerWithRoundingMode:NSRoundDown scale:2 raiseOnExactness:NO raiseOnOverflow:NO raiseOnUnderflow:NO raiseOnDivideByZero:YES];
    {
        NSString *subtractStr = @"1.0";
        NSString *oneStr = @"1.114";
        NSString *twoStr = @"1.116";
        NSDecimalNumber *oneNum = [NSDecimalNumber decimalNumberWithString:oneStr];
        NSDecimalNumber *twoNum = [NSDecimalNumber decimalNumberWithString:twoStr];
        NSDecimalNumber *subtractNum = [NSDecimalNumber decimalNumberWithString:subtractStr];
        
        // 减法
        oneNum = [oneNum decimalNumberBySubtracting:subtractNum withBehavior:roundDown];
        twoNum = [twoNum decimalNumberBySubtracting:subtractNum withBehavior:roundDown];
        NSLog(@"\n1.114 - 1.0 （保留两位小数）向下取整的结果为:%@\n1.116 - 1.0 （保留两位小数）向下取整的结果为:%@", oneNum, twoNum);
    }

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三：向上取整

    // 向上取整
    NSDecimalNumberHandler *roundUp = [NSDecimalNumberHandler decimalNumberHandlerWithRoundingMode:NSRoundUp scale:2 raiseOnExactness:NO raiseOnOverflow:NO raiseOnUnderflow:NO raiseOnDivideByZero:YES];
    {
        NSString *multiplyStr = @"1.0";
        NSString *oneStr = @"1.114";
        NSString *twoStr = @"1.116";
        NSDecimalNumber *oneNum = [NSDecimalNumber decimalNumberWithString:oneStr];
        NSDecimalNumber *twoNum = [NSDecimalNumber decimalNumberWithString:twoStr];
        NSDecimalNumber *multiplyNum = [NSDecimalNumber decimalNumberWithString:multiplyStr];
        
        // 乘法
        oneNum = [oneNum decimalNumberByMultiplyingBy:multiplyNum withBehavior:roundUp];
        twoNum = [twoNum decimalNumberByMultiplyingBy:multiplyNum withBehavior:roundUp];
        NSLog(@"\n1.114 * 1.0 （保留两位小数）向上取整的结果为:%@\n1.116 * 1.0 （保留两位小数）向上取整的结果为:%@", oneNum, twoNum);
    }

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四：NSRoundBankers

    // 在四舍五入的基础上，加上末尾数为5时，变成偶数的规则，例如小数点后两位， 1.215 时变为 1.22, 1.225 时变为 1.22
    NSDecimalNumberHandler *roundBankers = [NSDecimalNumberHandler decimalNumberHandlerWithRoundingMode:NSRoundBankers scale:2 raiseOnExactness:NO raiseOnOverflow:NO raiseOnUnderflow:NO raiseOnDivideByZero:YES];
    {
        NSString *dividStr = @"1.0";
        NSString *oneStr = @"1.115";
        NSString *twoStr = @"1.125";
        NSDecimalNumber *oneNum = [NSDecimalNumber decimalNumberWithString:oneStr];
        NSDecimalNumber *twoNum = [NSDecimalNumber decimalNumberWithString:twoStr];
        NSDecimalNumber *dividNum = [NSDecimalNumber decimalNumberWithString:dividStr];
        
        // 除法
        oneNum = [oneNum decimalNumberByDividingBy:dividNum withBehavior:roundBankers];
        twoNum = [twoNum decimalNumberByDividingBy:dividNum withBehavior:roundBankers];
        NSLog(@"\n1.115 / 1.0 （保留两位小数）NSRoundBankers的结果为:%@\n1.125 / 1.0 （保留两位小数）NSRoundBankers的结果为:%@", oneNum, twoNum);
    }

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