iOS开发中金钱货币的计算问题

ios开发中涉及到和货币价格计算相关的，都需要注意计算精度的问题。
服务器有可能返回的金钱是int float double string NSNumber, 单纯的float、double的计算都会出现精度问题，即使只是两位小数，也会出现误差。
苹果为我们提供了NSDecimalNumber的API来处理金钱的计算的问题：
1、字符串转float等不精确问题。
2、精确计算
3、保留小数位数
4、四舍五入及其它的保留小数位数规则

NSDecimalNumber的使用

**1.把相应的类型转换成NSDecimalNumber类型 **

    //1、NSString -> NSDecimalNumber
    NSDecimalNumber *num1 = [NSDecimalNumber decimalNumberWithString:@"100"];
    NSLog(@"%@",num1);
    
    //2、NSNumber -> NSDecimalNumber(基础类型的话，先转成NSNumber)
    NSDecimalNumber *num2 = [NSDecimalNumber decimalNumberWithDecimal:[[NSNumber numberWithInt:50] decimalValue]];
    NSLog(@"%@",num2);

2.相关运算
加+

/*加 +
     - (NSDecimalNumber *)decimalNumberByAdding:(NSDecimalNumber *)decimalNumber;
     - (NSDecimalNumber *)decimalNumberByAdding:(NSDecimalNumber *)decimalNumber withBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;//带保留小数位数
     */
    NSDecimalNumber *num3 = [num1 decimalNumberByAdding:num2];
    NSLog(@"num3 == %@",num3);

减 -

    /*减 -
     - (NSDecimalNumber *)decimalNumberBySubtracting:(NSDecimalNumber *)decimalNumber;
     - (NSDecimalNumber *)decimalNumberBySubtracting:(NSDecimalNumber *)decimalNumber withBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;//带保留小数位数
     */
    NSDecimalNumber *num4 = [num1 decimalNumberBySubtracting:num2];
    NSLog(@"num4 == %@",num4);

乘 *

    /*乘 *
     - (NSDecimalNumber *)decimalNumberByMultiplyingBy:(NSDecimalNumber *)decimalNumber;
     - (NSDecimalNumber *)decimalNumberByMultiplyingBy:(NSDecimalNumber *)decimalNumber withBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;//带保留小数位数
     */
    NSDecimalNumber *num5 = [num1 decimalNumberByMultiplyingBy:num2];
    NSLog(@"num5 == %@",num5);

除 \

    /*除 \
     - (NSDecimalNumber *)decimalNumberByDividingBy:(NSDecimalNumber *)decimalNumber;
     - (NSDecimalNumber *)decimalNumberByDividingBy:(NSDecimalNumber *)decimalNumber withBehavior:(nullable id <NSDecimalNumberBehaviors>)behavior;//带保留小数位数
     */
    NSDecimalNumber *num6 = [num1 decimalNumberByDividingBy:num2];
    NSLog(@"num6 == %@",num6);

比较

    /*比较
     - (NSComparisonResult)compare:(NSNumber *)decimalNumber;
     */
    NSComparisonResult result = [num1 compare:num2];
    NSLog(@"result == %ld",(long)result);

保留小数及相关参数含义

 // 保留小数
+ (instancetype)decimalNumberHandlerWithRoundingMode:(NSRoundingMode)roundingMode scale:(short)scale raiseOnExactness:(BOOL)exact raiseOnOverflow:(BOOL)overflow raiseOnUnderflow:(BOOL)underflow raiseOnDivideByZero:(BOOL)divideByZero;

//1、 NSRoundingMode
    // Rounding policies :
    // Original
    //    value 1.2  1.21  1.25  1.35  1.27
    // Plain    1.2  1.2   1.3   1.4   1.3
    // Down     1.2  1.2   1.2   1.3   1.2
    // Up       1.2  1.3   1.3   1.4   1.3
    // Bankers  1.2  1.2   1.2   1.4   1.3
    
    /* typedef NS_ENUM(NSUInteger, NSRoundingMode) {
     NSRoundPlain,   // Round up on a tie        四舍五入
     NSRoundDown,    // Always down == truncate  向下保留
     NSRoundUp,      // Always up                向上保留
     NSRoundBankers  // on a tie round so last digit is even 貌似是：
     
     if(四舍五入位 == 5)（四舍五入位（5）+  保留位 ）%2 == 0 ？ 入 ： 舍 ；
     
     if(四舍五入位 ！= 5) 遵从四舍五入
     };
     */
    //2、scale 保留小数个数
    //3、其它：溢出等是否报错

    NSDecimalNumber *num_1 = [NSDecimalNumber decimalNumberWithString:@"1.2"];
    NSDecimalNumber *num_2 = [NSDecimalNumber decimalNumberWithString:@"3.1415926"];
    
    NSDecimalNumberHandler *handel = [NSDecimalNumberHandler decimalNumberHandlerWithRoundingMode:NSRoundBankers scale:2 raiseOnExactness:NO raiseOnOverflow:NO raiseOnUnderflow:NO raiseOnDivideByZero:NO];
        
    NSDecimalNumber *num_3 = [num_1 decimalNumberByMultiplyingBy:num_2 withBehavior:handel];
    NSLog(@"num_3 == %@",num_3);

3.对金钱进行千分位和保留两位小数处理

3.1使用NSNumberFormatter去格式化

    NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
    formatter.numberStyle = kCFNumberFormatterCurrencyAccountingStyle;
    NSString *newAmount = [formatter stringFromNumber:[NSNumber numberWithLong:[@"10000" integerValue]]];
    NSLog(@"newAmount==%@",newAmount);

/*
typedef CF_ENUM(CFIndex, CFNumberFormatterStyle) {  // number format styles
    kCFNumberFormatterNoStyle = 0,
    kCFNumberFormatterDecimalStyle = 1,
    kCFNumberFormatterCurrencyStyle = 2,
    kCFNumberFormatterPercentStyle = 3,
    kCFNumberFormatterScientificStyle = 4,
    kCFNumberFormatterSpellOutStyle = 5,
    kCFNumberFormatterOrdinalStyle CF_ENUM_AVAILABLE(10_11, 9_0) = 6,
    kCFNumberFormatterCurrencyISOCodeStyle CF_ENUM_AVAILABLE(10_11, 9_0) = 8,
    kCFNumberFormatterCurrencyPluralStyle CF_ENUM_AVAILABLE(10_11, 9_0) = 9,
    kCFNumberFormatterCurrencyAccountingStyle CF_ENUM_AVAILABLE(10_11, 9_0) = 10,
};
*/

3.2使用分类方法处理

- (NSString *)positiveFormat:(NSString *)text{
   if(!text || [text floatValue] == 0){
       return @"0.00";
   }
   if (text.floatValue < 1000) {
       return  [NSString stringWithFormat:@"%.2f",text.floatValue];
   };
   
   NSNumberFormatter *numberFormatter = [[NSNumberFormatter alloc] init];
   [numberFormatter setPositiveFormat:@",###.00;"];
   return [numberFormatter stringFromNumber:[NSNumber numberWithDouble:[text doubleValue]]];
}