方法一

    CGFloat fl1 = 1.23;
    CGFloat fl2 = 1.34;
    CGFloat fl3 = 1.35;
    CGFloat fl4 = 1.36;
    CGFloat fl5 = 1.44;
    CGFloat fl6 = 1.45;
    CGFloat fl7 = 1.46;
    
    NSLog(@"fl1 = %.1f, fl2 = %.1f, fl3 = %.1f, fl4 = %.1f, fl5 = %.1f, fl6 = %.1f, fl7 = %.1f",fl1,fl2,fl3,fl4,fl5,fl6,fl7);

打印结果:
fl1 = 1.2, fl2 = 1.3, fl3 = 1.4, fl4 = 1.4, fl5 = 1.4, fl6 = 1.4, fl7 = 1.5

从上边的打印结果可以看出当前面为1.3的时候最后一位为四舍五入, 当前面为1.4的时候最后一位为五舍六入, 这是因为该机制为当要保留的最后一位数为奇数时会采用四舍五入, 当要保留的最后一位为偶数时采用五舍六入;
如果想要更多的舍入精度可以采用以下方法二

方法二

这个方法涉及到两个类:NSDecimalNumber & NSDecimalNumberHandler

// 四舍五入涉及到的NSDecimalNumberHandler
// 类方法: + (instancetype)decimalNumberHandlerWithRoundingMode:(NSRoundingMode)roundingMode scale:(short)scale raiseOnExactness:(BOOL)exact raiseOnOverflow:(BOOL)overflow raiseOnUnderflow:(BOOL)underflow raiseOnDivideByZero:(BOOL)divideByZero;
/*
参数讲解:
RoundingMode  四舍五入的标准.
原始值: 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
    NSRoundPlain,   // Round up on a tie    严格遵守四舍五入
    NSRoundDown,    // Always down == truncate    直接舍去
    NSRoundUp,      // Always up                                 直接进一位
    NSRoundBankers  // on a tie round so last digit is even   进位标准同方法一

     scale : 需要保留的精度。
     raiseOnExactness : 为YES时在处理精确时如果有错误，就会抛出异常。
     raiseOnOverflow  : YES时在计算精度向上溢出时会抛出异常，否则返回。
     raiseOnUnderflow : YES时在计算精度向下溢出时会抛出异常，否则返回.
     raiseOnDivideByZero : YES时。当除以0时会抛出异常，否则返回。
*/
    NSDecimalNumber *num1 = [NSDecimalNumber decimalNumberWithString:@"1.34"];
    NSDecimalNumber *num2 = [NSDecimalNumber decimalNumberWithString:@"1.35"];
    NSDecimalNumber *num3 = [NSDecimalNumber decimalNumberWithString:@"1.44"];
    NSDecimalNumber *num4 = [NSDecimalNumber decimalNumberWithString:@"1.45"];
    NSDecimalNumberHandler *numHandler = [NSDecimalNumberHandler decimalNumberHandlerWithRoundingMode:NSRoundBankers scale:1 raiseOnExactness:NO raiseOnOverflow:NO raiseOnUnderflow:NO raiseOnDivideByZero:NO];
    
    NSString *str1 = [[num1 decimalNumberByRoundingAccordingToBehavior:numHandler] stringValue];
    NSString *str2 = [[num2 decimalNumberByRoundingAccordingToBehavior:numHandler] stringValue];
    NSString *str3 = [[num3 decimalNumberByRoundingAccordingToBehavior:numHandler] stringValue];
    NSString *str4 = [[num4 decimalNumberByRoundingAccordingToBehavior:numHandler] stringValue];
    NSLog(@"\n%@,%@",str1,str2);
    NSLog(@"\n%@,%@",str3,str4);
打印结果:
str1 = 1.3,str2 = 1.4
str3 = 1.4,str4 = 1.4

补充:
NSDecimalNumber 同样可以进行加减乘除运算:
- (NSDecimalNumber *)decimalNumberByAdding:(NSDecimalNumber *)decimalNumber;
- (NSDecimalNumber *)decimalNumberBySubtracting:(NSDecimalNumber *)decimalNumber;
- (NSDecimalNumber *)decimalNumberByMultiplyingBy:(NSDecimalNumber *)decimalNumber;
- (NSDecimalNumber *)decimalNumberByDividingBy:(NSDecimalNumber *)decimalNumber;

如有错误欢迎批评指正.
更多详细内容可参考苹果官方文档:
https://developer.apple.com/reference/foundation/nsdecimalnumber