fractional part

+ (NSNumberFormatter *)numberFormatter {
    static NSNumberFormatter *_numberFormatter = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _numberFormatter = [[NSNumberFormatter alloc] init];
        [_numberFormatter setNumberStyle:NSNumberFormatterDecimalStyle];
        [_numberFormatter setLocale:[NSLocale currentLocale]];
        _numberFormatter.usesGroupingSeparator = NO;
        _numberFormatter.maximumFractionDigits = 1;
    });
    return _numberFormatter;
}

- (NSNumber *)amountValueFromString:(NSString *)string {
    NSNumberFormatter *numberFormatter = [AppHelper numberFormatter];
    NSNumber *numberValue = [numberFormatter numberFromString:string];
    return numberValue;
}

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
    NSString *newString = [textField.text stringByReplacingCharactersInRange:range withString:string];
    NSNumber *amountValue = [self amountValueFromString:newString];
    double fractionPart = [amountValue doubleValue] - floor([amountValue doubleValue]);
    BOOL fractionPartTooPrecise = fractionPart != 0 && fractionPart < 0.1;
    
    ...
}