UITextField只容許輸入正數

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    if ([textField.text isEqualToString:@"0"] &&
        string.length)
    {
        if (![string isEqualToString:@"."])
        {
            return NO;
        }
        else
        {
            return YES;
        }
    }
    
    NSString *tmpString = [textField.text stringByAppendingString:string];
    
    if (![tmpString isNumber] && string.length)
    {
        if ([tmpString isFloatValue_fs])
        {
            return YES;
        }
        
        return NO;
    }
    
    return YES;
}