textfield设置只能输入的字符长度

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string;

{

    if ([string isEqualToString:@"\n"])

    {

        return YES;

    }

    NSString * toBeString = [textField.text stringByReplacingCharactersInRange:range withString:string];

    

    if (creatNewpwdTextFied == textField)

    {

        if ([toBeString length] > 16) {

            textField.text = [toBeString substringToIndex:16];

            UIAlertView *alert1 = [[UIAlertView alloc] initWithTitle:@"提示" message:@"请输入6至16位密码" delegate:self cancelButtonTitle:@"确定" otherButtonTitles:nil, nil] ;

            [alert1 show];

            return NO;

        }

    }

    return YES;

}