Stateless validators

Oleg Posyniak · January 4, 2017

php

Any developer is able to write validator. For date, time, URL, no matter. But what’s a correct way to implement it?

Let’s imagine next interface:

<?php

interface ValidatorInterface {

    public function isValid($value);
}

What’s obvious implementation here?

<?php

class TestValidator implements ValidatorInterface {

    public function isValid($value) {
        if (false === $value) {
            return false
        }

        return true;
    }
}

Ok, that’s make sense. We can either return true or false. But, let’s add one more thing:

<?php

class TestValidator implements ValidatorInterface {

    public function isValid($value) {
        if (fasle === $value) {
            return false
        }

        if (0 === $value) {
            return false
        }

        return true;
    }
}

Here we face with some kind of problem: we can not determine why our value is not valid. Let’s check how big guys do this:

https://github.com/zendframework/zend-validator/blob/master/src/ValidatorInterface.php

<?php

namespace Zend\Validator;

interface ValidatorInterface
{
    /**
     * Returns true if and only if $value meets the validation requirements
     *
     * If $value fails validation, then this method returns false, and
     * getMessages() will return an array of messages that explain why the
     * validation failed.
     *
     * @param  mixed $value
     * @return bool
     * @throws Exception\RuntimeException If validation of $value is impossible
     */
    public function isValid($value);

    /**
     * Returns an array of messages that explain why the most recent isValid()
     * call returned false. The array keys are validation failure message identifiers,
     * and the array values are the corresponding human-readable message strings.
     *
     * If isValid() was never called or if the most recent isValid() call
     * returned true, then this method returns an empty array.
     *
     * @return array
     */
    public function getMessages();
}

Ok, that’s make sense. We collect all occurred errors into array and call getMessages() afterwards.

Stateless

What is stateless? It’s possibility ob object to not have any state. Stateless gives us ability to use Dependency Injections of our validators. Zend Validator is not stateless, because any time when validator is called - it will collect errors or other metadata. So is it possible to write stateless validator with possibility to determine an error?

Yes

<?php

class TestValidator implements ValidatorInterface {

    public function isValid($value) {
        if (fasle === $value) {
            throw new \Exception('Value is not false.');
        }

        if (0 === $value) {
            throw new \Exception('Value is not 0.');
        }

        return true;
    }
}

With exceptions we can not store any kind of information, but this also doesn’t allow us to expect for multiple errors. For such cases we can make decomposition of TestValidator into NotFalseValidator and NotZeroValidator. Also we cannot call such methods as isValid, because another developer would rather expect boolean as return parameter. I would call it validate.

Pros

  • Doesn’t contains any state - can be called in DI;
  • Does not require factories.

Cons

  • Can not validate more than 1 error at the same time;
  • isValid is not correct method name. It can be either validate;
  • Requires copy-paste of try-catches in an kind of calls.

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