[Chase media_entity] Convert validation to Constraints API

1. +++ b/src/Plugin/Validation/Constraint/ItemsCountConstraint.php
   @@ -0,0 +1,34 @@
   +  public function __construct($options = null) {
   +    parent::__construct($options);
   +  }
   

   We don't need to override constructor.

2. +++ b/src/Tests/ConstraintsTest.php
   @@ -0,0 +1,84 @@
   +class ConstraintsTest extends WebTestBase {
   

   It would be nicer to have unit test for this.

1. +++ b/src/Plugin/Validation/Constraint/ItemsCountConstraintValidator.php
   @@ -0,0 +1,31 @@
   diff --git a/src/Tests/ConstraintsTest.php b/src/Tests/ConstraintsTest.php
   
   diff --git a/src/Tests/ConstraintsTest.php b/src/Tests/ConstraintsTest.php
   new file mode 100644
   
   new file mode 100644
   index 0000000..17033b3
   
   index 0000000..17033b3
   --- /dev/null
   
   --- /dev/null
   +++ b/src/Tests/ConstraintsTest.php
   
   +++ b/src/Tests/ConstraintsTest.php
   +++ b/src/Tests/ConstraintsTest.php
   @@ -0,0 +1,122 @@
   
   @@ -0,0 +1,122 @@
   +<?php
   +
   +/**
   + * @file
   + * Contains \Drupal\media_entity_slideshow\Tests\ConstraintsTest.
   + */
   +
   +namespace Drupal\media_entity_slideshow\Tests;
   +
   +use Drupal\media_entity_slideshow\Plugin\Validation\Constraint\ItemsCountConstraint;
   +use Drupal\media_entity_slideshow\Plugin\Validation\Constraint\ItemsCountConstraintValidator;
   +use Drupal\Tests\UnitTestCase;
   +
   +/**
   + * Tests media_entity_slideshow constraints.
   + *
   + * @group media_entity
   + */
   

   PHPUnit tests are usually located in module_root/tests/src and namespaced \Drupal\Tests\module_name\...

2. +++ b/src/Tests/ConstraintsTest.php
   @@ -0,0 +1,122 @@
   +  /**
   +   * Provides test data for testValidation().
   +   */
   +  public function dataProvider() {
   +    return [
   +      'invalid data' => ['', 1],
   +      'valid data' => ['Some text', 0]
   +    ];
   +  }
   

   I think that we don't need data provider since there will always be just two options.

3. +++ b/src/Tests/ConstraintsTest.php
   @@ -0,0 +1,122 @@
   +  /**
   

   Leave an empty line at the beginning and end of the class definition.

4. +++ b/src/Tests/ConstraintsTest.php
   @@ -0,0 +1,122 @@
   +    $this->property = $value == '' ? NULL : $value;
   

   Let's make argument optional and default it to NULL. This way it won't be needed to do empty string check any more.

Committed. Thank you!