Improve the state validation to take the previous value into account.

src/Plugin/Field/FieldType/StateItem.php

@@ -7,8 +7,6 @@
 
 namespace Drupal\commerce_workflow\Plugin\Field\FieldType;
 
-use Drupal\Component\Utility\Random;
-use Drupal\Core\Field\FieldDefinitionInterface;
 use Drupal\Core\Field\FieldItemBase;
 use Drupal\Core\Field\FieldStorageDefinitionInterface;
 use Drupal\Core\Form\FormStateInterface;
@@ -16,6 +14,7 @@ use Drupal\Core\Session\AccountInterface;
 use Drupal\Core\StringTranslation\TranslationWrapper;
 use Drupal\Core\TypedData\DataDefinition;
 use Drupal\Core\TypedData\OptionsProviderInterface;
+use Drupal\Core\Validation\Plugin\Validation\Constraint\AllowedValuesConstraint;
 
 /**
  * Plugin implementation of the 'state' field type.
@@ -37,6 +36,13 @@ class StateItem extends FieldItemBase implements OptionsProviderInterface {
    */
   protected static $workflows = [];
 
+  /**
+   * The initial value, used to validate state changes.
+   *
+   * @var string
+   */
+  protected $initialValue;
+
   /**
    * {@inheritdoc}
    */
@@ -62,6 +68,23 @@ class StateItem extends FieldItemBase implements OptionsProviderInterface {
     return $properties;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function getConstraints() {
+    $constraints = parent::getConstraints();
+    // Replace the 'AllowedValuesConstraint' constraint with the 'State' one.
+    foreach ($constraints as $key => $constraint) {
+      if ($constraint instanceof AllowedValuesConstraint) {
+        unset($constraints[$key]);
+      }
+    }
+    $manager = \Drupal::typedDataManager()->getValidationConstraintManager();
+    $constraints[] = $manager->create('State', []);
+
+    return $constraints;
+  }
+
   /**
    * {@inheritdoc}
    */
@@ -98,6 +121,8 @@ class StateItem extends FieldItemBase implements OptionsProviderInterface {
    * {@inheritdoc}
    */
   public function isEmpty() {
+    // Note that in this field's case the value will never be empty
+    // because of the default returned in applyDefaultValue().
     return $this->value === NULL || $this->value === '';
   }
 
@@ -112,6 +137,32 @@ class StateItem extends FieldItemBase implements OptionsProviderInterface {
     return $this;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function setValue($values, $notify = TRUE) {
+    if (empty($this->initialValue)) {
+      // Track the initial field value to allow isValid() to validate changes.
+      $this->initialValue = $values['value'];
+    }
+    parent::setValue($values, $notify);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function postSave($update) {
+    $this->initialValue = $this->value;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isValid() {
+    $allowed_states = $this->getAllowedStates($this->initialValue);
+    return isset($allowed_states[$this->value]);
+  }
+
   /**
    * {@inheritdoc}
    */
@@ -146,36 +197,49 @@ class StateItem extends FieldItemBase implements OptionsProviderInterface {
    * {@inheritdoc}
    */
   public function getSettableOptions(AccountInterface $account = NULL) {
+    // $this->value is unpopulated due to https://www.drupal.org/node/2629932
+    $field_name = $this->getFieldDefinition()->getName();
+    $value = $this->getEntity()->get($field_name)->value;
+    $allowed_states = $this->getAllowedStates($value);
+    $state_labels = array_map(function ($state) {
+      return $state->getLabel();
+    }, $allowed_states);
+
+    return $state_labels;
+  }
+
+  /**
+   * Gets the next allowed states for the current field value.
+   *
+   * @param string $value
+   *   The field value, representing the state id.
+   *
+   * @return \Drupal\commerce_workflow\Plugin\Workflow\WorkflowState[]
+   *   The allowed states.
+   */
+  protected function getAllowedStates($value) {
     $workflow = $this->getWorkflow();
     if (!$workflow) {
       // The workflow is not known yet, the field is probably being created.
       return [];
     }
-    $entity = $this->getEntity();
-    // $this->value is unpopulated due to https://www.drupal.org/node/2629932
-    $field_name = $this->getFieldDefinition()->getName();
-    $value = $entity->get($field_name)->value;
-
-    $state_labels = [
+    $allowed_states = [
       // The current state is always allowed.
-      $value => $workflow->getState($value)->getLabel(),
+      $value => $workflow->getState($value),
     ];
-    $transitions = $workflow->getAllowedTransitions($value, $entity);
+    $transitions = $workflow->getAllowedTransitions($value, $this->getEntity());
     foreach ($transitions as $transition) {
       $state = $transition->getToState();
-      $state_labels[$state->getId()] = $state->getLabel();
+      $allowed_states[$state->getId()] = $state;
     }
 
-    return $state_labels;
+    return $allowed_states;
   }
 
   /**
-   * Gets the workflow used by the current field.
-   *
-   * @return \Drupal\commerce_workflow\Plugin\Workflow\WorkflowInterface|false
-   *   The workflow, or FALSE if unknown at this time.
+   * {@inheritdoc}
    */
-  protected function getWorkflow() {
+  public function getWorkflow() {
     $field_definition = $this->getFieldDefinition();
     $definition_id = spl_object_hash($field_definition);
     if (!isset(static::$workflows[$definition_id])) {

src/Plugin/Field/FieldType/StateItemInterface.php

+<?php
+
+/**
+ * @file
+ * Contains \Drupal\commerce_workflow\Plugin\Field\FieldType\StateItemInterface.
+ */
+
+namespace Drupal\commerce_workflow\Plugin\Field\FieldType;
+
+use Drupal\Core\TypedData\OptionsProviderInterface;
+
+/**
+ * Defines the interface for state item fields.
+ */
+interface StateItemInterface extends OptionsProviderInterface {
+
+  /**
+   * Gets the workflow used by the current field.
+   *
+   * @return \Drupal\commerce_workflow\Plugin\Workflow\WorkflowInterface|false
+   *   The workflow, or FALSE if unknown at this time.
+   */
+  public function getWorkflow();
+
+  /**
+   * Gets whether the current state is valid.
+   *
+   * Drupal separates field validation into a separate step, allowing an
+   * invalid state to be set before validation is invoked. At that point
+   * validation has no access to the previous value, so it can't determine
+   * if the transition is allowed. Thus, the field item must track the state
+   * changes internally, and answer via this method if the current state is
+   * valid.
+   *
+   * @see \Drupal\commerce_workflow\Plugin\Validation\Constraint\StateConstraintValidator
+   *
+   * @return bool
+   *   TRUE if the current state is valid, FALSE otherwise.
+   */
+  public function isValid();
+
+}

src/Plugin/Validation/Constraint/StateConstraint.php

+<?php
+
+/**
+ * @file
+ * Contains \Drupal\commerce_workflow\Plugin\Validation\Constraint\StateConstraint.
+ */
+
+namespace Drupal\commerce_workflow\Plugin\Validation\Constraint;
+
+use Symfony\Component\Validator\Constraint;
+
+/**
+ * Ensures the validity of the specified state.
+ *
+ * The state must exist on the used workflow, and be in the allowed transitions.
+ *
+ * @Constraint(
+ *   id = "State",
+ *   label = @Translation("State", context = "Validation")
+ * )
+ */
+class StateConstraint extends Constraint {
+
+  /**
+   * The default violation message.
+   *
+   * @var string
+   */
+  public $message = "The state '@state' is invalid.";
+
+}

src/Plugin/Validation/Constraint/StateConstraintValidator.php

+<?php
+
+/**
+ * @file
+ * Contains \Drupal\commerce_workflow\Plugin\Validation\Constraint\StateConstraintValidator.
+ */
+
+namespace Drupal\commerce_workflow\Plugin\Validation\Constraint;
+
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+
+/**
+ * Validates the State constraint.
+ *
+ * @see \Drupal\commerce_workflow\Plugin\Field\FieldType\StateItemInterface::isValid()
+ */
+class StateConstraintValidator extends ConstraintValidator {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validate($value, Constraint $constraint) {
+    if (!$value->getEntity()->isNew() && !$value->isValid()) {
+      $this->context->addViolation($constraint->message, ['@state' => $value->value]);
+    }
+  }
+
+}