@@ -17,7 +17,7 @@
  *
  * @todo .patternkit-editor-target .card all: initial
  *
- * cspell:ignore iconlib
+ * cspell:ignore iconlib startval
  */
 
 import {patternkitEditorCKEditor5} from './patternkit.jsoneditor.ckeditor5.js';
@@ -49,8 +49,14 @@ patternkitEditorArray(jQuery, Drupal, JSONEditor);
 
      this.data = {
        schema: JSON.parse(settings.schemaJson),
-       starting: JSON.parse(settings.startingJson)
      };
+
+     // Only assign a starting value if a non-empty value was provided,
+     // otherwise don't provide a value so a default may be assumed.
+     const startVal = settings.startingJson;
+     if (startVal !== '{}' && Boolean(startVal) === true) {
+       this.data.starting = JSON.parse(startVal)
+     }
    }
 
    /**
@@ -143,8 +149,17 @@ patternkitEditorArray(jQuery, Drupal, JSONEditor);
      // Initialize the editor with a JSON schema.
      let config = {
        schema: this.data.schema,
-       refs: {}
+       refs: {},
      };
+
+     // Conditionally specify the start value only if data already exists.
+     // Starting with an empty string or object results in all form fields
+     // starting hidden by default.
+     if (this.data.starting) {
+       config.startval = this.data.starting;
+     }
+
+
      window.patternkitEditor = new JSONEditor(this.editor_root.getElementById('editor_holder'), config);
      window.patternkitEditor.patternkit = this;
 
 
@@ -135,6 +135,11 @@ services:
       - '@entity_type.manager'
       - '@keyvalue.expirable'
       - '@module_handler'
+  patternkit.helper.validation:
+    class: Drupal\patternkit\Utility\PatternValidationHelper
+    arguments:
+      - '@patternkit.schema.context_builder'
+      - '@patternkit.schema.schema_factory'
   patternkit.helper.layout:
     class: Drupal\patternkit\LayoutHelper
     arguments:
 
@@ -9,6 +9,7 @@
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
 use Drupal\Core\Render\Element\FormElementBase;
 use Drupal\patternkit\Entity\PatternInterface;
+use Drupal\patternkit\Exception\SchemaValidationException;
 use Drupal\patternkit\Exception\UnknownPatternException;
 use Drupal\patternkit\Form\PatternLibraryJSONForm;
 use Drupal\patternkit\PatternLibraryPluginManager;
@@ -21,7 +22,7 @@
  * Properties:
  * - #pattern_name: The name of the pattern to be loaded and used for the form.
  * - #pattern_entity: (optional) The loaded Pattern entity for the form.
- * - #data: (optional) A serialized string of JSON values for the form.
+ * - #default_value: (optional) A serialized string of JSON values for the form.
  * - #config: (optional) An associative array of configuration options for
  *     JSONEditor behaviors.
  *
@@ -33,6 +34,7 @@
  *   '#config' => [
  *     'disablePropertiesButtons' => FALSE,
  *   ],
+ *   '#default_value' => '{"text": "My text value"}',
  * ];
  * @endcode
  *
@@ -71,18 +73,24 @@ public function getInfo(): array {
       '#pre_render' => [
         [$this, 'loadPattern'],
         [$this, 'buildEditorForm'],
+        [static::class, 'preRenderGroup'],
       ],
       '#process' => [
+        [static::class, 'processAjaxForm'],
+        [static::class, 'processGroup'],
         [static::class, 'processEditorForm'],
       ],
       '#theme_wrappers' => ['form_element'],
+      '#input' => TRUE,
       '#pattern_name' => NULL,
       '#pattern_entity' => NULL,
       '#schema' => '',
-      // Pattern instance configuration values.
-      '#data' => '{}',
       // Editor configuration options to be overridden.
       '#config' => [],
+      // Validate upon submissions.
+      '#element_validate' => [
+        [static::class, 'validatePatternConfig'],
+      ],
     ];
   }
 
@@ -204,7 +212,8 @@ public function buildEditorForm(array $element): array {
     $editor_overrides = $element['#config'] ?? [];
 
     // Assemble the full instance settings for the editor.
-    $settings = $this->editorSettings($element['#schema'], $element['#data'] ?? '{}', $editor_overrides);
+    $value = $element['#value'] ?? $element['#default_value'] ?? '{}';
+    $settings = $this->editorSettings($element['#schema'], $value, $editor_overrides);
 
     // Load the editor from the pattern's library plugin.
     $pattern = $element['#pattern_entity'];
@@ -240,19 +249,75 @@ public function buildEditorForm(array $element): array {
    *   The updated render element.
    */
   public static function processEditorForm(&$element, FormStateInterface $form_state, array $form) {
+    // Retrieve the form-submitted value if available, otherwise, default to the
+    // original element data.
+    $data = $form_state->getValue($element['#parents']);
+
+    // Update the element data value to persist it to the next form build.
+    // This ensures even if it was loaded from a previous form submission, the
+    // element initialization data is synchronized with the submitted value.
+    $element['#value'] = $data;
+
     // Add a hidden field to capture configuration from the JSONEditor form.
     $element['instance_config'] = [
       '#type' => 'hidden',
       '#attributes' => [
         'id' => 'schema_instance_config',
       ],
-      '#default_value' => $element['#data'] ?? '{}',
+      '#value' => $data,
       '#parents' => $element['#parents'],
     ];
 
     return $element;
   }
 
+  /**
+   * Validates the pattern configuration.
+   *
+   * @param array $element
+   *   The render element being processed.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current form state.
+   * @param array $form
+   *   The full form.
+   *
+   * @see \Drupal\patternkit\Element\JsonEditorForm::getInfo()
+   */
+  public static function validatePatternConfig(&$element, FormStateInterface $form_state, array $form): void {
+    $value = $form_state->getValue($element['#parents']);
+    $pattern = $element['#pattern_entity'];
+    $schema = $pattern->getSchema();
+    $validationHelper = \Drupal::service('patternkit.helper.validation');
+    $validationHelper->validateContent($schema, $value);
+    if ($validationHelper->hasExceptions()) {
+
+      $message = 'Content has failed to validate. See errors below.';
+
+      // Log additional information if debug mode is enabled.
+      /** @var \Drupal\patternkit\PatternkitEnvironment $environment */
+      $environment = \Drupal::service('patternkit.environment');
+      if ($environment->isDebug()) {
+        $exceptions = $validationHelper->getExceptions();
+        $exception = array_shift($exceptions);
+        if ($exception instanceof SchemaValidationException && $previous = $exception->getPrevious()) {
+          $message .= "\n" . $previous->getMessage();
+        }
+        else {
+          $message .= "\n" . $exception->getMessage();
+        }
+      }
+
+      $form_state->setError($element, $message);
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function valueCallback(&$element, $input, FormStateInterface $form_state) {
+    return is_string($input) ? $input : NULL;
+  }
+
   /**
    * Statically load the configured settings for default JSON Editor behaviors.
    *
@@ -285,22 +350,8 @@ protected function defaultEditorSettings(): array {
         $config_object = $this->configFactory->get(PatternLibraryJSONForm::SETTINGS);
         $config_array = isset($config_object) ? $config_object->get() : [];
 
-        // Map config names to JS-friendly identifiers.
         // @todo Remove this once internal config names are renamed.
-        $mapped_config = [];
-        $map = PatternLibraryJSONForm::JSON_EDITOR_SETTINGS_MAP;
-        array_walk($config_array, function ($value, $key) use (&$mapped_config, $map) {
-          if (array_key_exists($key, $map)) {
-            // Relabel the value if a mapped key is found.
-            $mapped_config[$map[$key]] = $value;
-          }
-          else {
-            // Maintain the existing key if no mapped key is found.
-            $mapped_config[$key] = $value;
-          }
-        });
-
-        $this->config = $mapped_config;
+        $this->config = $this->mapConfigNames($config_array);
       }
 
       $this->defaults = $this->config + [
@@ -316,4 +367,33 @@ protected function defaultEditorSettings(): array {
     return $this->defaults;
   }
 
+  /**
+   * Map server-side configuration values to JS names.
+   *
+   * @param mixed $config_array
+   *   The server-side configuration values to be mapped to JS names.
+   *
+   * @return array
+   *   An array of config mapped to JS-friendly names.
+   *
+   * @todo Remove this once internal config names are renamed.
+   */
+  private function mapConfigNames(mixed $config_array): array {
+    // Map config names to JS-friendly identifiers.
+    $mapped_config = [];
+    $map = PatternLibraryJSONForm::JSON_EDITOR_SETTINGS_MAP;
+    array_walk($config_array, function ($value, $key) use (&$mapped_config, $map) {
+      if (array_key_exists($key, $map)) {
+        // Relabel the value if a mapped key is found.
+        $mapped_config[$map[$key]] = $value;
+      }
+      else {
+        // Maintain the existing key if no mapped key is found.
+        $mapped_config[$key] = $value;
+      }
+    });
+
+    return $mapped_config;
+  }
+
 }
@@ -377,10 +377,18 @@ protected function handleBlockFormUpdate(FormStateInterface $form_state): void {
 
     $pattern = $this->updatePattern($pattern, $base_pattern);
 
+    // Capture updated pattern data in form state for reference.
     $form_state->set('pattern', $pattern);
     $form_state->set('schema_updated', TRUE);
     $form_state->set('configuration', $this->getConfiguration());
-    $form_state->setRebuild(TRUE);
+
+    // Update the form state values to be properly reflected.
+    $block_entity = $this->getBlockEntity();
+    // Access data field value directly to get the serialized version.
+    $form_state->setValue('configuration', $block_entity->data->value);
+
+    // Set the form to be rebuilt to reflect the changes.
+    $form_state->setRebuild();
   }
 
   /**
@@ -412,8 +420,8 @@ public function updatePattern(?PatternInterface $pattern = NULL, ?PatternInterfa
     $pattern ??= $this->getPatternEntity();
     $base_pattern ??= $this->getBasePattern();
 
-    // If a base pattern is provided we assume it is always for the same pattern
-    // asset.
+    // If a base pattern is provided, we assume it is always for the same
+    // pattern asset.
     assert($pattern->getAssetId() === $base_pattern->getAssetId());
 
     // Stop here if no update is available.
@@ -759,7 +767,7 @@ protected function editorForm(array &$form, FormStateInterface $form_state): voi
     $form['configuration'] = [
       '#type' => 'jsoneditor_form',
       '#pattern_entity' => $pattern,
-      '#data' => $data ?? '{}',
+      '#default_value' => $data ?? '{}',
     ];
   }
 
 
@@ -0,0 +1,54 @@
+<?php
+
+namespace Drupal\patternkit\Utility;
+
+use Drupal\patternkit\PatternValidationTrait;
+use Drupal\patternkit\Schema\ContextBuilder;
+use Drupal\patternkit\Schema\SchemaFactory;
+
+/**
+ * A service for validating pattern content against schemas.
+ *
+ * This service provides a centralized way for other classes to validate
+ * pattern content against JSON schemas. It wraps the PatternValidationTrait
+ * functionality in a service that can be easily injected into other classes.
+ *
+ * @internal This service may be replaced or rewritten in the future and does
+ *   not guarantee backward compatibility.
+ */
+class PatternValidationHelper {
+
+  use PatternValidationTrait {
+    hasExceptions as public;
+    getExceptions as public;
+    validateContent as public;
+    validatePattern as public;
+  }
+
+  /**
+   * Constructs a PatternValidationHelper object.
+   *
+   * @param \Drupal\patternkit\Schema\ContextBuilder $contextBuilder
+   *   The schema context builder service.
+   * @param \Drupal\patternkit\Schema\SchemaFactory $schemaFactory
+   *   The schema factory service.
+   */
+  public function __construct(
+    ContextBuilder $contextBuilder,
+    SchemaFactory $schemaFactory,
+  ) {
+    $this->setContextBuilder($contextBuilder);
+    $this->setSchemaFactory($schemaFactory);
+  }
+
+  /**
+   * Clear all stored exceptions.
+   *
+   * This method allows the service to be reused for multiple validations
+   * without carrying over exceptions from previous validation attempts.
+   */
+  public function clearExceptions(): void {
+    $this->exceptions = [];
+  }
+
+}
@@ -289,7 +289,7 @@ public function testProcessEditorForm() {
       '#type' => 'jsoneditor_form',
       '#pattern_name' => 'my_pattern',
       '#schema' => $random->string(50),
-      '#data' => $random->string(50),
+      '#default_value' => $random->string(50),
       '#config' => (array) $random->object(),
       '#parents' => [$random->string(), $random->string()],
     ];
@@ -298,11 +298,15 @@ public function testProcessEditorForm() {
     $element_copy = $render_element;
 
     // Mock additional arguments for the method.
-    $form_state = $this->prophesize(FormStateInterface::class);
+    $form_state = $this->getMockBuilder(FormStateInterface::class)
+      ->getMock();
+    $form_state->method('getValue')
+      ->with($render_element['#parents'])
+      ->willReturn($render_element['#default_value']);
     $form = [];
 
     // Run the process method for testing.
-    $result = $this->element->processEditorForm($element_copy, $form_state->reveal(), $form);
+    $result = $this->element->processEditorForm($element_copy, $form_state, $form);
 
     // Expect all pre-existing values to be persisted.
     $this->assertArraySubset($render_element, $result);
@@ -313,7 +317,7 @@ public function testProcessEditorForm() {
       '#attributes' => [
         'id' => 'schema_instance_config',
       ],
-      '#default_value' => $render_element['#data'],
+      '#value' => $render_element['#default_value'],
       '#parents' => $render_element['#parents'],
     ];
     $this->assertArrayHasKey('instance_config', $result);