Revert "Issue #2563843 by heddn, jhodgdon, mradcliffe, swentel: MapItem...

Revert "Issue #2563843 by heddn, jhodgdon, mradcliffe, swentel: MapItem FieldType isn't used, documented, or tested: remove it"

Turns out contrib was making wide use of this.

This reverts commit 18026477.

core/lib/Drupal/Core/Field/Plugin/Field/FieldType/MapItem.php

+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Field\Plugin\Field\FieldType\MapItem.
+ */
+
+namespace Drupal\Core\Field\Plugin\Field\FieldType;
+
+use Drupal\Core\Field\FieldStorageDefinitionInterface;
+use Drupal\Core\Field\FieldItemBase;
+use Drupal\Core\TypedData\DataDefinition;
+
+/**
+ * Defines the 'map' entity field type.
+ *
+ * @FieldType(
+ *   id = "map",
+ *   label = @Translation("Map"),
+ *   description = @Translation("An entity field for storing a serialized array of values."),
+ *   no_ui = TRUE
+ * )
+ */
+class MapItem extends FieldItemBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
+    // The properties are dynamic and can not be defined statically.
+    return array();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function schema(FieldStorageDefinitionInterface $field_definition) {
+    return array(
+      'columns' => array(
+        'value' => array(
+          'type' => 'blob',
+          'size' => 'big',
+          'serialize' => TRUE,
+        ),
+      ),
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function toArray() {
+    // The default implementation of toArray() only returns known properties.
+    // For a map, return everything as the properties are not pre-defined.
+    return $this->getValue();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setValue($values, $notify = TRUE) {
+    $this->values = array();
+    if (!isset($values)) {
+      return;
+    }
+
+    if (!is_array($values)) {
+      if ($values instanceof MapItem) {
+        $values = $values->getValue();
+      }
+      else {
+        $values = unserialize($values);
+      }
+    }
+
+    $this->values = $values;
+
+    // Notify the parent of any changes.
+    if ($notify && isset($this->parent)) {
+      $this->parent->onChange($this->name);
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function __get($name) {
+    if (!isset($this->values[$name])) {
+      $this->values[$name] = array();
+    }
+
+    return $this->values[$name];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function __set($name, $value) {
+    if (isset($value)) {
+      $this->values[$name] = $value;
+    }
+    else {
+      unset($this->values[$name]);
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function mainPropertyName() {
+    // A map item has no main property.
+    return NULL;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isEmpty() {
+    return empty($this->values);
+  }
+
+}