Added route and controller for adding activity to case

src/Controller/ActivityController.php

@@ -0,0 +1,35 @@
+<?php
+
+namespace Drupal\zencrm\Controller;
+
+use Drupal\Core\Controller\ControllerBase;
+
+/**
+ * Class ActivityController.
+ */
+class ActivityController extends ControllerBase {
+
+  /**
+   * Displays a form for creating a activity. 
+   * The type of activity and the case are prepopulated.
+   *
+   * @return form for creating a activity
+   */
+  public function createActivityForCase($case_id, $activity_type_id) {
+    error_log( "$case_id, $activity_type_id");
+    $values = array(
+      'type' => $activity_type_id,
+      'case_entity' => $case_id,
+    );
+
+    $activity = \Drupal::entityTypeManager()
+      ->getStorage('activity')
+      ->create($values);
+
+    $form = \Drupal::entityTypeManager()
+      ->getFormObject('activity', 'default')
+      ->setEntity($activity);
+
+    return \Drupal::formBuilder()->getForm($form);
+  }
+}

src/Plugin/Block/ActivityCreator.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace Drupal\zencrm\Plugin\Block;
+
+use Drupal\Core\Block\BlockBase;
+
+/**
+ * Provides a 'ActivityCreator' block.
+ * Block contains links for creating activities with.
+ * The links open an entity create form in a popup, passing in the activity type and case id in the url.
+ *
+ * @Block(
+ *  id = "activity_creator",
+ *  admin_label = @Translation("Activity creator"),
+ * )
+ */
+class ActivityCreator extends BlockBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build() {
+    $case_id = \Drupal::routeMatch()->getParameter('case_entity')->id();
+    $markup = "";    
+
+    $activity_types = \Drupal::service('entity_type.bundle.info')->getBundleInfo('activity');
+    foreach($activity_types as $activity_type_id => $type) {
+      $label = $type['label'];
+      $markup .= "<p><a class='use-ajax' data-dialog-type='modal' href='/zencrm/activity/$case_id/add/$activity_type_id?destination=/zencrm/case/$case_id'>Add a $label Activity</a></p>";
+    }
+    return [
+      '#cache' => [
+         'max-age' => 0,
+       ],
+      '#markup' => "<div class='zencrm_creation_links'>$markup</div>"
+    ];
+
+  }
+
+}