Automatically order events when a talk is saved

To ensure that the event dates for a talk are always in the correct
order, and to make the editing experience easier, this change
automatically re-orders the events on a talk node to be based on the
event date.

Fixes #74

web/modules/custom/custom/tests/modules/custom_test/config/install/field.field.paragraph.event.field_name.yml

@@ -0,0 +1,18 @@
+langcode: en
+status: true
+dependencies:
+  config:
+    - field.storage.paragraph.field_name
+    - paragraphs.paragraphs_type.event
+id: paragraph.event.field_name
+field_name: field_name
+entity_type: paragraph
+bundle: event
+label: 'Event name'
+description: ''
+required: true
+translatable: false
+default_value: {  }
+default_value_callback: ''
+settings: {  }
+field_type: string

web/modules/custom/custom/tests/modules/custom_test/config/install/field.storage.paragraph.field_name.yml

@@ -0,0 +1,20 @@
+langcode: en
+status: true
+dependencies:
+  module:
+    - paragraphs
+id: paragraph.field_name
+field_name: field_name
+entity_type: paragraph
+type: string
+settings:
+  max_length: 255
+  is_ascii: false
+  case_sensitive: false
+module: core
+locked: false
+cardinality: 1
+translatable: true
+indexes: {  }
+persist_with_no_fields: false
+custom_storage: false

web/modules/custom/custom/tests/src/Kernel/EventsAreReorderedByDateTest.php

@@ -0,0 +1,113 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\custom\Kernel;
+
+use Carbon\Carbon;
+use Drupal\paragraphs\ParagraphInterface;
+
+final class EventsAreReorderedByDateTest extends TalksTestBase {
+
+  public function testCreatingNode() {
+    $events = [
+      $this->createEvent([
+        'field_date' => Carbon::today()->addWeeks(2),
+        'field_name' => 'Drupal Bristol',
+      ]),
+      $this->createEvent([
+        'field_date' => Carbon::yesterday(),
+        'field_name' => 'DrupalCamp London',
+      ]),
+      $this->createEvent([
+        'field_date' => Carbon::tomorrow(),
+        'field_name' => 'PHP UK conference',
+      ]),
+      $this->createEvent([
+        'field_date' => Carbon::today()->addMonths(3),
+        'field_name' => 'CMS Philly',
+      ]),
+      $this->createEvent([
+        'field_date' => Carbon::today()->subYear(),
+        'field_name' => 'PHP South Wales',
+      ]),
+    ];
+
+    $talk = $this->createTalk([
+      'field_events' => $events,
+    ]);
+
+    $this->assertSame(
+      [
+        'PHP South Wales',
+        'DrupalCamp London',
+        'PHP UK conference',
+        'Drupal Bristol',
+        'CMS Philly',
+      ],
+      $talk->getEvents()
+        ->map(fn(ParagraphInterface $event) => $event->get('field_name')
+          ->getString())
+        ->toArray()
+    );
+  }
+
+  public function testUpdatingNode() {
+    $events = [
+      $this->createEvent([
+        'field_date' => Carbon::today()->addWeeks(2),
+        'field_name' => 'Drupal Bristol',
+      ]),
+      $this->createEvent([
+        'field_date' => Carbon::yesterday(),
+        'field_name' => 'DrupalCamp London',
+      ]),
+      $this->createEvent([
+        'field_date' => Carbon::today()->addMonths(3),
+        'field_name' => 'CMS Philly',
+      ]),
+      $this->createEvent([
+        'field_date' => Carbon::today()->subYear(),
+        'field_name' => 'PHP South Wales',
+      ]),
+    ];
+
+    $talk = $this->createTalk([
+      'field_events' => $events,
+    ]);
+
+    $this->assertSame(
+      [
+        'PHP South Wales',
+        'DrupalCamp London',
+        'Drupal Bristol',
+        'CMS Philly',
+      ],
+      $talk->getEvents()
+        ->map(fn(ParagraphInterface $event) => $event->get('field_name')
+          ->getString())
+        ->toArray()
+    );
+
+    $talk->addEvent($this->createEvent([
+      'field_date' => Carbon::tomorrow(),
+      'field_name' => 'PHP UK conference',
+    ]));
+    $talk->save();
+
+    $this->assertSame(
+      [
+        'PHP South Wales',
+        'DrupalCamp London',
+        'PHP UK conference',
+        'Drupal Bristol',
+        'CMS Philly',
+      ],
+      $talk->getEvents()
+        ->map(fn(ParagraphInterface $event) => $event->get('field_name')
+          ->getString())
+        ->toArray()
+    );
+  }
+
+}

web/modules/custom/custom/tests/src/Kernel/TalksTestBase.php

@@ -4,7 +4,7 @@ declare(strict_types=1);
 
 namespace Drupal\Tests\custom\Kernel;
 
-use Drupal\Core\Entity\EntityInterface;
+use Drupal\custom\Entity\Node\Talk;
 use Drupal\KernelTests\Core\Entity\EntityKernelTestBase;
 use Drupal\node\Entity\Node;
 use Drupal\paragraphs\Entity\Paragraph;
@@ -33,6 +33,7 @@ abstract class TalksTestBase extends EntityKernelTestBase {
   ];
 
   protected function createEvent(array $overrides = []): ParagraphInterface {
+    /** @var \Drupal\paragraphs\ParagraphInterface $event */
     $event = Paragraph::create(array_merge([
       'type' => 'event',
     ], $overrides));
@@ -40,7 +41,7 @@ abstract class TalksTestBase extends EntityKernelTestBase {
     return tap($event)->save();
   }
 
-  protected function createTalk(array $overrides = []): EntityInterface {
+  protected function createTalk(array $overrides = []): Talk {
     $talk = Node::create(array_merge([
       'title' => 'Test Driven Drupal',
       'type' => 'talk',