Introduce hooks for registering patterns for email editor

To get rid of hardcoded MailPoet-specific patterns from the editor package
We need to introduce an API to register the patterns and their categories.

Patterns have to be defined as extenders of Abstract_Pattern.

[MAILPOET-6243]

packages/php/email-editor/tests/integration/Engine/Patterns/Dummy_Test_Pattern.php

@@ -0,0 +1,43 @@
+<?php
+/**
+ * This file is part of the MailPoet plugin.
+ *
+ * @package MailPoet\EmailEditor
+ */
+
+declare(strict_types = 1);
+namespace MailPoet\EmailEditor\Engine\Patterns;
+
+/**
+ * Dummy test pattern
+ */
+class Dummy_Test_Pattern extends Abstract_Pattern {
+	/**
+	 * Name of the pattern
+	 *
+	 * @var string
+	 */
+	protected $name = 'dummy-test-pattern';
+	/**
+	 * Namespace of the pattern
+	 *
+	 * @var string
+	 */
+	protected $namespace = 'dummy';
+	/**
+	 * Get the pattern content
+	 *
+	 * @return string
+	 */
+	public function get_content(): string {
+		return '<!-- wp:paragraph --><p>Test pattern</p><!-- /wp:paragraph -->';
+	}
+	/**
+	 * Get the pattern title
+	 *
+	 * @return string
+	 */
+	public function get_title(): string {
+		return 'Test pattern title';
+	}
+}

packages/php/email-editor/tests/integration/Engine/Patterns/Patterns_Test.php

@@ -0,0 +1,92 @@
+<?php
+/**
+ * This file is part of the MailPoet plugin.
+ *
+ * @package MailPoet\EmailEditor
+ */
+
+declare(strict_types = 1);
+namespace MailPoet\EmailEditor\Engine\Patterns;
+
+require_once 'Dummy_Test_Pattern.php';
+
+/**
+ * Integration test for Patterns class
+ */
+class Patterns_Test extends \MailPoetTest {
+	/**
+	 * Patterns instance
+	 *
+	 * @var Patterns
+	 */
+	private $patterns;
+	/**
+	 * Set up before each test
+	 */
+	public function _before() {
+		parent::_before();
+		$this->patterns = $this->di_container->get( Patterns::class );
+		$this->cleanup_patterns();
+	}
+
+	/**
+	 * Test that the patterns added via filter are registered in WP_Block_Patterns_Registry
+	 */
+	public function testItRegistersPatterns() {
+		$pattern = new Dummy_Test_Pattern();
+		add_filter(
+			'mailpoet_email_editor_block_patterns',
+			function ( $patterns ) use ( $pattern ) {
+				$patterns[] = $pattern;
+				return $patterns;
+			}
+		);
+		$this->patterns->initialize();
+		$block_patterns = \WP_Block_Patterns_Registry::get_instance()->get_all_registered();
+		$block_pattern  = array_pop( $block_patterns );
+		$this->assertEquals( 'dummy/dummy-test-pattern', $block_pattern['name'] );
+		$this->assertEquals( $pattern->get_content(), $block_pattern['content'] );
+		$this->assertEquals( $pattern->get_title(), $block_pattern['title'] );
+	}
+
+	/**
+	 * Test that the pattern categories added via filter are registered in WP_Block_Patterns_Registry
+	 */
+	public function testItRegistersPatternCategories() {
+		add_filter(
+			'mailpoet_email_editor_block_pattern_categories',
+			function ( $categories ) {
+				$pattern_category = array(
+					'name'        => 'mailpoet-test',
+					'label'       => 'MailPoet',
+					'description' => 'A collection of email template layouts.',
+				);
+				$categories[]     = $pattern_category;
+				return $categories;
+			}
+		);
+		$this->patterns->initialize();
+		$categories = \WP_Block_Pattern_Categories_Registry::get_instance()->get_all_registered();
+		$category   = array_pop( $categories );
+		$this->assertEquals( 'mailpoet-test', $category['name'] );
+		$this->assertEquals( 'MailPoet', $category['label'] );
+		$this->assertEquals( 'A collection of email template layouts.', $category['description'] );
+	}
+
+	/**
+	 * Clean registered patterns and categories
+	 */
+	private function cleanup_patterns() {
+		$registry       = \WP_Block_Patterns_Registry::get_instance();
+		$block_patterns = $registry->get_all_registered();
+		foreach ( $block_patterns as $pattern ) {
+			$registry->unregister( $pattern['name'] );
+		}
+
+		$categories_registry = \WP_Block_Pattern_Categories_Registry::get_instance();
+		$categories          = $categories_registry->get_all_registered();
+		foreach ( $categories as $category ) {
+			$categories_registry->unregister( $category['name'] );
+		}
+	}
+}