Understanding Drupal 8 Plugins System

About Me?

• Full stack Developer at Srijan Technologies PVT LTD
• Working with Drupal 6, 7 and 8
• I also love Javascript and CSS
• Hobbyist photographer, biker and runner

@msankhala             @MutantMahesh        @msankhala

Prerequisites

• Basic understanding of Drupal 7
• Object Oriented Programming concepts

Disclaimer

• I am not a Drupal 8 expert, but i'll try my best to explain D8 plugin system

What's in the talk

• What were plugins in Drupal 7
• What is Plugin in Drupal 8 ?
• Advantage of using plugins
• PSR-4, Annotation, Dependency injection, Service Container
• What all the things to think about when you think about Plugins?
• Available plugins in Drupal 8
• How to create plugin (Demo)
• How to create your custom plugin type. (Demo)

What were plugins in Drupal 7

• In Drupal 7 core there was nothing like plugins.
• Plugins were introduced by contributed module like Ctools, Views, panels etc
• These plugins helps you to create reusable custom functionality.
• Other module need to rely on these contributed module to develop something like plugin

<?php

/**
 * Implements hook_ctools_plugin_directory().
 */
function MY_MODULE_ctools_plugin_directory($owner, $plugin_type) {
  
  if ($owner == 'panels' && $plugin_type == 'YOUR-PLUGIN-TYPE') {
    return "plugins/$plugin_type";
  }
}

// YOUR-PLUGIN-TYPE
1. content_type 
2. access(visibility rules)
3. context
4. relationships
5. arguments 

D7 declare your own ctools plugin

D7 ctools content type plugin

D7 ctools access plugin

Drupal 7 panels plugins

1. row
2. display
3. display_extender
4. style
5. argument default
6. argument validator
7. access
8. query
9. cache
10. pager
11. exposed_form 
12. localization.

Drupal 7 views plugins

Drupal 7 views plugins

Drupal 7 views plugins directory

What is plugin in D8

• Plugin is a reusable functionality encapsulated in a class which implements one or more specific interfaces. 
• Evolved from ctools and Views plugins but implementation is different.
• Plugins replaces _info hooks and few other implementation hooks.
• Example: hook_block_info() and hook_block_view(), _configure(), _save() replaced by Block plugins.

What is plugin in D8

• Plugins system is a drupal specific design pattern to solve a recurring problem of developing reusable functionality with different configuration in different context.
• A module can contain multiple plugins of different type.
• Plugin is implemented as a class.
• All the plugins are implemented as classes but not all classes are plugins.

Plugin Manager and IDs

• Every plugin has a type, which is responsible for its own type of functionality.
• Every plugin type has a manager - registered as a service (available from the container) and used to find and instantiate the desired plugin instance(s)
• Each plugin has an ID, which may be in its definition, or generated as a derivative
• For a given plugin ID one single class will be used for all plugin instances using that plugin ID
• A plugin instance is specified by the combination of plugin ID and its configuration values, potentially coming from a ConfigEntity

Advantage of using Plugins

• Definition + implementation at one place
• Plugins are lazy loaded
• Code is unified
• Plugins are extensible
• Plugins are consistent
• interchangeable within plugin types
• Reusable across different projects

Few concepts used in Drupal 8 core

1. PSR-0 / PSR-4
2. Annotation
3. Dependency Injection
4. Service Container

PSR-0 / PSR-4

http://www.php-fig.org/faqs  

http://www.php-fig.org/psr/psr-4/

PSR-0 vs PSR-4

http://www.php-fig.org/faqs  

http://www.php-fig.org/psr/psr-4/

class under namespace according to PSR-4

<?php

/**
 * @file
 * Contains \Drupal\search\Plugin\Block\SearchBlock.
 */

namespace Drupal\search\Plugin\Block;

use Drupal\Core\Access\AccessResult;
use Drupal\Core\Session\AccountInterface;
use Drupal\Core\Block\BlockBase;

/**
 * Provides a 'Search form' block.
 *
 * @Block(
 *   id = "search_form_block",
 *   admin_label = @Translation("Search form"),
 *   category = @Translation("Forms")
 * )
 */
class SearchBlock extends BlockBase {

}

• Annotations are meta-data that can be embedded in source code.
• Annotation extends the PHP reflection API.
• Annotation let you examine and modify structure and behaviour of object at runtime.

Annotation

<?php
/**
 * Simple Demo class to show php reflection class.
 */
class DCampHyderabad {
  public $location;

  function __construct() {
    $this->location = 'IIIT';
  }

  function getLocation() {
    return $this->location;
  }
}

$ref = new \ReflectionClass('DCampHyderabad');
echo $ref->getDocComment() . PHP_EOL . PHP_EOL;
print_r($ref->getMethods());

PHP Reflection API example:

// Output:

/**
 * Simple Demo class to show php reflection class.
 */

Array
(
    [0] => ReflectionMethod Object
        (
            [name] => __construct
            [class] => DCampHyderabad
        )

    [1] => ReflectionMethod Object
        (
            [name] => getLocation
            [class] => DCampHyderabad
        )

)

Dependency Injection

Its 'D' in SOLID rule / Inversion of Control

class ShoppingCart {
  protected $mailer;

  function __construct()
  {
    $this->mailer = new Mailer();
  }
}

class ShoppingCart {
  protected $mailer;

  function __construct($mailer)
  {
    $this->foo = $mailer;
  }
}

$mailer = new Mailer();
$cart = new ShoppingCart($mailer);

Service Container

Service container are responsible for instantiating classes with all its dependency.

# core/modules/search/search.services.yml
search.search_page_repository:
  class: Drupal\search\SearchPageRepository
  arguments: ['@config.factory', '@entity.manager']

/* Service Container */
$instance = \Drupal::service('search.search_page_repository');

Drupal 8 Plugins System components

Plugin Manager

Discovery

Factory

Mapper

DiscoveryInterface

FactoryInterface

MapperInterface

Drupal 8 Plugins System components

$pluginManager->getDefination();
// $this->discovery->getDefination();

$pluginManager->createInstance(plugin_id, configuration);
// $this->factory->createInstance(plugin_id, configuration);

$pluginManager->getInstance(options);
// $this->mapper->getInstance(options);

Available Plugins in Drupal 8

• Block
• Field
• FieldWidget
• FieldFormatter
• TextFilter
• Action
• Menus
• Validation
• ImageEffect
  
  and many more.....

Plugins Discovery in core

• YAML based: MenuLink, LocalTask, LocalAction, ContextualLink
• Annotation, some config, but no config entity: ImageToolkit, Archiver
• Annotation and config entity (many) including: Block, ViewsDisplay, ImageEffect, SearchPlugin
•  Not a pure Plugin but uses Annotation discovery: Entity (Node, User, etc.)

Plugins Discovery in core

MyPluginManager::discovery = new AnnotatedClassDiscovery(‘Plugin/Block’, $namespaces,
  'Drupal\block\Annotation\Block');

Annotated Plugin Discovery

Hook Based Plugins Discovery

MyPluginManager::discovery = new HookDiscovery($this->moduleHandler, 'block_info');

YAML Based Plugin Discovery

MyPluginManager::discovery = new YamlDiscovery('blocks', 
  $module_handler->getModuleDirectories());

Static Plugin Discovery

MyPluginManager::discovery = new StaticDiscovery();

Plugin Factories in core

return new $plugin_class($configuration, $plugin_id, $plugin_definition);

The Default Factory

The Container Factory

return $plugin_class::create(\Drupal::getContainer(), $configuration, $plugin_id, 
  $plugin_definition);

What all the things to think about when you think about Plugins?

• What is my plugin type. (In most cases D8 core plugin type)
• What is my plugin annotation class. (In most case D8 core plugin annotation)
• Which discovery class i am going to use for my plugin discovery (AnnotatedClassDiscovery)
• Which factory class i am going to use for my plugin instantiation. (ContainerFactory)
• Is my plugin going to have derivative.

How to create D8 Plugin / Custom Plugin Type

Demo

Resources

Unraveling the Drupal 8 Plugin System

https://drupalize.me/blog/201409/unravelling-drupal-8-plugin-system

Plugin API overview

https://www.drupal.org/docs/8/api/plugin-api/plugin-api-overview

Drupal 8 Plugin System - Helior Colorado

https://www.youtube.com/watch?v=2o5uY-iOoMo

Peter Wolanin: Drupal 8, where did the code go? From info hook to plugin

https://www.youtube.com/watch?v=GewUz0qwzCo

Questions

Find me on Twitter @MutantMahesh