7.md

Lesson 7: Interfaces

New Keywords:

• interface
• implements
• instanceOf

Interfaces closely resemble abstract classes, but cannot contain concrete methods or properties. In order to create an interface, use the interface keyword.

<?php

interface EntityInterface {
  // ... Other constants or methods.
  public function id();
  public function label();
}

You can then implement an interface with another class by using the implements keyword. Just as concrete classes can contain additional properties and methods to augment an abstract class, so can implementing classes contain more properties/methods than declared in the interfaces; i.e., implementing classes need to comply with the interface, and they can add any other thing they need. Interfaces can also extend other interfaces.

<?php

abstract class Entity implements EntityInterface {
  // ... Other constants or methods.
  
  public function id() {
    return isset($this->id) ? $this->id : NULL;
  } 

  public function label() {
    // Processing to return the label.
    return $label;
  }
}

Constants

You can't use variables inside of an interface, but you can use constants.

<?php

interface NodeInterface {
  const NOT_PUBLISHED = 0;
  const PUBLISHED = 1;

  // ... Other constants or methods.
}

Those constants can be accessed via the scope resolution operator :: inside of an implementing class. For example:

<?php

$node->set('status', NodeInterface::PUBLISHED);

instanceOf

The instanceof operator is used to find out if an object is an instantiated instance of a class. This can be useful to make sure that a parameter or variable is a particular instance of an object before using it. It is particularly useful to check against an interface, which is more generic than an implementing class.

<?php

function hook_entity_presave(EntityInterface $entity) {
  if ($entity instanceof ContentEntityInterface && $entity->isTranslatable()) {
    $route_match = \Drupal::routeMatch();
    \Drupal::service('content_translation.synchronizer')->synchronizeFields($entity, $entity->language()->getId(), $route_match->getParameter('source_langcode'));
  }
}

Drupal Examples:

Entities are a great place to look at interfaces. An interface allows developers wishing to create a custom entity to do so simply by adhering to the interface.

Drupal's content entities (NodeInterface, 'CommentInterface, UserInterface, etc.) extend the ContentEntityInterface and in that way provide a unified way to access all of the content entities and provide a consistent API for module developers.

Exercise:

• Create a person interface that declares the methods "work()" and "sayHi()".
• Modify the abstract class Person so it implements that interface.
• Instantiate an object from a concrete class.
• Review what errors does PHP give when a method declared in the interface is missing on the implementing class.