Concept: What Are Hooks? for Drupal 8, 9, and 10

Hooks in Drupal enable modules to alter or extend the behavior of Drupal core or other modules. By implementing functions with specific names, modules can intervene at various points in Drupal's execution flow. This tutorial introduces hooks, their implementation, and their significance in module interaction.

In this tutorial, we'll:

• Define what hooks are.
• Explore how hooks are implemented in modules.
• Understand when modules should define and invoke new hooks.

By the end of this tutorial, you'll understand the concept of hooks as a means to alter Drupal's behavior.

Goal

Learn how hooks enable modules to customize and extend Drupal functionality.

Prerequisites

• Concept: What Are Modules?
• Concept: Anatomy of a Module

Understanding hooks

Hooks are one of the ways that module's can write PHP code that ties into Drupal's runtime and alter, extend, or enhance, existing functionality.

From an implementation perspective, hooks are specially-named PHP functions called at specific times to alter or add to the base behavior. Each hook has a unique name (like hook_entity_load()), a defined set of parameters, and a defined return value.

As a module developer you'll implement hooks that fall into 3 categories:

• Informational hooks: Gather data from modules, such as permissions or menu items.
• Alter hooks: Modify data provided by other modules.
• Action hooks: Respond to events within Drupal, like entity creation.

Implementing hooks

Implement hooks by following these steps:

1. Find the hook: Determine which hook you need to implement.
2. Function naming: Name your function using the pattern MODULE_NAME_hookname(), replacing MODULE_NAME with your module's name.
3. Coding: Write the function's logic within your module's MODULE_NAME.module file.

Example

Here's an example of implementing hook_form_alter() in the anytown module. This code would live in the anytown.module PHP file:

/**
 * Implements hook_form_alter(). 
 */
function anytown_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  // Custom logic to alter forms.
}

It's best practice to document hook implementations with an "Implements HOOK." comment.

Defining and invoking hooks

You can define and invoke hooks in your module to let other modules interact with its functionality. This helps create modular, extensible code that others can easily extend. Defining custom hooks involves:

• Choosing a unique name.
• Documenting the hook's purpose, parameters, and expected return values in a MODULE_NAME.api.php file.
• Invoking the hook at appropriate times in your module's code using the module_handler service.

You should define a new hook when:

• Your code performs a critical action like saving a record to the database, or processing a response from a third-party API.
• Any time you're gathering a list of "things" that someone might want to add to.
• Any time you want to let other code alter a list or definition before you use it.

Learn more in Define and Invoke a New Hook.

Recap

Hooks are essential for Drupal module development, offering a way to alter core functionality or integrate with other modules. Understanding how to implement and invoke hooks is key to building flexible code that can use and provide integration points with other modules in the system.

Further your understanding

• What are the steps required to implement a hook?
• What scenarios in your module's custom logic could benefit from invoking hooks?

Additional resources

• What Are Hooks? (Drupalize.Me)
• Hooks (api.drupal.org)