Part VI of the practical guide to creating basic Drupal 8 modules
From .info to tests, just the basics

To add a new project to drupal.org: https://www.drupal.org/node/add/project-module

Hooks are one of the ways modules interact with other contributed modules or Drupal core subsystems. Hooks are used for a variety of tasks, including preprocessing variables for template files (hook_preprocess()), modifying information lists (hook_tokens_alter(), hook_views_data_alter()), and manipulating forms (hook_form_alter()), among other things. This page lists all hooks provided by core.

This documentation is for modules. For information about themes, see Adding stylesheets (CSS) and JavaScript (JS) to a Drupal 8 theme.

In Drupal 8, stylesheets (CSS) and JavaScript (JS) are loaded using the same system for both modules (code) and themes: the asset library. Asset libraries can include one or more CSS resources, one or more JS resources, and one or more JS settings.

Drupal 8 introduces many new APIs and includes only some of the APIs from Drupal 7/6. This section provides a detailed overview of the APIs available in Drupal 8.

Before diving into the documentation below, you might want to review the fundamentals and prerequisites as well as the Drupal 8.x module development guides, which explain some of the concepts and terminology assumed in the API documentation here.

API Features

Organized by the least frequently used APIs:

Authentication Provider Services
Implement \Drupal\Core\Authentication\AuthenticationProviderInterface and use the service tag 'authentication_provider'.

_auth Option on Routes
The default authentication manager (see below) allows developers to restrict the set of allowed authentication mechanisms to a specified subset by defining _auth in the route options.
Example: _auth: ['basic_auth', 'cookie']

Overview

Blocks in Drupal 8 actually consist of two separate API structures to create a user interface similar to previous iterations of Drupal. These two APIs are the Block Plugin API, which is a reusable standalone API, and the Block Entity API, which is a Drupal 8-specific implementation for placing blocks and controlling their visibility.

Cache API Drupal documentation.

The Cache API has been significantly improved in Drupal 8. The following sections explore each feature in more detail.

For a quick overview, see the Cache API documentation page.

Cacheability Metadata

Everything that is rendered directly or used to determine what to render provides cacheability metadata — from access results to entities and URLs.

Cacheability metadata consists of three properties:

Cache contexts = (request) contextual dependencies

Cache contexts are similar to the HTTP Vary header.

Why?

Cache contexts provide a way to create context-dependent variants of something that needs to be cached. It makes caching logic easier to read and avoids repeating the same conditional logic wherever context-based variation is needed.

Examples: