The SitePoint PHP blog has a new tutorial posted today showing how to create custom field formatters in a Drupal 8 application. Custom formatters allow you to enhance the current functionality of objects in the application and extend them with additional functionality.
With the introduction of annotated plugins, a lot has changed in Drupal 8. We have a more streamlined approach to describing and discovering pieces of functionality that extend the core. Along with many other components, the former Field API (part of the larger and consolidated Entity API) is now based on plugins. In this tutorial we will go through defining a custom field formatter for an existing field (image). What we want to achieve is to make it possible to display an image with a small caption below it. This caption will be the title value assigned to the image if one exists.
They start with a new custom module, starting with just the YAML configuration. Then they help you create the field formatter as a plugin in the "Plugin/Field/FieldFormatter" namespace (code included). They explain how this code works and show how to add it as a hook to make it available to the template layer. Finally they show it in use and how it places the title value into the image caption in the result.