How to create a custom Autocomplete using the Drupal 8 Form API

In this article, I will not explain how to customise/alter an Autocomplete Field Widget - which should only be used on from using the Drupal Admin UI.

Here I will try to expose you a step by step guide which explains how you can create a custom Autocomplete field using the Drupal 8 Form API Core feature - An autocomplete that you could use in your own Drupal frontend applications.

If you are looking for resources which explains how to implement Views to alter an Autocomplete Field, please refer to this excellent guide.

Truth can only be found in one place: the code

• Robert C. Martin, Clean Code: A Handbook of Agile Software Craftsmanship

Step 1- The autocomplete form element

The other day, I was asked to create a custom Autocomplete for a project which uses a custom Form build using the Drupal 8 Form API.

At first sight, you may be interested to use the #entity_autocomplete field type - it seems exactly what you need.
Unfortunately, this is not. Indeed, the #entity_autocomplete doesn't allow you any customisation.

So, you will need the old folk's #textfield and his cousin attribute #autocomplete_route_name

The #autocomplete_route_name attribute will allow you to define a route to handle the autocomplete business logic (data you will return given the user input).

You also may add the #autocomplete_route_parameters attribute, this one gives you the possibility to send a fixed unalterable parameter to your #autocomplete_route_name, you may use it to fix the number of results to return.

Step 2 - Define autocomplete route

Now you know how to create the autocomplete form, but you will need a route to manage the logic which will fetch data & return them.
How? By simply adding the reference to the route — where data will get retrieved from — to your my_module.routing.yml file:

Be careful to use the same route name (here my_module.autocomplete.articles) in your previous #autocomplete_route_name.
Also, be sure to change permission according to your own needs.

Step 3 - Add Controller and return JSON response

Now having a routing & a form, you have to define your custom controller, with the handleAutocomplete method.
Well, it's precisely this method that makes sure that the proper data gets collected and properly formatted once requested by Drupal.

Let's dig deeper and see how we can precisely deal with specific JSON response for our textfield element.

1. Setup an ArticleAutoCompleteController class file under my_module/src/Controller/ArticleAutoCompleteController.php;
2. Then, extend the ControllerBase class and setup your handle method (in our case ::handleAutocomplete see my_module.routing.yml).

That's pretty much all the hocus-pocus that you need to have an autocomplete based on a textfield of Drupal 8.

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Sources

For the most curious of you, here are some sources of additional information that inspired the creation of this article.

Purushotam Rai (24 November, 2016). Implementing #autocomplete in Drupal 8 with Custom Callbacks
See on https://www.qed42.com/blog/autocom...

Stijn Berkers (28 February, 2018). How to produce a custom auto-complete field in drupal 8
See on https://lucius.digital/en/blog/drupal...

Make-me-alive (4 May, 2014). Adding autocomplete for text-field
See on https://drupal.stackexchange.com/.../..

Stijn Berkers (18 July, 2018). How to add autocomplete to text fields in drupal 8: defining a custom route
See on https://www.optasy.com/blog/how-add-autocom...