Drupal's Form API is a set of interfaces, utility classes, and conventions that when combined together allow module developers to create forms that collect, validate, and process user-submitted data. The Form API is closely related to the Render API.

Understanding the flow--or life cycle--of a Drupal form will help you better understand how to build new forms or influence existing ones. This tutorial will help you understand the complete life cycle of a Drupal form: receiving the request from a browser, displaying a page with a form, rendering the form as HTML, handling the submitted form, validating input, handling errors, and processing data. We'll point out the common places that module developers might want to inject additional functionality into the process and link to tutorials with more details about how to do so.

Each form is defined by a controller, a class that implements the \Drupal\Core\Form\FormInterface. Form controllers declare the unique ID of the form, the $form array that describes the content of the form, how to validate the form, and what to do with the data collected.

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Module developers can add new elements to a form by adding their definition to the $form array in the buildForm() method of their controller or via an implementation of hook_form_alter(). Doing so requires knowing the element #type, and details about any element-type-specific properties.

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Drupal core provides a couple dozen different input #type elements that can be added to forms. This includes one for every standard HTML5 input element, and some Drupal-specific ones that encapsulate more complex interactions like uploading files. But how can you know what elements exist? Where do you find information about what Render API properties each element uses?

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When a form is submitted you'll need to check the data input by the user to ensure that it matches certain constraints, and to raise errors when necessary. Is the email address in the proper format? Is the title field long enough? Does the ASIN ID entered match a valid Amazon product? This process is called validation and is handled by a combination of the validateForm() method of a form controller, and validation callbacks.

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When your module defines the form and the form controller, you can add your validation logic as part of the form controller. This is done via the implementation of a validateForm() method. The FormBuilder service will automatically invoke this method at the appropriate time during the process of submitting a form. While the validateForm() method is required by \Drupal\Core\Form\FormInterface, an empty method will fulfill that requirement. It's up to you to provide appropriate validation code inside the method.

When working with forms that are not created by your code, where you're not implementing the form controller but rather interacting with the form via implementations of hook_form_alter(), you can use the #validate property of the root form element to add additional validation logic in the form of a callback function or method.

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When defining a new FormElement plugin, or if your constraints are only relevant for a single element, you can use the #element_validate property of an individual element to specify additional validation callbacks.

In this tutorial we'll:

• Add an #element_validate callback to a single element in a form to perform validation of just that element.

By the end of this tutorial you should know how to add #element_validate callbacks to any form element in Drupal.

You probably created a form with the intent of collecting user input and then doing something with that input. Using the submitForm() method of our form class we can access the validated, and sanitized, user input data and make use of it for our own needs. We might do things like save the collected data as configuration, update an entity, or use it as part of a search query.

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Sometimes you need to add additional processing of input to forms where your module doesn't implement the form controller. In order to do this you can use the #submit property of the root level $form element, or of a specific button on a form, to add one or more callbacks. These functions, or methods, will be automatically called when the Form API is processing a submitted form and give your custom code an opportunity to do whatever it needs to do.

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When you create a custom form for Drupal and your module defines the form controller, the best way to handle processing of submitted data is via the submitForm() method of your controller. This method is called automatically by the Form API during the process of handling a user-submitted form. It can be used to save incoming data to the database, trigger a workflow based on user input, and instruct the Form API where to send the user after form processing has completed.

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Eventually you'll want to do something with the information your form collects beyond just printing it to the screen. It's generally considered a best practice to keep business logic out of your form controller so that it can be reused. In order to accomplish that you'll generally define your business logic in a service, and then call out to that service from your form controller. Or, you can make use of one of the existing services provided by Drupal core to save data.

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Forms are used for both collecting new data, and editing existing data. In order to allow users to modify existing data you need to pre-populate the elements on the form with the data you previously stored.

In this tutorial we'll look at the ways in which forms can be pre-populated with existing data, including:

You'll often need to make minor, or major, alterations to an existing form provided by another module. The Form API allows you to alter any existing form through a series of hooks without having to change the existing module's code at all. This is probably one of the most powerful features of the Drupal Form API. Knowing how to implement and leverage hook_form_alter() and its variations is an essential skill for any module developer.

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Forms can be displayed as the main content of a route or by using the form_builder service to retrieve and display a form directly. Which one you choose will depend on where on the page you want the form to appear.

In this tutorial we'll:

• Go over the two different ways a form can be retrieved and displayed
• Demonstrate how to display a form inside a custom block using the form_builder service

By the end of this tutorial you'll know how to retrieve a form and have it rendered anywhere on the page.

Asynchronous JavaScript And XML (Ajax) is a programming practice for building more complex, dynamic webpages using a technology known as XMLHttpRequest. It allows you to asynchronously perform server-side operations without requiring a refresh, thus allowing for more complex user interaction and, in some cases, improved user experience.

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A common use of Ajax is to alter a form by adding, removing, or updating parts of the form in response to actions taken by the user. The resulting altered form is still eventually submitted with a traditional HTTP POST request. For example, one might update the options available in a city dropdown field after someone has chosen a value in the country dropdown, or add an additional textfield for collecting a person's name when the user clicks an "Add another person" button.

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Using Ajax allows you to create forms that are submitted to the server, and processed, without requiring a page reload.

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As a developer, within a module, you can define settings for the module and provide a configuration form for administrators to update the values of those settings. In this tutorial, we'll create a configuration settings form for a module and define default values for each setting. We'll use Drupal Console, a command-line utility for Drupal to do some code scaffolding and speed up the process.

The Render API consists of two parts: structured arrays that provide data and hints about how that data should be rendered, and a rendering pipeline that can be used to render these arrays into various output formats. Understanding at least the basics of how the Render API works, the difference between elements and properties, and the concept of callback functions is an integral part of learning Drupal.

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