I recently worked on a Drupal 8 project which involved a lot of theming of form elements. That sounds simple right? Just edit some templates and preprocess functions and you’re done. That’s basically true but, like many things, it can be confusing and frustrating at first when things don’t quite behave as expected.
While I don’t think it’s necessary to know all the inner workings of the rendering system unless that is something that really interests you, you do need to know the basic operation, especially such things as the order of execution. I learned a lot by studying how a simple form element is rendered and setting myself an exercise to modify the result.
I’m assuming that the reader knows how to build a form class and create a controller and route to it. There are lots of tutorials online for that. Drupal Console is a great tool for generating this sort of boilerplate code.
Let’s build a test form with a simple textfield.
Show that on a page and use theme debugging to examine the hooks and templates.
There are three theme hooks and templates involved.
The outer hook is which has a template of Contained inside it is the output from two other templates. These are:
- The label from hook and a template of .
- The actual input element itself from and a template of .
These all follow a slightly different path through the rendering pipeline.
Data returned from is used to set default values in the final render array based on the #type of ‘textfield’. Among the values set is the #theme of .
The core templates are in . I went there expecting to find but there is no such file. The rendering code recognizes double underscores in the hook name and progressively slices parts off the end looking for a less specific template. In this case it finds .
The elegance of this is that renders all the basic form elements because they’re all just an input tag. The only difference between a text field and a radio button are the input tag’s attributes. Of course if you want to do something special, you can create in your theme and that will be used instead. It’s effectively a build-in theme suggestion without having to implement .
Now that we have the basic input tag rendered, how does the label get rendered and associated? A look at the render array after the defaults are set based on the #type will show that is a theme wrapper. This is rendered next and is supplied with the already rendered result of the input element. There is a function in form.inc which, among other things, constructs a render array for the label based on values such as the #title of the textfield in the form definition. This render array is given a #theme value of and is supplied to in a variable ‘label’ which is rendered in the template with . Drupal’s Twig extensions give the ability to render either a string or a render array with .
All the usual theming techniques are available to us. The most obvious being custom templates or the following preprocess functions. We can modify the variables supplied to the template, add new variables etc. Substitute your own theme name.
The order of execution is important and usually explains the reason why something doesn’t have the desired effect. Let’s say you want an additional CSS class on the input tag. Perhaps you’re already working in . You start looking at the $variables array and see the existing classes in . It’s tempting to try:
But that has no affect because it’s too late, the input tag has already been rendered. The place to do it is in mytheme_preprocess_input, before the tag is rendered. The correct code there is:
Preprocess functions in core copy important variables to the top level. If you need to drill down into to find what you want to change, that’s a good sign that you’re in the wrong function.
A learning exercise: Wrapping labels around the element
I wanted to give my form elements the option of wrapping the label around the element. This seems to be a common pattern for checkboxes.
I’ll leave it to my front end developer colleagues to argue the pros and cons of the two patterns. If it means anything, the popular Bootstrap framework uses the wrapped version. I don’t think either way is right or wrong but it provides a good exercise in using the knowledge described above. I went through several iterations of how to do this before getting to the following, which I think is a reasonably elegant solution that avoids major changes to templates.
Let’s invent a new boolean property, for a form element to use when defining a form. A checkbox might look like:
We need to customize the rendering of . I implemented to “suggest” using a different template when we want a wrapped label.
Now we can implement a preprocess function for this template.
The significant line here is the first one, which assigns a new #theme to the label. This will let us use a different template. Moving from a key of label to label_open is not strictly necessary but is done in the interests of code clarity. We’re going to render a “label open”, i.e., a label without a closing tag, so let’s not call it a label. We also copy the title to the top level so it can be easily rendered in our wrapper template rather than in the template.
Here’s the main containing div part of my .
And here’s the main part of my .
And there you have it. Labels are wrapped or not depending on a new attribute in the form array.
Default theme implementation for a form element.
- attributes: HTML attributes for the containing element.
- errors: (optional) Any errors for this form element, may not be set.
- prefix: (optional) The form element prefix, may not be set.
- suffix: (optional) The form element suffix, may not be set.
- required: The required marker, or empty if the associated form element is not required.
- type: The type of the element.
- name: The name of the element.
- label: A rendered label element.
- label_display: Label display setting. It can have these values:
- before: The label is output before the element. This is the default. The label includes the #title and the required marker, if #required.
- after: The label is output after the element. For example, this is used for radio and checkbox #type elements. If the #title is empty but the field is #required, the label will contain only the required marker.
- invisible: Labels are critical for screen readers to enable them to properly navigate through forms but can be visually distracting. This property hides the label for everyone except screen readers.
- attribute: Set the title attribute on the element to create a tooltip but output no label element. This is supported only for checkboxes and radios in \Drupal\Core\Render\Element\CompositeFormElementTrait::preRenderCompositeFormElement(). It is used where a visual label is not needed, such as a table of checkboxes where the row and column provide the context. The tooltip will include the title and required marker.
- description: (optional) A list of description properties containing:
- content: A description of the form element, may not be set.
- attributes: (optional) A list of HTML attributes to apply to the description content wrapper. Will only be set when description is set.
- description_display: Description display setting. It can have these values:
- before: The description is output before the element.
- after: The description is output after the element. This is the default value.
- invisible: The description is output after the element, hidden visually but available to screen readers.
- disabled: True if the element is disabled.
- title_display: Title display setting.
- Theme system overview
- Functions and templates for the user interface that themes can override.
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Drupal core provides a couple dozen different input 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?
In this tutorial we'll:
- Define what are and how they relate to the Render API
- Find a list of all available input element types, additional documentation and usage examples
- See examples of the most common element types
By the end of this tutorial you should be able to discover the different types of elements you can add to a array and find usage examples for each.
Provide a reference page for quickly finding information about available form element types.
List of form element types:
- Button (): Provides an action button form element.
- Checkbox (): Provides a form element for a single checkbox.
- Checkboxes (): Provides a form element for a set of checkboxes.
- Color (): Provides a form element for choosing a color.
- Date (): Provides a form element for date selection.
- Datelist (): Provides a datelist element which consists of a set of select elements pre-configured for choosing a date.
- Datetime (): Provides a datetime element.
- Email (): Provides a form input element for entering an email address.
- EntityAutocomplete (): Provides an entity autocomplete form element.
- File (): Provides a form element for uploading a file.
- Hidden (): Provides a form element for an HTML 'hidden' input element.
- ImageButton (): Provides a form element for a submit button with an image.
- Item (): Provides a display-only form element with an optional title and description.
- LanguageConfiguration(): Defines an element for language configuration for a single field.
- LanguageSelect (): Provides a form element for selecting a language.
- MachineName (): Provides a machine name form element that consists of a textfield for human readable input, and another textfield that automatically generates a machine name based on the input.
- ManagedFile (): Provides an AJAX/progress aware widget for uploading and saving a file. Files are saved as entities and managed by Drupal.
- Number (): Provides a form element for numeric input, with special numeric validation.
- Password (): Provides a form element for entering a password, with hidden text.
- PasswordConfirm (): Provides a form element for double-input of passwords.
- PathElement (): Provides a form element to enter a path which can be optionally validated and stored as either a value object or an array containing a route name and route parameters pair.
- Radio (): Provides a form element for a single radio button.
- Radios (): Provides a form element for a set of radio buttons.
- Range (): Provides a slider for input of a number within a specific range.
- Search (): Provides an HTML5 input element with type of "search".
- Select (): Provides a form element for a drop-down menu or scrolling selection box.
- Submit (): Provides a form submit button.
- Table (): Provides a render element for a table.
- Tableselect (): Provides a form element for a table with radios or checkboxes in left column.
- Tel (): Provides a form element for entering a telephone number.
- Textarea (): Provides a form element for input of multiple-line text.
- Textfield (): Provides a one-line text field form element.
- Token (): Stores token data in a hidden form field. This is generally used to protect against cross-site forgeries. A token element is automatically added to each Drupal form so you generally do not need to add one yourself.
- Url (): Provides a form element for input of a URL, with built in validation for URL formatting.
- Value (): Provides a form element for storage of internal information.
- VerticalTabs (): Provides a render element for vertical tabs in a form.
- Weight (): Provides a form element for input of a weight. Weights are integers used to indicate ordering, with larger numbers later in the order.
Form elements explained
The method of a form controller returns an associative array, usually named , that defines the markup and input elements your form is composed of. Each element in the array consists of a set of properties, and possible nested child elements, that define the details Drupal uses to generate the HTML version of the form.
These arrays are known as render arrays, and it's a good idea to be familiar with their structure, and the related terminology.
Render arrays that define a form can make use of all standard Render API types, as well as the Form API-specific types. The latter are used primarily to define input and control elements on a form. These are used for the key of elements in a array, and also dictate which additional properties can be used for that element.
In addition to the set of default properties available for all elements, elements all have the following properties, as well as element-type-specific properties.
|#after_build (array)||Array of callables or function names, which are called after the element is built. Arguments: , .|
|#ajax (array)||Array of elements to specify Ajax behavior. See the Ajax API topic for more information.|
|#array_parents (string, read-only)||Array of names of all the element's parents (including itself) in the render array. See also , .|
|#default_value||Default value for the element. See also .|
|#description (string)||Help or description text for the element. In an ideal user interface, the should be enough to describe the element, so most elements should not have a description. If you do need one, make sure it is translated. If it is not already wrapped in a safe markup object, it will be filtered for XSS safety.|
|#disabled (bool)||If , the element is shown but does not accept user input.|
|#element_validate (array)||Array of callables or function names, which are called to validate the input. Arguments: , , .|
|#field_prefix (string)||Prefix to display before the HTML input element. Should be translated, normally. If it is not already wrapped in a safe markup object, it will be filtered for XSS safety.|
|#field_suffix (string)||Suffix to display after the HTML input element. Should be translated, normally. If it is not already wrapped in a safe markup object, will be filtered for XSS safety.|
|#input (bool, internal)||Whether or not the element accepts input.|
|#parents (string, read-only)||Array of names of the element's parents for purposes of getting values out of . See also , .|
|#process (array)||Array of callables or function names, which are called during form building. Arguments: , , .|
|#processed (bool, internal)||Set to when the element is processed.|
|#required (bool)||Whether or not input is required on the element.|
|#title (string)||Title of the form element. Should be translated.|
|#title_display (string)||Where and how to display the . Possible values: before: Label goes before the element (default for most elements). after: Label goes after the element (default for radio elements). invisible: Label is there but is made invisible using CSS. attribute: Make it the title attribute (hover tooltip).|
|#tree (bool)||if the values of this element and its children should be hierarchical in ; if the values should be flat. See also , .|
|#value_callback (callable)||Callable or function name, which is called to transform the raw user input to the element's value. Arguments:, , .|
You can find a complete list of the render element types provided by Drupal core at https://api.drupal.org/api/drupal/elements. Pay special attention to types, and note that you can click through to the class that defines the element type for additional documentation on element type specific properties and in most cases a usage example.
In this tutorial we listed all of the form element types provided by Drupal core and linked to the documentation for each. We also looked at the list of properties that are available for all form elements regardless of type. View the documentation for each individual element type to see documentation of type-specific properties.
Further your understanding
- Check out the list of properties that are available for all render element types. Remember, those also apply to form elements.
Custom form field in Drupal 8
It so happens that when creating forms, the developer lacks all the predefined field types that are in the Form API. In this case, you can write your custom form element based on the Drupal 8 FormElement class.
Consider the development of such field based on a time field. And then on the basis of the new field we will create a field that allows you to enter a time interval within one day.
HTML5 has a type of time field that allows you to enter hours, minutes and seconds:
The step parameter indicates the increment of minutes (in this case, 15), if it is less than 60, then it will be possible to enter also seconds.
Note: you can make this field with time input using the built-in Drupal type datetime (you need to check if the Datetime module is enabled).
But we want more control over our field.
So, create a module that will display the form. Create Time.php in the src/Element folder. This file will contain a time field.
We need to specify the name of our type (time) — this is what will be written in '#type' when building the form, declare the getInfo method, which describes the parameters of the element and defines the methods for validation, rendering.
In the '#process' section, we defined the processTime method. In this method, we can set default parameters for the field and process the parameters of the element, which we get from the field description when we create the form.
In getInfo, there are two options for theme:
Theme for the field output. We can create our own theme (we will do it for the second field — interval), but here we use a standard template for the text field and a standard wrapper for the form element. They are all defined in the core.
To set the input field attributes, we define the preRenderTime method:
For an element, it is necessary to determine how the default value will be formed and how the value will generally be assigned to the element. For this you need a method valueCallback:
Here, by default, we assign the passed default value, and if the value is already in the field, then check it for compliance with the time format, clearing the field if the format is incorrect.
The last thing to do is to write the validation method specified in getInfo in the '#element_validate' section:
Element is ready! Now we can set this field in the form:
Now, on the basis of the created element, create a field that allows you to enter a time interval. It's all the same here, in this example you can see how to make composite elements, as well as how to use your templates to display the field.
Create element Timerange:
As you can see in the processRange method, the element now contains two fields: start and end, both of the time type. When assigning a value to a field, we now have an associative array. It is easy to cope with validation, the main thing is not to forget that the value of the field is an array with the start and end keys.
As templates for the field output, we specified timerange_form and timerange_wrapper. Preprocess hooks are taken from the Drupal's core:
And a bit of CSS so that these two fields stand side by side:
Now you can set the field like this:
You can view the sources of the core elements in core/lib/Drupal/Core/Render/Element.
Drupal 8Form APIAlena Parfireva
8 elements drupal form
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