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19.

All about Form Themes

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There's just one problem left with our registration form - it looks terrible! It does not look like our original form, which was styled pretty nicely. One of the trickiest things to do with the form system is to style, or theme your forms. The system is super powerful: we just need to unlock its potential!

Adding Attributes to the
Tag

Here's the goal: make our form render the same markup that we had before. Let's start with something simple: the form tag had a class called form-signin. Google for "Symfony form function reference". Hey, we know this page! It lists all of the functions that we can call to render each part of our form! And it will give us a clue about how we can customize each part.

For example, the first argument to form_start() is called view. When you see "view" on this page, it's referring to your form variable. The really important argument is the second one: variables. We saw this before: almost every function has this mysterious variables argument. This is an array of, literally, Twig variables! They're used to render each part of the form.

For example, there is apparently a variable called method that you can set to control the method attribute on the form. But, it's not as simple as: every variable becomes an attribute. For example, we can't just pass a class variable to add a class attribute.

Scroll all the way down to the bottom of this page. Remember this table? It shows the most common variables that we can override. One of the most important ones is attr. Let's try that one! Add a second argument - an array, with an attr key set to another array with class set to form-signin. Phew! And while we're here, we also had an <h1> before. Add that right at the beginning of the form.

48 lines | templates/security/register.html.twig
// ... lines 1 - 14
{{ form_start(registrationForm, {
'attr': {'class': 'form-signin'}
}) }}
<h1 class="h3 mb-3 font-weight-normal">Register</h1>
// ... lines 19 - 48

Two small steps forward! Let's try it! Oh, it's already, so much better. Heck, I can even see my agree to terms checkbox again!

The Core Bootstrap Form Theme

Now... things get more interesting. The original fields were just a label and an input. The input has a class on it, but otherwise, it's pretty basic. But the Bootstrap form theme renders everything inside of a form-group div. Then there's the <label> and the <input>. So, hmm: we need to change how all of this markup is rendered. To do that we need to dive deep: we need to learn how the form theme system works under the hood.

Earlier, we opened config/packages/twig.yaml and added a form_themes line that pointed to a core template called bootstrap_4_layout.html.twig. This... instantly, made everything pretty! But... what did that really do?

Whenever Symfony renders any part of your form, there is a Twig template deep in the core that contains the markup for that one piece: like the label, the widget or the errors. Once we know how this work, we can override it!

Press Shift+Shift to open this template: bootstrap_4_layout.html.twig. This is probably the strangest Twig template that you'll ever see. It's, huh, just a ton of blocks: block time_widget, percent_widget, file_widget and many, many more.

Form Theme Template & Block System

Here's how it works: every field has five different components: the row and the 4 things it contains: the widget label, errors and help. When you render each part, Symfony opens this template, selects the correct block for the thing it's rendering, and renders it like a mini-template. It passes all the variables into that block. Yea, it's a totally cool, but weird use of Twig.

Go back to our form class to see an example. Oh, I totally forgot! We can set email to EmailType::class. That will make it render as <input type="email"> instead of text. And that will give us some extra HTML5 validation.

54 lines | src/Form/UserRegistrationFormType.php
// ... lines 1 - 17
public function buildForm(FormBuilderInterface $builder, array $options)
{
$builder
->add('email', EmailType::class)
// ... lines 22 - 44
}
// ... lines 46 - 54

Back to the theming stuff. Here's the key: to render the "widget" part of an "email" field type, Symfony looks for a block called email_widget. That's the pattern: the block is named by combining the field type - email, an underscore, then the "part" - widget: email_widget. Ok... so let's find it! Search for the block email_widget.

Oh... boo - it doesn't exist? What? Actually, this block lives in another template that lives right next to this. I'll click the Form directory on top, then open a super important template called form_div_layout.html.twig.

This is Symfony's default form theme template. And even if you don't list this in your twig.yaml file, Symfony always uses it. What I mean is: when Symfony searches for a block - like email_widget - it will look in bootstrap_4_layout.html.twig first. But if it is not there, it will also look here.

Let's search again for email_widget. Boom! This is the block that's responsible for rendering the widget part of the email field. Want to find the block that renders the widget part of PasswordType? There it is: password_widget. Both of these execute another block - form_widget_simple - to do the real work.

So... cool! By understanding the naming system for these blocks, we can create our own form theme template and create blocks that override any part of any field type. Sweet!

But... there is a surprise ahead! What is the name of the block that Symfony looks for when rendering the label part of a password type field? We know! It's password_label, right? Search for that. It's not here! And you won't find it in bootstrap_4_layout.html.twig either.

To understand why, we need to learn a bit about field type hierarchy. Then, we'll be ready to create our own form theme. That's next!