LiveProp Data Types

LiveProps must be a value that can be sent to JavaScript. Supported values are scalars (int, float, string, bool, null), arrays (of scalar values), enums, DateTime objects, Doctrine entity objects, DTOs, or array of DTOs.

See Live Components for handling more complex data.

Debouncing

If the user types 5 characters really quickly, we don't want to send 5 Ajax requests. Fortunately, live components adds automatic debouncing: it waits for a 150ms pause between typing before sending an Ajax request to re-render. This is built in, so you don't need to think about it. But, you can delay via the debounce modifier:

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<input data-model="debounce(100)|max">

Lazy Updating on "change" of a Field

Sometimes, you might want a field to re-render only after the user has changed an input and moved to another field. Browsers dispatch a change event in this situation. To re-render when this event happens, use the on(change) modifier:

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<input data-model="on(change)|max">

Deferring a Re-Render Until Later

Other times, you might want to update the internal value of a property, but wait until later to re-render the component (e.g. until a button is clicked). To do that, use norender modifier:

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<input data-model="norender|max">

Now, as you type, the max "model" will be updated in JavaScript, but it won't, yet, make an Ajax call to re-render the component. Whenever the next re-render does happen, the updated max value will be used.

Using name="" instead of data-model

If you're building a form (more on forms later), instead of adding data-model to every field, you can instead rely on the name attribute.

To activate this, you must add a data-model attribute to the <form> element:

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<div {{ attributes }}>
    <form data-model="*">
        <input
            name="max"
            value="{{ max }}"
        >

        // ...
    </form>
</div>

The * value of data-model is not necessary, but is commonly used. You can also use the normal modifiers, like data-model="on(change)|*" to, for example, only send model updates for the change event of each field inside.

Model Updates don't work when External JavaScript Changes a Field

Suppose you use a JavaScript library that sets the value of a field for you: for example a "date picker" library that hides the native <input data-model="publishAt"> field and sets it behind-the-scenes when the user selects a date.

In this case, the model (e.g. publishAt) will probably not update correctly because JavaScript doesn't trigger the normal change event. To fix this, you'll need to "hook" into the JavaScript library and set the model directly (or trigger a change event on the data-model field). See manually trigger an element change.

Writable Object Properties or Array Keys

By default, the user can't change the properties of an entity LiveProp You can allow this by setting writable to property names that should be writable. This also works as a way to make only some keys of an array writable:

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use App\Entity\Post;

#[AsLiveComponent]
class EditPost
{
    #[LiveProp(writable: ['title', 'content'])]
    public Post $post;

    #[LiveProp(writable: ['allow_markdown'])]
    public array $options = ['allow_markdown' => true, 'allow_html' => false];
}

Now post.title, post.content or options.allow_markdown can be used like normal model names:

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<div {{ attributes }}>
    <input data-model="post.title">
    <textarea data-model="post.content"></textarea>

    Allow Markdown?
    <input type="checkbox" data-model="options.allow_markdown">

    Preview:
    <div>
        <h3>{{ post.title }}</h3>
        {{ post.content|markdown_to_html }}
    </div>
</div>

Any other properties on the object (or keys on the array) will be read-only.

For arrays, you can set writable: true to allow any key in the array to be changed, added or removed:

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#[AsLiveComponent]
class EditPost
{
    // ...

    #[LiveProp(writable: true)]
    public array $options = ['allow_markdown' => true, 'allow_html' => false];

    #[LiveProp(writable: true)]
    public array $todoItems = ['Train tiger', 'Feed tiger', 'Pet tiger'];
}

Checkboxes, Select Elements Radios & Arrays

Checkboxes can be used to set a boolean or an array of strings:

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#[AsLiveComponent]
class EditPost
{
    #[LiveProp(writable: true)]
    public bool $agreeToTerms = false;

    #[LiveProp(writable: true)]
    public array $foods = ['pizza', 'tacos'];
}

In the template, setting a value attribute on the checkbox will set that value on checked. If no value is set, the checkbox will set a boolean value:

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<input type="checkbox" data-model="agreeToTerms">

<input type="checkbox" data-model="foods[]" value="pizza">
<input type="checkbox" data-model="foods[]" value="tacos">
<input type="checkbox" data-model="foods[]" value="sushi">

select and radio elements are a bit easier: use these to either set a single value or an array of values:

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#[AsLiveComponent]
class EditPost
{
    // ...

    #[LiveProp(writable: true)]
    public string $meal = 'lunch';

    #[LiveProp(writable: true)]
    public array $foods = ['pizza', 'tacos'];
}

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<input type="radio" data-model="meal" value="breakfast">
<input type="radio" data-model="meal" value="lunch">
<input type="radio" data-model="meal" value="dinner">

<select data-model="foods" multiple>
    <option value="pizza">Pizza</option>
    <option value="tacos">Tacos</option>
    <option value="sushi">Sushi</option>
</select>

LiveProp Date Formats

If you have a writable LiveProp that is some sort of DateTime instance, you can control the format of the model on the frontend with the format option:

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#[LiveProp(writable: true, format: 'Y-m-d')]
public ?\DateTime $publishOn = null;

Now you can bind this to a field on the frontend that uses that same format:

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<input type="date" data-model="publishOn">

Allowing an Entity to be Changed to Another

What if, instead of changing a property on an entity, you want to allow the user to switch the entity to another? For example:

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<select data-model="post">
    {% for post in posts %}
        <option data-model="{{ post.id }}">{{ post.title }}</option>
    {% endfor %}
</select>

To make the post property itself writable, use writable: true:

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use App\Entity\Post;

#[AsLiveComponent]
class EditPost
{
    #[LiveProp(writable: true)]
    public Post $post;
}

If you want the user to be able to change the Post and certain properties, use the special LiveProp::IDENTITY constant:

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use App\Entity\Post;

#[AsLiveComponent]
class EditPost
{
    #[LiveProp(writable: [LiveProp::IDENTITY, 'title', 'content'])]
    public Post $post;
}

Note that being able to change the "identity" of an object is something that works only for objects that are dehydrated to a scalar value (like persisted entities, which dehydrate to an id).

Using DTO's on a LiveProp

You can also use a DTO (i.e. data transfer object / any simple class) with LiveProp as long as the property has the correct type:

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class ComponentWithAddressDto
{
    public AddressDto $addressDto;
}

To work with a collection of DTOs, specify the collection type inside PHPDoc:

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class ComponentWithAddressDto
{
    /**
     * @var AddressDto[]
    /*
    public array $addressDtoCollection;
}

Here is how the (de)hydration of DTO objects works:

• All "properties" (public properties or fake properties via getter/setter methods) are read & dehydrated. If a property is settable but not gettable (or vice versa), an error will be thrown.
• The PropertyAccess component is used to get/set the value, which means getter and setter methods are supported, in addition to public properties.
• The DTO cannot have any constructor arguments.

If this solution doesn't fit your need there are two others options to make this work:

Hydrating with the Serializer

To hydrate/dehydrate through Symfony's serializer, use the useSerializerForHydration option:

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class ComponentWithAddressDto
{
    #[LiveProp(useSerializerForHydration: true)]
    public AddressDto $addressDto;
}

You can also set a serializationContext option on the LiveProp.

Hydrating with Methods: hydrateWith & dehydrateWith

You can take full control of the hydration process by setting the hydrateWith and dehydrateWith options on LiveProp:

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class ComponentWithAddressDto
{
    #[LiveProp(dehydrateWith: 'dehydrateAddress', hydrateWith: 'hydrateAddress')]
    public AddressDto $addressDto;

    public function dehydrateAddress(AddressDto $address)
    {
        return [
            'street' => $address->street,
            'city' => $address->city,
            'state' => $address->state,
        ];
    }

    public function hydrateAddress($data): AddressDto
    {
        return new AddressDto($data['street'], $data['city'], $data['state']);
    }
}

Hydration Extensions

If you frequently hydrate/dehydrate the same type of object, you can create a custom hydration extension to make this easier. For example, if you frequently hydrate a custom Food object, a hydration extension might look like this:

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use App\Model\Food;
use Symfony\UX\LiveComponent\Hydration\HydrationExtensionInterface;

class FoodHydrationExtension implements HydrationExtensionInterface
{
    public function supports(string $className): bool
    {
        return is_subclass_of($className, Food::class);
    }

    public function hydrate(mixed $value, string $className): ?object
    {
        return new Food($value['name'], $value['isCooked']);
    }

    public function dehydrate(object $object): mixed
    {
        return [
            'name' => $object->getName(),
            'isCooked' => $object->isCooked(),
        ];
    }
}

If you're using autoconfiguration, you're done! Otherwise, tag the service with live_component.hydration_extension.

JavaScript Component Hooks

The JavaScript Component object has a number of hooks that you can use to run code during the lifecycle of your component. To hook into the component system from Stimulus:

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// assets/controllers/some-custom-controller.js
// ...
import { getComponent } from '@symfony/ux-live-component';

export default class extends Controller {
    async initialize() {
        this.component = await getComponent(this.element);

        this.component.on('render:finished', (component) => {
            // do something after the component re-renders
        });
    }
}

The following hooks are available (along with the arguments that are passed):

• connect args (component: Component)
• disconnect args (component: Component)
• render:started args (html: string, response: BackendResponse, controls: { shouldRender: boolean })
• render:finished args (component: Component)
• response:error args (backendResponse: BackendResponse, controls: { displayError: boolean })
• loading.state:started args (element: HTMLElement, request: BackendRequest)
• loading.state:finished args (element: HTMLElement)
• model:set args (model: string, value: any, component: Component)

Adding a Stimulus Controller to your Component Root Element

To add a custom Stimulus controller to your root component element:

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<div {{ attributes.defaults(stimulus_controller('my-controller', { someValue: 'foo' })) }}>

Adding and Removing Classes or Attributes

Instead of hiding or showing an entire element, you could add or remove a class:

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<!-- add this class when loading -->
<div data-loading="addClass(opacity-50)">...</div>

<!-- remove this class when loading -->
<div data-loading="removeClass(opacity-50)">...</div>

<!-- add multiple classes when loading -->
<div data-loading="addClass(opacity-50 disabled)">...</div>

Sometimes you may want to add or remove an attribute when loading. That can be accomplished with addAttribute or removeAttribute:

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<!-- add the "disabled" attribute when loading -->
<div data-loading="addAttribute(disabled)">...</div>

You can also combine any number of directives by separating them with a space:

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<div data-loading="addClass(opacity-50) addAttribute(disabled)">...</div>

Finally, you can add the delay modifier to not trigger the loading changes until loading has taken longer than a certain amount of time:

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<!-- Add class after 200ms of loading -->
<div data-loading="delay|addClass(opacity-50)">...</div>

<!-- Show after 200ms of loading -->
<div data-loading="delay|show">Loading</div>

<!-- Show after 500ms of loading -->
<div data-loading="delay(500)|show">Loading</div>

Targeting Loading for a Specific Action

To only toggle the loading behavior when a specific action is triggered, use the action() modifier with the name of the action - e.g. saveForm():

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<!-- show only when the "saveForm" action is triggering -->
<span data-loading="action(saveForm)|show">Loading</span>
<!-- multiple modifiers -->
<div data-loading="action(saveForm)|delay|addClass(opacity-50)">...</div>

Targeting Loading When a Specific Model Changes

You can also toggle the loading behavior only if a specific model value was just changed using the model() modifier:

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<input data-model="email" type="email">

<span data-loading="model(email)|show">
    Checking if email is available...
</span>

<!-- multiple modifiers & child properties -->
<span data-loading="model(user.email)|delay|addClass(opacity-50)">...</span>

Actions & Services

One really neat thing about component actions is that they are real Symfony controllers. Internally, they are processed identically to a normal controller method that you would create with a route.

This means that, for example, you can use action autowiring:

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// src/Components/RandomNumber.php
namespace App\Components;

// ...
use Psr\Log\LoggerInterface;

class RandomNumber
{
    // ...

    #[LiveAction]
    public function resetMax(LoggerInterface $logger)
    {
        $this->max = 1000;
        $logger->debug('The min/max were reset!');
    }

    // ...
}

Actions & Arguments

You can also provide custom arguments to your action:

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<form>
    <button
        data-action="live#action"
        data-action-name="addItem(id={{ item.id }}, itemName=CustomItem)"
    >Add Item</button>
</form>

In your component, to allow each argument to be passed, we need to add the #[LiveArg()] attribute:

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// src/Components/ItemList.php
namespace App\Components;

// ...
use Psr\Log\LoggerInterface;
use Symfony\UX\LiveComponent\Attribute\LiveArg;

class ItemList
{
    // ...
    #[LiveAction]
    public function addItem(#[LiveArg] int $id, #[LiveArg('itemName')] string $name)
    {
        $this->id = $id;
        $this->name = $name;
    }
}

Normally, the argument name in PHP - e.g. $id - should match the argument name used in Twig id={{ item.id }}. But if they don't match, you can pass an argument to LiveArg, like we did with itemName.

Actions and CSRF Protection

When you trigger an action, a POST request is sent that contains a X-CSRF-TOKEN header. This header is automatically populated and validated. In other words… you get CSRF protection without any work.

Your only job is to make sure that the CSRF component is installed:

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$ composer require symfony/security-csrf

If you want to disable CSRF for a single component you can set csrf option to false:

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namespace App\Twig\Components;

use Symfony\UX\LiveComponent\Attribute\AsLiveComponent;
use Symfony\UX\LiveComponent\Attribute\LiveProp;

#[AsLiveComponent(csrf: false)]
class MyLiveComponent
{
    // ...
}

Actions, Redirecting and AbstractController

Sometimes, you may want to redirect after an action is executed (e.g. your action saves a form and then you want to redirect to another page). You can do that by returning a RedirectResponse from your action:

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// src/Components/RandomNumber.php
namespace App\Components;

// ...
use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;

class RandomNumber extends AbstractController
{
    // ...

    #[LiveAction]
    public function resetMax()
    {
        // ...

        $this->addFlash('success', 'Max has been reset!');

        return $this->redirectToRoute('app_random_number');
    }

    // ...
}

You probably noticed one interesting trick: to make redirecting easier, the component now extends AbstractController! That is totally allowed, and gives you access to all of your normal controller shortcuts. We even added a flash message!

Rendering an Entire Form in a Component

Suppose you have a PostType form class that's bound to a Post entity and you'd like to render this in a component so that you can get instant validation as the user types:

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namespace App\Form;

use App\Entity\Post;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;

class PostType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('title')
            ->add('slug')
            ->add('content')
        ;
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults([
            'data_class' => Post::class,
        ]);
    }
}

Great! In the template for some page (e.g. an "Edit post" page), render a PostForm component that we will create next:

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{# templates/post/edit.html.twig #}
{% extends 'base.html.twig' %}

{% block body %}
    <h1>Edit Post</h1>

    {{ component('PostForm', {
        initialFormData: post,
    }) }}
{% endblock %}

Ok: time to build that PostForm component! The Live Components package comes with a special trait - ComponentWithFormTrait - to make it easy to deal with forms:

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namespace App\Twig\Components;

use App\Entity\Post;
use App\Form\PostType;
use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\Form\FormInterface;
use Symfony\UX\LiveComponent\Attribute\AsLiveComponent;
use Symfony\UX\LiveComponent\Attribute\LiveProp;
use Symfony\UX\LiveComponent\ComponentWithFormTrait;
use Symfony\UX\LiveComponent\DefaultActionTrait;

#[AsLiveComponent]
class PostForm extends AbstractController
{
    use DefaultActionTrait;
    use ComponentWithFormTrait;

    /**
     * The initial data used to create the form.
     */
    #[LiveProp]
    public Post $initialFormData = null;

    protected function instantiateForm(): FormInterface
    {
        // we can extend AbstractController to get the normal shortcuts
        return $this->createForm(PostType::class, $this->initialFormData);
    }
}

The trait forces you to create an instantiateForm() method, which is used each time the component is rendered via AJAX. To recreate the same form as the original, we pass in the initialFormData property and set it as a LiveProp.

The template for this component will render the form, which is available as form thanks to the trait:

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{# templates/components/PostForm.html.twig #}
<div {{ attributes }}>
    {{ form_start(form) }}
        {{ form_row(form.title) }}
        {{ form_row(form.slug) }}
        {{ form_row(form.content) }}

        <button>Save</button>
    {{ form_end(form) }}
</div>

That's it! The result is incredible! As you finish changing each field, the component automatically re-renders - including showing any validation errors for that field! Amazing!

How this works:

1. The ComponentWithFormTrait has a $formValues writable LiveProp containing the value for every field in your form.
2. When the user changes a field, that key in $formValues is updated and an Ajax request is sent to re-render.
3. During that Ajax call, the form is submitted using $formValues, the form re-renders, and the page is updated.

Build the "New Post" Form Component

The previous component can already be used to edit an existing post or create a new post. For a new post, either pass in a new Post object to initialFormData, or omit it entirely to let the initialFormData property default to null:

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{# templates/post/new.html.twig #}
{# ... #}

{{ component('PostForm', {
    form: form
}) }}

Submitting the Form via a LiveAction

The simplest way to handle your form submit is directly in your component via a LiveAction:

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// ...
use Doctrine\ORM\EntityManagerInterface;
use Symfony\UX\LiveComponent\Attribute\LiveAction;

class PostForm extends AbstractController
{
    // ...

    #[LiveAction]
    public function save(EntityManagerInterface $entityManager)
    {
        // Submit the form! If validation fails, an exception is thrown
        // and the component is automatically re-rendered with the errors
        $this->submitForm();

        /** @var Post $post */
        $post = $this->getForm()->getData();
        $entityManager->persist($post);
        $entityManager->flush();

        $this->addFlash('success', 'Post saved!');

        return $this->redirectToRoute('app_post_show', [
            'id' => $post->getId(),
        ]);
    }
}

Next, tell the form element to use this action:

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{# templates/components/PostForm.html.twig #}
{# ... #}

{{ form_start(form, {
    attr: {
        'data-action': 'live#action',
        'data-action-name': 'prevent|save'
    }
}) }}

Now, when the form is submitted, it will execute the save() method via Ajax. If the form fails validation, it will re-render with the errors. And if it's successful, it will redirect.

Submitting with a Normal Symfony Controller

If you prefer, you can submit the form via a Symfony controller. To do this, create your controller like normal, including the submit logic:

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// src/Controller/PostController.php
class PostController extends AbstractController
{
    #[Route('/admin/post/{id}/edit', name: 'app_post_edit')]
    public function edit(Request $request, Post $post, EntityManagerInterface $entityManager): Response
    {
        $form = $this->createForm(PostType::class, $post);
        $form->handleRequest($request);

        if ($form->isSubmitted() && $form->isValid()) {
            // save, redirect, etc
        }

        return $this->render('post/edit.html.twig', [
            'post' => $post,
            'form' => $form,
        ]);
    }
}

If validation fails, you'll want the live component to render with the form errors instead of creating a fresh form. To do that, pass the form variable into the component:

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{# templates/post/edit.html.twig #}
{{ component('PostForm', {
    initialFormData: post,
    form: form
}) }}

Using Form Data in a LiveAction

Each time an Ajax call is made to re-render the live component the form is automatically submitted using the latest data.

However, there are two important things to know:

1. When a LiveAction is executed, the form has not yet been submitted.
2. The initialFormData property is not updated until after the form is submitted.

If you need to access the latest data in a LiveAction, you can manually submit the form:

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// ...

#[LiveAction]
public function save()
{
    // $this->initialFormData will *not* contain the latest data yet!

    // submit the form
    $this->submitForm();

    // now you can access the latest data
    $post = $this->getForm()->getData();
    // (same as above)
    $post = $this->initialFormData;
}

Dynamically Updating the Form In a LiveAction

When an Ajax call is made to re-render the live component (whether that's due to a model change or a LiveAction), the form is submitted using a $formValues property from ComponentWithFormTrait that contains the latest data from the form.

Sometimes, you need to update something on the form dynamically from a LiveAction. For example, suppose you have a "Generate Title" button that, when clicked, will generate a title based on the content of the post.

To do this, you must update the $this->formValues property directly before the form is submitted:

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// ...

#[LiveAction]
public function generateTitle()
{
    // this works!
    // (the form will be submitted automatically after this method, now with the new title)
    $this->formValues['title'] = '... some auto-generated-title';

    // this would *not* work
    // $this->submitForm();
    // $post = $this->getForm()->getData();
    // $post->setTitle('... some auto-generated-title');
}

This is tricky. The $this->formValues property is an array of the raw form data on the frontend and contains only scalar values (e.g. strings, integers, booleans and arrays). By updating this property, the form will submit as if the user had typed the new title into the form. The form will then be re-rendered with the new data.

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$this->formValues['author'] = $author->getId();

Why not just update the $post object directly? Once you submit the form, the "form view" (data, errors, etc for the frontend) has already been created. Changing the $post object has no effect. Even modifying $this->initialFormData before submitting the form has no effect: the actual, submitted title would override that.

Form Rendering Problems

For the most part, rendering a form inside a component works beautifully. But there are a few situations when your form may not behave how you want.

A) Text Boxes Removing Trailing Spaces

If you're re-rendering a field on the input event (that's the default event on a field, which is fired each time you type in a text box), then if you type a "space" and pause for a moment, the space will disappear!

This is because Symfony text fields "trim spaces" automatically. When your component re-renders, the space will disappear… as the user is typing! To fix this, either re-render on the change event (which fires after the text box loses focus) or set the trim option of your field to false:

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public function buildForm(FormBuilderInterface $builder, array $options)
{
    $builder
        // ...
        ->add('content', TextareaType::class, [
            'trim' => false,
        ])
    ;
}

B) PasswordType loses the password on re-render

If you're using the PasswordType, when the component re-renders, the input will become blank! That's because, by default, the PasswordType does not re-fill the <input type="password"> after a submit.

To fix this, set the always_empty option to false in your form:

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public function buildForm(FormBuilderInterface $builder, array $options)
{
    $builder
        // ...
        ->add('plainPassword', PasswordType::class, [
            'always_empty' => false,
        ])
    ;
}

Resetting the Form

After submitting a form via an action, you might want to "reset" the form back to its initial state so you can use it again. Do that by calling resetForm() in your action instead of redirecting:

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#[LiveAction]
public function save(EntityManagerInterface $entityManager)
{
    // ...

    $this->resetForm();
}

Using Actions to Change your Form: CollectionType

Symfony's CollectionType can be used to embed a collection of embedded forms including allowing the user to dynamically add or remove them. Live components make this all possible while writing zero JavaScript.

For example, imagine a "Blog Post" form with an embedded "Comment" forms via the CollectionType:

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namespace App\Form;

use App\Entity\BlogPost;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\CollectionType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;

class BlogPostFormType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('title', TextType::class)
            // ...
            ->add('comments', CollectionType::class, [
                'entry_type' => CommentFormType::class,
                'allow_add' => true,
                'allow_delete' => true,
                'by_reference' => false,
            ])
        ;
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(['data_class' => BlogPost::class]);
    }
}

Now, create a Twig component to render the form:

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namespace App\Twig;

use App\Entity\BlogPost;
use App\Entity\Comment;
use App\Form\BlogPostFormType;
use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\Form\FormInterface;
use Symfony\UX\LiveComponent\Attribute\AsLiveComponent;
use Symfony\UX\LiveComponent\Attribute\LiveAction;
use Symfony\UX\LiveComponent\ComponentWithFormTrait;
use Symfony\UX\LiveComponent\DefaultActionTrait;

#[AsLiveComponent]
class BlogPostCollectionType extends AbstractController
{
    use ComponentWithFormTrait;
    use DefaultActionTrait;

    #[LiveProp]
    public initialFormData $post;

    protected function instantiateForm(): FormInterface
    {
        return $this->createForm(BlogPostFormType::class, $this->initialFormData);
    }

    #[LiveAction]
    public function addComment()
    {
        // "formValues" represents the current data in the form
        // this modifies the form to add an extra comment
        // the result: another embedded comment form!
        // change "comments" to the name of the field that uses CollectionType
        $this->formValues['comments'][] = [];
    }

    #[LiveAction]
    public function removeComment(#[LiveArg] int $index)
    {
        unset($this->formValues['comments'][$index]);
    }
}

The template for this component has two jobs: (1) render the form like normal and (2) include links that trigger the addComment() and removeComment() actions:

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<div{{ attributes }}>
    {{ form_start(form) }}
        {{ form_row(form.title) }}

        <h3>Comments:</h3>
        {% for key, commentForm in form.comments %}
            <button
                data-action="live#action"
                data-action-name="removeComment(index={{ key }})"
                type="button"
            >X</button>

            {{ form_widget(commentForm) }}
        {% endfor %}
        </div>

        {# avoid an extra label for this field #}
        {% do form.comments.setRendered %}

        <button
            data-action="live#action"
            data-action-name="addComment"
            type="button"
        >+ Add Comment</button>

        <button type="submit" >Save</button>
    {{ form_end(form) }}
</div>

Done! Behind the scenes, it works like this:

A) When the user clicks "+ Add Comment", an Ajax request is sent that triggers the addComment() action.

B) addComment() modifies formValues, which you can think of as the raw "POST" data of your form.

C) Still during the Ajax request, the formValues are "submitted" into your form. The new key inside of $this->formValues['comments'] tells the CollectionType that you want a new, embedded form.

D) The form is rendered - now with another embedded form! - and the Ajax call returns with the form (with the new embedded form).

When the user clicks removeComment(), a similar process happens.

Using LiveCollectionType

The LiveCollectionType uses the same method described above, but in a generic way, so it needs even less code. This form type adds an 'Add' and a 'Delete' button for each row by default, which work out of the box thanks to the LiveCollectionTrait.

Let's take the same example as before, a "Blog Post" form with an embedded "Comment" forms via the LiveCollectionType:

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namespace App\Form;

use App\Entity\BlogPost;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Symfony\UX\LiveComponent\Form\Type\LiveCollectionType;

class BlogPostFormType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('title', TextType::class)
            // ...
            ->add('comments', LiveCollectionType::class, [
                'entry_type' => CommentFormType::class,
            ])
        ;
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(['data_class' => BlogPost::class]);
    }
}

Now, create a Twig component to render the form:

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namespace App\Twig;

use App\Entity\BlogPost;
use App\Form\BlogPostFormType;
use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\Form\FormInterface;
use Symfony\UX\LiveComponent\Attribute\AsLiveComponent;
use Symfony\UX\LiveComponent\Attribute\LiveProp;
use Symfony\UX\LiveComponent\DefaultActionTrait;
use Symfony\UX\LiveComponent\LiveCollectionTrait;

#[AsLiveComponent]
class BlogPostCollectionType extends AbstractController
{
    use LiveCollectionTrait;
    use DefaultActionTrait;

    #[LiveProp]
    public BlogPost $initialFormData;

    protected function instantiateForm(): FormInterface
    {
        return $this->createForm(BlogPostFormType::class, $this->initialFormData);
    }
}

There is no need for a custom template just render the form as usual:

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<div {{ attributes }}>
    {{ form(form) }}
</div>

This automatically renders add and delete buttons that are connected to the live component. If you want to customize how the buttons and the collection rows are rendered, you can use Symfony's built-in form theming techniques, but you should note that, the buttons are not part of the form tree.

Here are some examples of these techniques.

If you only want to customize some attributes, the simplest to use the options in the form type:

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// ...
$builder
    // ...
    ->add('comments', LiveCollectionType::class, [
        'entry_type' => CommentFormType::class,
        'label' => false,
        'button_delete_options' => [
            'label' => 'X',
            'attr' => [
                'class' => 'btn btn-outline-danger',
            ],
        ]
    ])
;

Inline rendering:

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<div {{ attributes }}>
    {{ form_start(form) }}
        {{ form_row(form.title)

        <h3>Comments:</h3>
        {% for key, commentForm in form.comments %}
            {# render a delete button for every row #}
            {{ form_row(commentForm.vars.button_delete, { label: 'X', attr: { class: 'btn btn-outline-danger' } }) }}

            {# render rest of the comment form #}
            {{ form_row(commentForm, { label: false }) }}
        {% endfor %}

        {# render the add button #}
        {{ form_widget(form.comments.vars.button_add, { label: '+ Add comment', class: 'btn btn-outline-primary' }) }}

        {# render rest of the form #}
        {{ form_row(form) }}

        <button type="submit" >Save</button>
    {{ form_end(form) }}
</div>

Override the specific block for comment items:

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{% form_theme form 'components/_form_theme_comment_list.html.twig' %}

<div {{ attributes }}>
    {{ form_start(form) }}

    {{ form_start(form) }}
        {{ form_row(form.title)

        <h3>Comments:</h3>
        <ul>
            {{ form_row(form.comments, { skip_add_button: true }) }}
        </ul>

        {# render rest of the form #}
        {{ form_row(form) }}

        <button type="submit" >Save</button>
    {{ form_end(form) }}
</div>

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{# templates/components/_form_theme_comment_list.html.twig #}
{%- block _blog_post_form_comments_entry_row -%}
    <li class="...">
        {{ form_row(form.content, { label: false }) }}
        {{ form_row(button_delete, { label: 'X', attr: { class: 'btn btn-outline-danger' } }) }}
    </li>
{% endblock %}

Override the generic buttons and collection entry:

The add and delete buttons are rendered as separate ButtonType form types and can be customized like a normal form type via the live_collection_button_add and live_collection_button_delete block prefix respectively:

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{% block live_collection_button_add_widget %}
    {% set attr = attr|merge({'class': attr.class|default('btn btn-ghost')}) %}
    {% set translation_domain = false %}
    {% set label_html = true %}
    {%- set label -%}
        <svg xmlns="http://www.w3.org/2000/svg" class="h-6 w-6" fill="none" viewBox="0 0 24 24" stroke="currentColor" stroke-width="2">
            <path stroke-linecap="round" stroke-linejoin="round" d="M12 6v6m0 0v6m0-6h6m-6 0H6"/>
        </svg>
        {{ 'form.collection.button.add.label'|trans({}, 'forms') }}
    {%- endset -%}
    {{ block('button_widget') }}
{% endblock live_collection_button_add_widget %}

To control how each row is rendered you can override the blocks related to the LiveCollectionType. This works the same way as the traditional collection type, but you should use live_collection_* and live_collection_entry_* as prefixes instead.

For example, by default the add button is placed after the items (the comments in our case). Let's move it before them.

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{%- block live_collection_widget -%}
    {%- if button_add is defined and not button_add.rendered -%}
        {{ form_row(button_add) }}
    {%- endif -%}
    {{ block('form_widget') }}
{%- endblock -%}

Now add a div around each row:

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{%- block live_collection_entry_row -%}
    <div>
        {{ block('form_row') }}
        {%- if button_delete is defined and not button_delete.rendered -%}
            {{ form_row(button_delete) }}
        {%- endif -%}
    </div>
{%- endblock -%}

As another example, let's create a general bootstrap 5 theme for the live collection type, rendering every item in a table row:

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{%- block live_collection_widget -%}
    <table class="table table-borderless form-no-mb">
        <thead>
        <tr>
            {% for child in form|last %}
                <td>{{ form_label(child) }}</td>
            {% endfor %}
            <td></td>
        </tr>
        </thead>
        <tbody>
            {{ block('form_widget') }}
        </tbody>
    </table>
    {%- if skip_add_button|default(false) is same as(false) and button_add is defined and not button_add.rendered -%}
        {{ form_widget(button_add, { label: '+ Add Item', attr: { class: 'btn btn-outline-primary' } }) }}
    {%- endif -%}
{%- endblock -%}

{%- block live_collection_entry_row -%}
    <tr>
        {% for child in form %}
            <td>{{- form_row(child, { label: false }) -}}</td>
        {% endfor %}
        <td>
            {{- form_row(button_delete, { label: 'X', attr: { class: 'btn btn-outline-danger' } }) -}}
        </td>
    </tr>
{%- endblock -%}

To render the add button later in the template, you can skip rendering it initially with skip_add_button, then render it manually after:

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<table class="table table-borderless form-no-mb">
    <thead>
        <tr>
            <td>Item</td>
            <td>Priority</td>
            <td></td>
        </tr>
    </thead>
    <tbody>
        {{ form_row(form.todoItems, { skip_add_button: true }) }}
    </tbody>
</table>

{{ form_widget(form.todoItems.vars.button_add, { label: '+ Add Item', attr: { class: 'btn btn-outline-primary' } }) }}

Resetting Validation Errors

If you want to clear validation errors (e.g. so you can reuse the form again), you can call the resetValidation() method:

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// ...
class EditUser
{
    // ...

    #[LiveAction]
    public function save()
    {
        // validate, save, etc

        // reset your live props to the original state
        $this->user = new User();
        $this->agreeToTerms = false;
        // clear the validation state
        $this->resetValidation();
    }
}

Emitting an Event

There are three ways to emit an event:

1. From Twig:

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<button
    data-action="live#emit"
    data-event="productAdded"
>

1. From your PHP component via ComponentToolsTrait:

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   use Symfony\UX\LiveComponent\ComponentToolsTrait;
   
   class MyComponent
   {
       use ComponentToolsTrait;
   
       #[LiveAction]
       public function saveProduct()
       {
           // ...
   
           $this->emit('productAdded');
       }
   }

2. From JavaScript, using your component:

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this.component.emit('productAdded');

Listen to Events

To listen to an event, add a method with a #[LiveListener] above it:

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#[LiveProp]
public int $productCount = 0;

#[LiveListener('productAdded')]
public function incrementProductCount()
{
    $this->productCount++;
}

Thanks to this, when any other component emits the productAdded event, an Ajax call will be made to call this method and re-render the component.

Behind the scenes, event listeners are also LiveActions <actions>, so you can autowire any services you need.

Passing Data to Listeners

You can also pass extra (scalar) data to the listeners:

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#[LiveAction]
public function saveProduct()
{
    // ...

    $this->emit('productAdded', [
        'product' => $product->getId(),
    ]);
}

In your listeners, you can access this by adding a matching argument name with #[LiveArg] in front:

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#[LiveListener('productAdded')]
public function incrementProductCount(#[LiveArg] int $product)
{
    $this->productCount++;
    $this->lastProduct = $data['product'];
}

And because event listeners are also actions, you can type-hint an argument with an entity name, just like you would in a controller:

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#[LiveListener('productAdded')]
public function incrementProductCount(#[LiveArg] Product $product)
{
    $this->productCount++;
    $this->lastProduct = $product;
}

Scoping Events

By default, when an event is emitted, it is sent to all components that are currently on the page. You can scope these in various ways:

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<button
    data-action="live#emitUp"
    data-event="productAdded"
>

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$this->emitUp('productAdded');

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<button
    data-action="live#emit"
    data-event="name(ProductList)|productAdded"
>

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$this->emit('productAdded', name: 'ProductList');

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<button
    data-action="live#emitSelf"
    data-event="productAdded"
>

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$this->emitSelf('productAdded');

Each component re-renders independent of one another

If a parent component re-renders, this won't, by default, cause any child components to re-render, but you can make it do that. Let's look at an example of a todo list component with a child that renders the total number of todo items:

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{# templates/components/TodoList.html.twig #}
<div {{ attributes }}>
    <input data-model="listName">

    {% for todo in todos %}
        ...
    {% endfor %}

    {{ component('TodoFooter', {
        count: todos|length
    }) }}
</div>

Suppose the user updates the listName model and the parent component re-renders. In this case, the child component will not re-render by design: each component lives in its own universe.

However, if the user adds a new todo item then we do want the TodoFooter child component to re-render: using the new count value. To trigger this, in the TodoFooter component, add the updateFromParent option:

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#[LiveComponent()]
class TodoFooter
{
    #[LiveProp(updateFromParent: true)]
    public int $count = 0;
}

Now, when the parent component re-renders, if the value of the count prop changes, the child will make a second Ajax request to re-render itself.

Child components keep their modifiable LiveProp values

What if the TodoFooter component in the previous example also has an isVisible LiveProp(writable: true) property which starts as true but can be changed (via a link click) to false. Will re-rendering the child when count changes cause this to be reset back to its original value? Nope! When the child component re-renders, it will keep the current value for all props, except for those that are marked as updateFromParent.

What if you do want your entire child component to re-render (including resetting writable live props) when some value in the parent changes? This can be done by manually giving your component a data-live-id attribute that will change if the component should be totally re-rendered:

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{# templates/components/TodoList.html.twig #}
<div {{ attributes }}>
    <!-- ... -->

    {{ component('TodoFooter', {
        count: todos|length,
        'data-live-id': 'todo-footer-'~todos|length
    }) }}
</div>

In this case, if the number of todos change, then the data-live-id attribute of the component will also change. This signals that the component should re-render itself completely, discarding any writable LiveProp values.

Actions in a child do not affect the parent

Again, each component is its own, isolated universe! For example, suppose your child component has:

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<button data-action="live#action" data-action-name="save">Save</button>

When the user clicks that button, it will attempt to call the save action in the child component only, even if the save action actually only exists in the parent. The same is true for data-model, though there is some special handling for this case (see next point).

Communicating with a Parent Component

There are two main ways to communicate from a child component to a parent component:

1. Emitting events

   The most flexible way to communicate: any information can be sent from the child to the parent.

2. Updating a parent model from a child

   Useful as a simple way to "synchronize" a child model with a parent model: when the child model changes, the parent model will also change.

Updating a Parent Model from a Child

Suppose a child component has a:

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<textarea data-model="value">

When the user changes this field, this will only update the value field in the child component… because (yup, we're saying it again): each component is its own, isolated universe.

However, sometimes this isn't what you want! Sometimes, when a child model changes, that should also update a model on the parent. To do this, pass a dataModel (or data-model) attribute to the child:

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{# templates/components/PostForm.html.twig #}
{{ component('TextareaField', {
    dataModel: 'content',
    error: _errors.get('content'),
}) }}

This does two things:

1. A prop called value will be passed into TextareaField set to content from the parent component (i.e. the same as manually passing value: content into the component).
2. When the value prop changes inside of TextareaField, the content prop will change on the parent component.

This result is that, when value changes, the parent component will also re-render, thanks to the fact that its content prop changed.

You can also specify the name of the child prop with the parentProp:childProp syntax. The following is the same as above:

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<!-- same as dataModel: 'content' -->
{{ component('TextareaField', {
    dataModel: 'content:value',
}) }}

If your child component has multiple models, separate each with a space:

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{{ component('TextareaField', {
    dataModel: 'user.firstName:first user.lastName:last',
}) }}

In this case, the child component will receive first and last props. And, when those update, the user.firstName and user.lastName models will be updated on the parent.

Full Embedded Component Example

Let's look at a full, complex example of an embedded component. Suppose you have an EditPost:

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namespace App\Twig\Components;

use App\Entity\Post;
use Doctrine\ORM\EntityManagerInterface;
use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\UX\LiveComponent\Attribute\AsLiveComponent;
use Symfony\UX\LiveComponent\Attribute\LiveAction;
use Symfony\UX\LiveComponent\Attribute\LiveProp;

#[AsLiveComponent]
final class EditPost extends AbstractController
{
    #[LiveProp(writable: ['title', 'content'])]
    public Post $post;

    #[LiveAction]
    public function save(EntityManagerInterface $entityManager)
    {
        $entityManager->flush();

        return $this->redirectToRoute('some_route');
    }
}

And a MarkdownTextarea:

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namespace App\Twig\Components;

use Symfony\UX\LiveComponent\Attribute\AsLiveComponent;
use Symfony\UX\LiveComponent\Attribute\LiveProp;

#[AsLiveComponent]
final class MarkdownTextarea
{
    #[LiveProp]
    public string $label;

    #[LiveProp]
    public string $name;

    #[LiveProp(writable: true)]
    public string $value = '';
}

In the EditPost template, you render the MarkdownTextarea:

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{# templates/components/EditPost.html.twig #}
<div {{ attributes }}>
    <form data-model="on(change)|*">
        <input
            type="text"
            name="post[title]"
            value="{{ post.title }}"
        >

        {{ component('MarkdownTextarea', {
            name: 'post[content]',
            dataModel: 'post.content:value',
            label: 'Content',
        }) }}

        <button
            data-action="live#action"
            data-action-name="save"
        >Save</button>
    </form>
</div>

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<div {{ attributes }} class="mb-3">
    <textarea
        name="{{ name }}"
        data-model="value"
    ></textarea>

    <div class="markdown-preview">
        {{ value|markdown_to_html }}
    </div>
</div>

Notice that MarkdownTextarea allows a dynamic name attribute to be passed in. This makes that component re-usable in any form.

Rendering Quirks with List of Elements

If you're rendering a list of elements in your component, to help LiveComponents understand which element is which between re-renders (i.e. if something re-orders or removes some of those elements), you can add a data-live-id attribute to each element

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{# templates/components/Invoice.html.twig #}
{% for lineItem in lineItems %}
    <div data-live-id="{{ lineItem.id }}">
        {{ lineItem.name }}
    </div>
{% endfor %}

Rendering Quirks with List of Embedded Components

Imagine your component renders a list of child components and the list changes as the user types into a search box... or by clicking "delete" on an item. In this case, the wrong children may be removed or existing child components may not disappear when they should.

To fix this, add a key prop to each child component that's unique to that component:

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{# templates/components/InvoiceCreator.html.twig #}
{% for lineItem in invoice.lineItems %}
    {{ component('InvoiceLineItemForm', {
        lineItem: lineItem,
        key: lineItem.id,
    }) }}
{% endfor %}

The key will be used to generate a data-live-id attribute, which will be used to identify each child component. You can also pass in a data-live-id attribute directly, but key is a bit more convenient.

Tricks with a Loop + a "New" Item

Let's get fancier. After looping over the current line items, you decide to render one more component to create a new line item. In that case, you can pass in a key set to something like new_line_item:

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{# templates/components/InvoiceCreator.html.twig #}
// ... loop and render the existing line item components

{{ component('InvoiceLineItemForm', {
    key: 'new_line_item',
}) }}

Imagine you also have a LiveAction inside of InvoiceLineItemForm that saves the new line item to the database. To be extra fancy, it emits a lineItem:created event to the parent:

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// src/Twig/InvoiceLineItemForm.php
// ...

#[AsLiveComponent]
final class InvoiceLineItemForm
{
    // ...

    #[LiveProp]
    #[Valid]
    public ?InvoiceLineItem $lineItem = null;

    #[PostMount]
    public function postMount(): void
    {
        if (!$this->lineItem) {
            $this->lineItem = new InvoiceLineItem();
        }
    }

    #[LiveAction]
    public function save(EntityManagerInterface $entityManager)
    {
        if (!$this->lineItem->getId()) {
            $this->emit('lineItem:created', $this->lineItem);
        }

        $entityManager->persist($this->lineItem);
        $entityManager->flush();
    }
}

Finally, the parent InvoiceCreator component listens to this so that it can re-render the line items (which will now contain the newly-saved item):

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// src/Twig/InvoiceCreator.php
// ...

#[AsLiveComponent]
final class InvoiceCreator
{
    // ...

    #[LiveListener('lineItem:created')]
    public function addLineItem()
    {
        // no need to do anything here: the component will re-render
    }
}

This will work beautifully: when a new line item is saved, the InvoiceCreator component will re-render and the newly saved line item will be displayed along with the extra new_line_item component at the bottom.

But something surprising might happen: the new_line_item component won't update! It will keep the data and props that were there a moment ago (i.e. the form fields will still have data in them) instead of rendering a fresh, empty component.

Why? When live components re-renders, it thinks the existing key: new_line_item component on the page is the same new component that it's about to render. And because the props passed into that component haven't changed, it doesn't see any reason to re-render it.

To fix this, you have two options:

1) Make the key dynamic so it will be different after adding a new item:

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{{ component('InvoiceLineItemForm', {
    key: 'new_line_item_'~lineItems|length,
}) }}

2) Reset the state of the InvoiceLineItemForm component after it's saved:

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// src/Twig/InvoiceLineItemForm.php
// ...

#[AsLiveComponent]
class InvoiceLineItemForm
{
    // ...

    #[LiveAction]
    public function save(EntityManagerInterface $entityManager)
    {
        $isNew = null === $this->lineItem->getId();

        $entityManager->persist($this->lineItem);
        $entityManager->flush();

        if ($isNew) {
            // reset the state of this component
            $this->emit('lineItem:created', $this->lineItem);
            $this->lineItem = new InvoiceLineItem();
            // if you're using ValidatableComponentTrait
            $this->clearValidation();
        }
    }
}

Passing Content (Blocks) to Components

Passing content via blocks to Live components works completely the same way you would pass content to Twig Components. Except with one important difference: when a component is re-rendered, any variables defined only in the "outside" template will not be available. For example, this won't work:

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{# templates/some_page.html.twig #}
{% set message = 'Variables from the outer part of the template are only available during  the initial render' %}

{% component Alert %}
    {% block content %}{{ message }}{% endblock %}
{% endcomponent %}

Local variables do remain available:

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{# templates/some_page.html.twig #}
{% component Alert %}
    {% block content %}
        {% set message = 'this works during re-rendering!' %}
        {{ message }}
    {% endblock %}
{% endcomponent %}

The Smart Re-Render Algorithm

When a component re-renders, the new HTML is "morphed" onto the existing elements on the page. For example, if the re-render includes a new class on an existing element, that class will be added to that element.

The rendering system is also smart enough to know when an element was changed by something outside of the LiveComponents system: e.g. some JavaScript that added a class to an element. In this case, the class will be preserved when the component re-renders.

The system doesn't handle every edge case, so here are some things to keep in mind:

• If JavaScript changes an attribute on an element, that change is preserved.
• If JavaScript adds a new element, that element is preserved.
• If JavaScript removes an element that was originally rendered by the component, that change will be lost: the element will be re-added during the next re-render.
• If JavaScript changes the text of an element, that change is lost: it will be restored to the text from the server during the next re-render.
• If an element is moved from one location in the component to another, that change is lost: the element will be re-added in its original location during the next re-render.

The Mystical data-live-id Attribute

The data-live-id attribute is mentioned several times throughout the documentation to solve various problems. It's usually not needed, but can be the key to solving certain complex problems. But what is it?

The data-live-id attribute is a unique identifier for an element or a component. It's used when a component re-renders and helps Live Components "connect" elements or components in the existing HTML with the new HTML. The logic works like this:

Suppose an element or component in the new HTML has a data-live-id="some-id" attribute. Then:

A) If there is an element or component with data-live-id="some-id" in the

existing HTML, then the old and new elements/components are considered to be the "same". For elements, the new element will be used to update the old element even if the two elements appear in different places - e.g. like if elements are moved or re-ordered. For components, because child components render independently from their parent, the existing component will be "left alone" and not re-rendered (unless some updateFromParent props have changed - see Live Components).

B) If there is not an element or component with data-live-id="some-id" in

the existing HTML, then the new element or component is considered to be "new". In both cases, the new element or component will be added to the page. If there is a component/element with a data-live-id attribute that is not in the new HTML, that component/element will be removed from the page.

Skipping Updating Certain Elements

If you have an element inside a component that you do not want to change when your component re-renders, you can add a data-live-ignore attribute:

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<input name="favorite_color" data-live-ignore>

But you should need this rarely if ever. Even if you write JavaScript that modifies an element, that changes is preserved (see Live Components).

Define another route for your Component

The default route for live components is /components/{_live_component}/{_live_action}. Sometimes it may be useful to customize this URL - e.g. so that the component lives under a specific firewall.

To use a different route, first declare it:

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# config/routes.yaml
live_component_admin:
    path: /admin/_components/{_live_component}/{_live_action}
    defaults:
        _live_action: 'get'

Then specify this new route on your component:

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// src/Components/RandomNumber.php
use Symfony\UX\LiveComponent\Attribute\AsLiveComponent;
use Symfony\UX\LiveComponent\DefaultActionTrait;

- #[AsLiveComponent]
+ #[AsLiveComponent(route: 'live_component_admin')]
  class RandomNumber
  {
      use DefaultActionTrait;
  }

Add a Hook on LiveProp Update

If you want to run custom code after a specific LiveProp is updated, you can do it by adding an onUpdated option set to a public method name on the component:

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#[AsLiveComponent]
class ProductSearch
{
    #[LiveProp(writable: true, onUpdated: 'onQueryUpdated')]
    public string $query = '';

    // ...

    public function onQueryUpdated($previousValue): void
    {
        // $this->query already contains a new value
        // and its previous value is passed as an argument
    }
}

As soon as the query LiveProp is updated, the onQueryUpdated() method will be called. The previous value is passed there as the first argument.

If you're allowing object properties to be writable, you can also listen to the change of one specific key:

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use App\Entity\Post;

#[AsLiveComponent]
class EditPost
{
    #[LiveProp(writable: ['title', 'content'], onUpdated: ['title' => 'onTitleUpdated'])]
    public Post $post;

    // ...

    public function onTitleUpdated($previousValue): void
    {
        // ...
    }
}