Form type lazy load support

[sovetski]

Feature Request

Q	A
New Feature	yes
RFC	yes
BC Break	no

Summary

In the templates\Form\fields.html.twig, we have this part:

        {%- if image_uri -%}
            <a href="{{ asset_helper is same as(true) ? asset(image_uri) : image_uri }}" download>
                <img src="{{ asset_helper is same as(true) ? asset(image_uri) : image_uri }}" alt="" />
            </a>
        {%- endif -%}

As the src attribute is hardcoded and we have not the ability to add a custom class name here, we can not use lazy image loading here.

I know that we generally use lazy load feature on a public pages where we have a lot of users, but sometimes, in admin pages where we can upload a lot of images, like 50+ images per product for ex, and we display them each time even if we don't see them, it means a lot of resources to consume for the server (imagine the website is a marketplace and people can edit their products X times, and sometimes they don't really need to see all images without scrolling).

My proposal is to add a new option to VichFileType like we have here: https://github.com/dustin10/VichUploaderBundle/blob/master/docs/form/vich_file_type.md

Maybe something like:

// ...
use Vich\UploaderBundle\Form\Type\VichFileType;

class Form extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options): void
    {
        // ...

        $builder->add('genericFile', VichFileType::class, [
            'required' => false,
            'allow_delete' => true,
            'delete_label' => '...',
            'download_uri' => '...',
            'download_label' => '...',
            'asset_helper' => true,
            'src_alternative' => 'data-src', // <---- HERE
            'class_name' => 'lazy', // <---- HERE
        ]);
    }
}

In my case I am using https://github.com/verlok/vanilla-lazyload but there are more packages to do it probably, so with the options above we give the ability to adapt it to each library.

If it is ok for the maintainers, I can do a PR. Or if it is not ok, this is understandable.

A little message to other devs: If this issue is ever closed and you want the same functionality, don't hesitate to create a new issue or add comment here to let maintainers know.

[garak]

You still have the possibility of using your custom template.
It doesn't seem of so general interest to justify a feature here.

[sovetski]

You still have the possibility of using your custom template.

The problem with templates, that they contains other logic that we don't need to change.
For example here fields.html.twig contains 52 lines, and if I copy this file to only edit 1 line, in the futur updates of the bundle, I have to check if the template changed and update it.

I completely understand that this is not the kind of problem that every person will encounter, I still created the issue in case it concerns other people, although I suspect not

[garak]

You don't need to redefine the entire template, overriding only a single block is possible (and advised)