[Form] BC break (maybe?) between v3.4.20 and v3.4.21

[simshaun]

Symfony version(s) affected: v.3.4.21

Description
Caused by #29307 (/cc @nicolas-grekas)

I've got a MainFormType that contains a collection of SubFormType. SubFormType has a hidden field that holds a JSON-encoded string (generated by JS in my actual use case). SubFormType transforms the string to an array on submit.

Everything has been working fine until 3.4.21 when the form started giving errors. I'm not really sure if this is a BC break or not, but it has definitely caused us some problems.

How to reproduce
I've created a repro branch at https://github.com/simshaun/symfony-standard/tree/form_bc_break_3.4.21

• Run phpunit to confirm errors.
• Downgrade to Symfony 3.4.20 and run phpunit again.

[xabbuh]

looks like the same as #29809

[simshaun]

Hm maybe. The difference being, instead of submitting an array structure I'm submitting a string then transforming it to an array in a form event listener. I guess both ways ultimately look the same to the form component. It's frustrating when a bugfix (was the scalar data issue really a bug or just a bad unintended feature?) causes problems like this.

For anyone else running into this problem, drop the PRE_SUBMIT form event listener and add a model transformer, e.g. below:

$builder->add(
    $builder->create('settings', HiddenType::class)
        ->addModelTransformer(new JsonTransformer())
);

<?php

namespace SomeRandomNamespace;

use Symfony\Component\Form\DataTransformerInterface;

class JsonTransformer implements DataTransformerInterface
{
    public function transform($data)
    {
        if (null === $data) {
            return null;
        }

        return json_encode($data);
    }

    public function reverseTransform($json)
    {
        if (!$json) {
            return null;
        }

        return json_decode($json, true);
    }
}

[nicolas-grekas]

It worked by chance - and it made any field accept arrays of values by just appending [] to the name of form inputs. Definitely not the definition of proper input filtering.
Using a data transformer is a nice idea - and a much more reliable one, thanks for sharing.

[stof]

Well, using a PRE_SUBMIT event listener means that the HiddenType receives an array as submitted value, while it expects a string as it data. The fact that it worked before was just luck (as it does not do anything special with its data except passing it around, it kind of worked). Btw, in case of an invalid form being re-displayed, it might even have been broken (as the template would then have to render the array) so I'm not even sure it fully worked before.

Using a model data transformer is exactly how your use case should be implemented.

[xabbuh]

I am closing here as this is not a bug as explained.