Contributed by Michael Käfer
in #45605.

The Symfony Form component is so stable and provides so many features, that
we rarely add new features to it. However, in Symfony 6.1 we're improving its
developer experience with a new feature to customize collection prototypes.
The CollectionType form field is used to render a collection of other fields
or forms. When the field allows to add new elements to the collection, it defines
a prototype option. This contains the HTML code needed to render the new empty
collection item (you've probably seen it as the data-prototype HTML attribute
of the element that wraps the entire collection).
The only drawback is that collection prototypes are not entirely configurable.
You can set their initial values with the prototype-data option, but that's
pretty much it. That's why in Symfony 6.1 we're adding a new option called
prototype_options so you can configure collection prototypes.
The options defined in prototype_options are passed to the form type specified
in the entry_type option when creating its prototype. In practice, this allows
to have different options depending on whether you are adding a new entry or
editing an existing entry:

$builder->add('names', CollectionType::class, [
'entry_type' => TextType::class,

// this is used when editing items in the collection
'entry_options' => [
'attr' => ['class' => 'item-edit'],
'help' => 'You cannot edit existing names.',
'disabled' => true,
],

// this is used when adding new items to the collection
'prototype_options' => [
'attr' => ['class' => 'item-add'],
'help' => 'Check out the rules to create new names',
'help_html' => true,
],
]);

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