Contributed by
Jules Pietri
in #18332.

ChoiceType is the most powerful Symfony form type and it's used to create select drop-downs, radio buttons and checkboxes. In Symfony 3.2 we added a new feature to improve its performance: lazy loading the choice values.

First, define the choice_loader option for the ChoiceType and then, use the new CallbackChoiceLoader class to set the PHP callable executed to get the list of choices:

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use Symfony\Component\Form\ChoiceList\Loader\CallbackChoiceLoader;
use Symfony\Component\Form\Extension\Core\Type\ChoiceType;

$builder->add('constants', ChoiceType::class, [
    'choice_loader' => new CallbackChoiceLoader(function() {
            return StaticClass::getConstants();
    },
]);

The CallbackChoiceLoader class implements ChoiceLoaderInterface, which is now also implemented in every ChoiceType subtype, such as CountryType, CurrencyType, LanguageType, LocaleType and TimezoneType.