Multiple Category Sources Field

Question

I want to utilise the Category field type but with the ability to choose from multiple category sources (as with Entries).

I noted in cms/src/fields/Categories.php that $this->allowMultipleSources is set to false. What is the simplest way of changing this to true without changing the source file?

Can I create a custom field that essentially just extends the original Category field but with $this->allowMultipleSources set to true?

Answer 1

Got there in the end and very simply through:

use Craft;
use craft\fields\BaseRelationField;
use craft\elements\Category;

class Categories extends BaseRelationField
{

    public static function displayName(): string
    {
        return Craft::t('app', 'Categories (multiple)');
    }

    protected static function elementType(): string
    {
        return Category::class;
    }

    public static function defaultSelectionLabel(): string
    {
        return Craft::t('app', 'Add a category');
    }
}

So simple

Answer 2

Isnt this possible with the modifyCategorySources hook?

-> https://docs.craftcms.com/v3/extend/updating-plugins.html#modifyentrysources-modifycategorysources-modifyassetsources-modifyusersources

I'm assuming you are using C3 by the namespace you mentioned. Correct me if this is wrong :).