Modifier classes should be used when static declaration is not applicable. For example, in cases when additional data should be loaded from database. Also, modifier is a place where you add validations to display only certain fields for certain product types.

In the run time, the form structure set in the modifier is merged with the configuration that comes from the product_form.xml configuration.

The Magento\Catalog\Ui\DataProvider\Product\Form\ProductDataProvider data provider class is responsible for data and metadata preparation for the product form. The pool of modifiers Magento\Catalog\Ui\DataProvider\Product\Form\Modifier\Pool (virtual type) is injected to this data provider using the __construct() method. The pool’s preference is defined in di.xml.

To add your custom modifier, you need to do the following:

1. Add the modifier code.
2. Add it to the modifiers’ pool in di.xml

Add your modifier

In your custom module directory, add the modifier class that implements the Magento\UI\DataProvider\ModifierInterface interface or extends the Magento\Catalog\Ui\DataProvider\Product\Form\Modifier\AbstractModifierclass. In your modifier, the modifyData() and the modifyMeta() methods must be implemented.

In the modifier class, you can add UI elements using the same structure as in the XML configuration.

For example:

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<?php

use Magento\Catalog\Ui\DataProvider\Product\Form\Modifier\AbstractModifier;
use Magento\Ui\Component\Form\Field;
use Magento\Ui\Component\Form\Fieldset;

class Example extends AbstractModifier
{
    /**
    * @param array $meta
    *
    * @return array
    */
    public function modifyMeta(array $meta): array
    {
        $meta['test_fieldset_name'] = [
            'arguments' => [
                'data' => [
                    'config' => [
                        'label' => __('Label For Fieldset'),
                        'sortOrder' => 50,
                        'collapsible' => true,
                        'componentType' => Fieldset::NAME
                    ]
                ]
            ],
            'children' => [
                'test_field_name' => [
                    'arguments' => [
                        'data' => [
                            'config' => [
                                'formElement' => 'select',
                                'componentType' => Field::NAME,
                                'options' => [
                                    ['value' => 'test_value_1', 'label' => 'Test Value 1'],
                                    ['value' => 'test_value_2', 'label' => 'Test Value 2'],
                                    ['value' => 'test_value_3', 'label' => 'Test Value 3'],
                                ],
                                'visible' => 1,
                                'required' => 1,
                                'label' => __('Label For Element')
                            ]
                        ]
                    ]
                ]
            ]
        ];

        return $meta;
    }

    /**
     * {@inheritdoc}
     */
    public function modifyData(array $data)
    {
        return $data;
    }
}

You can create nested structures of elements by adding them to the children key of any element.

Add modifier to the pool

In <your_module_dir>/etc/adminhtml/di.xml define your modifier as a dependency for Magento\Catalog\Ui\DataProvider\Product\Form\Modifier\Pool.

The following is an example of such a definition:

app/code/Magento/CatalogInventory/etc/adminhtml/di.xml:

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<virtualType name="Magento\Catalog\Ui\DataProvider\Product\Form\Modifier\Pool">
    <arguments>
        <argument name="modifiers" xsi:type="array">
            <item name="advancedInventory" xsi:type="array">
                <item name="class" xsi:type="string">Magento\CatalogInventory\Ui\DataProvider\Product\Form\Modifier\AdvancedInventory</item>
                <item name="sortOrder" xsi:type="number">20</item>
            </item>
        </argument>
    </arguments>
</virtualType>

The sortOrder parameter defines the order of invocation for your modifyData() and modifyMeta() methods among other these methods of other modifiers in the pool. If a modifier is first in a pool, its modifyData() and modifyMeta() are invoked with empty arguments.

To access product model within your modifier, it’s recommended to use an instance of Magento\Catalog\Model\Locator\LocatorInterface.

For reference, view the modifier classes in the Magento modules, for example:

• Magento/Catalog/Ui/DataProvider/Product/Form/Modifier/AdvancedPricing
• Magento/Catalog/Ui/DataProvider/Product/Form/Modifier/AttributeSet
• Magento/Catalog/Ui/DataProvider/Product/Form/Modifier/Eav
• Magento/ConfigurableProduct/Ui/DataProvider/Product/Form/Modifier/Data/AssociatedProducts

For reference about setting conditions for displaying certain elements for certain product types, view <Magento_Catalog_module_dir>/Ui/DataProvider/Product/Form/Modifier/Eav.php#L476.