What is the «backend_model» for a backend configuration's form field?

dmitry_fedyuk · November 15, 2015, 9:49pm

Having a backend_model specified, a backend config form field can employ a custom logic for a config value loading and saving.

An example:

github.com

magento/magento2/blob/2.2.0-RC1.1/app/code/Magento/Paypal/etc/adminhtml/system/express_checkout.xml#L48-L53


<field id="api_password" translate="label" type="obscure" sortOrder="40" showInDefault="1" showInWebsite="1">
    <label>API Password</label>
    <config_path>paypal/wpp/api_password</config_path>
    <backend_model>Magento\Config\Model\Config\Backend\Encrypted</backend_model>
    <attribute type="shared">1</attribute>
</field>

A backend model should implement the \Magento\Framework\App\Config\ValueInterface interface:

github.com

magento/magento2/blob/2.2.0-RC1.1/lib/internal/Magento/Framework/App/Config/ValueInterface.php#L17-L37


/**
 * Check if config data value was changed
 * @todo this method should be make as protected
 * @return bool
 */
public function isValueChanged();


/**
 * Get old value from existing config
 *
 * @return string
 */
public function getOldValue();


/**
 * Get value by key for new user data from <section>/groups/<group>/fields/<field>
 *
 * @param string $key
 * @return string
 */
public function getFieldsetDataValue($key);

Magento 2 checks whether the interface is implemented:

Step 1. Magento\Config\Block\System\Config\Form::getFieldData()

Step 2. Magento\Config\Model\Config\Structure\Element\Field::getBackendModel()

github.com

magento/magento2/blob/2.2.0-RC1.1/app/code/Magento/Config/Model/Config/Structure/Element/Field.php#L221-L221


return $this->_backendFactory->create($this->_data['backend_model']);

Step 3. Magento\Config\Model\Config\BackendFactory::create()

github.com

magento/magento2/blob/2.2.0-RC1.1/app/code/Magento/Config/Model/Config/BackendFactory.php#L36-L38


if (!$model instanceof \Magento\Framework\App\Config\ValueInterface) {
    throw new \InvalidArgumentException('Invalid config field backend model: ' . $modelName);
}

But implementing the \Magento\Framework\App\Config\ValueInterface is not enough for a backend_model: Logical error: Magento 2 checks whether a backend config form field’s «backend_model» implements the \Magento\Framework\App\Config\ValueInterface interface only, but actually calls the «backend_model» for much more methods (absent in the interface).

Usually a backend_model is an instance of the \Magento\Framework\App\Config\Value class.

dmitry_fedyuk · November 17, 2015, 5:18am