What is reverseMetadataMerge in UI Components?

Question

When I look at the documentation of UI Components, I notice the following:

<item name="reverseMetadataMerge" xsi:type="boolean">true</item>

Now, most arguments are pretty self-explaining in what they do, or I already know it, but what does reverseMetadataMerge do?

Answer 1

The setting you mention is used in vendor/magento/framework/View/Element/UiComponentFactory.php

 $reverseMerge = isset($componentArguments['data']['reverseMetadataMerge'])
            && $componentArguments['data']['reverseMetadataMerge'];
        $bundleComponents = $this->mergeMetadata($identifier, $bundleComponents, $reverseMerge);

this will merge the already defined metadata in the vendor/magento/module-customer/view/base/ui_component/customer_form.xml with some other metadata defined in the constructor of the data provider.

protected function mergeMetadata($identifier, array $bundleComponents, $reverseMerge = false)
{
    $dataProvider = $this->getDataProvider($identifier, $bundleComponents);
    if ($dataProvider instanceof DataProviderInterface) {
        $metadata = [
            $identifier => [
                'children' => $dataProvider->getMeta(),
            ],
        ];
        $bundleComponents = $this->mergeMetadataItem($bundleComponents, $metadata, $reverseMerge);
    }

    return $bundleComponents;
}

In the case of the documentation your data provider is vendor/magento/module-customer/Model/Customer/DataProvider.php that inside the constructor has

 $this->meta['customer']['children'] = $this->getAttributesMeta(
        $this->eavConfig->getEntityType('customer')
    );
    $this->meta['address']['children'] = $this->getAttributesMeta(
        $this->eavConfig->getEntityType('customer_address')
    );

that adds some other attributes. In the case of the customer the attributes are the ones shown in blue color in the following image