Fix merging nested <var /> in view.xml

[torreytsui]

Nested <var /> in view.xml cannot be merged

Problem

The view.xsd defined that <var /> element can contain unlimited <var /> children.

<!-- magento/framework/Config/etc/view.xsd -->
<!-- ... -->
    <xs:complexType name="varType" mixed="true">
        <xs:sequence minOccurs="0" maxOccurs="unbounded">
            <xs:element name="var" type="varType"/>
        </xs:sequence>
        <!-- ... -->
    </xs:complexType>
<!-- ... -->

Despite of the nested <var /> definition in xsd, it is not fully configured in the \Magento\Framework\Config\View, that is, /view/vars/var does not support nested <var />.

// \Magento\Framework\Config\View::getIdAttributes
protected function getIdAttributes()
    {
        $idAttributes = [
            '/view/vars' => 'module',
+           '/view/vars/(var/)*var' => 'name',
-            '/view/vars/var' => 'name',
            '/view/exclude/item' => ['type', 'item'],
        ];
        foreach ($this->xpath as $attribute) {
            if (is_array($attribute)) {
                foreach ($attribute as $key => $id) {
                    if (count($id) > 1) {
                        $idAttributes[$key] = array_values($id);
                    } else {
                        $idAttributes[$key] = array_shift($id);
                    }
                }
            }
        }
        return $idAttributes;
    }

Bug reproduction

Steps

1. Create a module with a default etc/view.xml which contain more than one nested var

   <!-- Awesome_Module/etc/view.xml -->
   <?xml version="1.0"?>
   <view xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/view.xsd">
       <vars module="Awesome_Module">
           <var name="awesome">
               <var name="first">default</var>
               <var name="second">defatul</var>
           </var>
       </vars>
   </view>

2. In the local view.xml, overwrite the default settings.

   <!-- app/design/frontend/Awesome/theme/etc/view.xml -->
   ...
   +    <vars module="Awesome_Module">
   +        <var name="awesome">
   +            <var name="first">This is awesome</var>
   +            <var name="second">This is also awesome</var>
   +        </var>
   +    </vars>
   ...

3. Call \Magento\Framework\View\Element\AbstractBlock::getVar from anywhere

<?php echo $block->getVar('awesome/first'); ?>

Expect Result

This is awesome

Actual Result

Exception #0 (Magento\Framework\Exception\LocalizedException): More than one node matching the query: /view/vars[@module='Awesome_Module']/var[@name='awesome']/var

Workaround

For the time being, a workaround can be applied while this PR or other relevant fix is waiting to merge. Add below fragment into any di.xml

    <!-- Workaround to fix nested <var /> merge issue -->
    <type name="Magento\Framework\Config\View">
        <arguments>
            <argument name="xpath" xsi:type="array">
                <item name="vars" xsi:type="array">
                    <item name="/view/vars/(var/)*var" xsi:type="array">
                        <item name="id" xsi:type="string">name</item>
                    </item>
                </item>
            </argument>
        </arguments>
    </type>

[okorshenko]

@torreytsui thank you for preparing excellent Pull Request. Your contribution successfully merged to develop branch! Thank you