Magento 2 di.xml merging with arguments

Question

I've been working through Magento U's Fundamentals of Magento 2 Development course, and I'm currently working on the second unit, which covers request processing. This morning, I was finishing the videos and exercises on routers in Magento 2. One of the exercises that was provided involved writing a custom router.

I added the router to the routerlist with the following code in etc/di.xml:

<type name="Magento\Framework\App\RouterList">
    <arguments>
        <argument name="routerList" xsi:type="array">
            <item name="training" xsi:type="array">
                <item name="class" xsi:type="string">Module\Router\Name</item>
                <item name="disable" xsi:type="boolean">false</item>
                <item name="sortOrder" xsi:type="string">70</item>
            </item>
        </argument>
    </arguments>
</type>

I was surprised, when I ran the application, to find that the router wasn't working as expected. I stepped into the code with Xdebug and realized that Magento 2 wasn't even loading the router into the RouterList. After doing a bit of Googling, I realized that I had put the type in etc/di.xml, while others were putting the type declaration in etc/frontend/di.xml. I didn't think this would make a difference, but I moved my file into the frontend area, and to my chagrin, it worked.

I was under the impression that Magento 2 merged the DI files together, including the global and the applicable area-specific files; however, if I were correct, my first attempt would have worked.

So, how does Magento 2 handle merging the global and area-specific di.xml files together, specifically when processing types, and why didn't declaring the code in the etc/di.xml work, when etc/frontend/di.xml did?

Answer 1

See the question/answer Marius linked to: https://magento.stackexchange.com/a/139912/1905

The simple answer is that DI of the same scope gets merged together. DI of a higher scope overwrites a lower scope.

That means values set in etc/frontend/di.xml will be combined with other values set in etc/frontend/di.xml files, but will overwrite values set in etc/di.xml files (when you're in a frontend area). In your example, the core router list is defined in frontend, so that's why your addition didn't take effect when you had it in the global scope.