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Google for "Drupal Devel": it will lead you to a project on Drupal.org that has some nice tools in it. What we're after is the webprofiler module. Copy the installation link address so we can install it! In the future, you should be able to install modules via Composer. More on that later.
For now, click install new module, paste the URL, press the install button and
wait with great anticipation! When it finishes, follow the enable newly added modules
link. I'm only interested in the web profiler. Check its box and press the install
button. It'll also ask me to install the devel, module which is fine. Hit continue...
and watch closely.
On the very next request a really cool web debug toolbar pops up at the bottom. This is the oracle of information about the request that was just executed - in this case - a request to this admin page. It shows us tons of stuff like database queries, who is authenticated, stats about the configuration, the css and js that's being loaded, the route and controller that's being used for this page and - somewhere in there - I'm pretty sure it knows where my car keys are.
Before you go crazy with this, go back down on this page to the webprofiler. Expand
it and click "Configure". Here, you can see that there's even more information
that you can display if you want to. Check the boxes for Events, Routing and Services
and then press the "Save configuration" button.
Ooo look: a few new things on the toolbar. If you click any icon, you'll go to a new page called the profiler. It turns out that the web debug toolbar was just a short summary of the information about the last request. The profiler has tons more.
If you want to see all of the routes, click the Routing tab. This is the same list that the Drupal console showed us.
There are lots of other treats in here: it's like a Drupal Halloween! For example, Performance Timing checks how fast the frontend of your site is rendering. The timeline is probably my favorite... and for some reason it's broken in this version. Wah wah. It normally shows you this great graph of how long it takes each part of Drupal to execute. It's great for profiling, but also great to see what all the magic layers of Drupal are.
If you follow the documentation for the webprofiler, you also need to install a
couple of JavaScript libraries to help the profiler do its job. But it seems to work
pretty well without them, so I skipped that part to save us time.
Now that we have this, click on the admin "Structure" page. Obviously, this page comes from Drupal. But how does it work? Go down to the toolbar and hover over the 200 status code. Ha! This tells us exactly what controller renders this page:
Drupal\System\Controller\SystemController::systemAdminMenuBlockPage()
If you see the D\s\C stuff, that stands for Drupal\System\Controller. The web
profiler tries to shorten things: just Hover over this syntax to see the full class
name.
If you wanted to reverse engineer this page, you could! I'll use the keyboard shortcut
shift+shift to search the project for SystemController. Here's the class! Now,
look around for the method systemAdminMenuBlockPage(). And this is the actual
function that renders the admin "Structure" page:
| ... lines 1 - 24 | |
| class SystemController extends ControllerBase { | |
| ... lines 26 - 184 | |
| /** | |
| * Provides a single block from the administration menu as a page. | |
| */ | |
| public function systemAdminMenuBlockPage() { | |
| return $this->systemManager->getBlockContents(); | |
| } | |
| ... lines 191 - 343 | |
| } |
In fact, if you add return new Response('HI!') and refresh, it'll completely replace
the page! Try this and see if your co-workers can figure out what's going on!
We don't know yet what this systemManager thing is or how to debug it, but we're
going there next.
I just think it's really cool that we can see exactly what's going on in the page, dive into the core code and find out how things work.
// composer.json
{
"require": {
"composer/installers": "^1.0.21", // v1.0.21
"wikimedia/composer-merge-plugin": "^1.3.0" // dev-master
}
}