Named Autowiring

Let's start with a challenge: let's pretend that, to help us debug something, we want to log some messages from inside MarkdownHelper. Okay: logging is work and work is done by services.... so let's go see if our project has a logger!

Find your terminal and run our favorite command:

php bin/console debug:autowiring log

There it is! We can use LoggerInterface to get a service whose id is monolog.logger. Notice that there are a bunch of loggers listed here. Ignore the others for now: we'll talk about them in a few minutes.

In MarkdownHelper, how do we get access to a service that we need? It's the same process every time: add another constructor argument: LoggerInterface $logger. To create a new property and set it, I'm going to use a PhpStorm shortcut. With my cursor on the argument, I'll hit Alt+Enter and select "Initialize properties":

Nice! But it's not magic: that just created the property and set it down here.

In parse(), let's add some very important code: if stripos($source, 'cat') !== false, then say $this->logger and... let's use, ->info('Meow!'):

Let's take it for a spin! Move over, refresh... then click any link on the web debug toolbar to jump into the profiler. In the "Logs" section... there's our message!

Multiple Logger Services

The true reason we're doing this - other than to practice dependency injection - is to talk about how we can work with the many logger services in the container. Symfony's logging library - called Monolog - has this concept of logger channels. They're... kind of like logger categories and their useful because you can send logs from different channels to different files.

This is what you're seeing inside of debug:autowiring: each logger "channel" is actually its own, unique logger service. The question is: we know how to get the "main" logger service, but how could we autowire one of these other loggers if we needed to?

Creating a Logger Channel

Logging channels are not a super important concept - but it's a great example of this problem. To see how to handle it, let's add our own logger channel. The logger services comes from a bundle called MonologBundle. By adding a little config to that bundle, we can get a shiny new logger channel.

In config/packages/, create a new file called monolog.yaml. Inside say monolog: and below, set channels: to an array. Let's create one new channel called markdown:

By the way, if you're surprised that there was no monolog.yaml file by default, there actually is: there's one in the dev/ directory and another in prod/. Loggers behave pretty differently in dev versus prod. Thanks to this new file, the markdown channel will exist in all environments.

Anyways, now find your terminal and run debug:autowiring:

php bin/console debug:autowiring log

Yes! The bundle created a new service for us called monolog.logger.markdown.

Named Autowiring

So back to my original question: how can we get access to this logger? Well, this is already telling us! This says that if we type-hint an argument with LoggerInterface and name the argument $markdownLogger, it will pass us the monolog.logger.markdown service.

Ok, let's try it! Back in MarkdownHelper, rename the argument from $logger to $markdownLogger... and update the variable name below:

Let's see what difference this makes. When we reload, it still works... but open up the profiler and go to the Logs section. Yes! There it is! It says "channel": markdown. For this tutorial, I'm not really concerned about how or why we would use a different logger channel. The point is: this proves that we just fetched one of the other logger services.

The whole reason this works is because MonologBundle is smart: it sets up "autowiring aliases" for each channel. Basically, it makes sure that we can autowire the main logger with the type-hint or any of the other loggers with a type-hint and argument name combination. It sets all of that up for us, so we can just take advantage of it.

But what if it hadn't done that? Or, what if we needed to access one of the many lower-level services in the container that cannot be autowired? This is the last missing piece of the autowiring puzzle. Let's talk about it next.