> Symfony 7 > Dependency Injection #[Attributes]
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08.

Enforce Named Autowiring with Target

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In the last chapter, we added logging! Check out the "Logs" profile panel. Notice this little "app" tag? Symfony's logger, Monolog has the concept of log channels, which are like categories for your log messages. The default channel is "app", but you can create others.

Adding a New Log Channel

Let's see if we can add a new channel called "button". Open config/packages/monolog.yaml. Under channels, add a new one called button:

64 lines | config/packages/monolog.yaml
monolog:
channels:
// ... line 3
- button
// ... lines 5 - 64

Perfect! Close this file.

We want LoggerRemote to use this new channel... but... how do we do that?

We're autowiring the LoggerInterface service. That gives us the main logger: the one that logs to the "app" channel. To log to the new channel, we need to autowire a different logger service.

Spin over to your terminal and run:

bin/console debug:autowiring logger

The logger argument filters the results down to services with "logger" in the name. The first is LoggerInterface: this is the service you get if using standard autowiring, like we currently are.

But there are multiple logger services. The others all have an argument, including a new one called buttonLogger. This is the service we want! To grab this service, we need to use named autowiring.

Named Autowiring

In LoggerRemote, rename the argument to $buttonLogger:

38 lines | src/Remote/LoggerRemote.php
// ... lines 1 - 8
final class LoggerRemote implements RemoteInterface
{
public function __construct(
private LoggerInterface $buttonLogger,
// ... line 13
) {
}
// ... lines 16 - 36
}

Now, head back to our app... refresh the page, and... press the "Channel Up" button. We've watched enough Sponge Bob for one day. Pop into the last request's profile and check out the "Logs" panel. Sweet! We're now logging to the button channel!

This is working like it's supposed to, but I'd like to talk about a problem that can occur when using named autowiring. Imagine, a year from now, we rename this variable to something simpler like just $logger:

40 lines | src/Remote/LoggerRemote.php
// ... lines 1 - 9
final class LoggerRemote implements RemoteInterface
{
public function __construct(
// ... line 13
private LoggerInterface $logger,
// ... line 15
) {
}
// ... lines 18 - 38
}

Jump back to our app, refresh the page, and press a button. It seems like it still works, but check out the "Logs" profile panel. The message is there, but it sneakily switched back the "app" channel. How dare it!

When we renamed the argument, we broke named autowiring and switched back to standard autowiring. Maybe this isn't super problematic for logging, but imagine something like a database or cache service being autowired this way. That could be trouble... and I'm generally anti-trouble.

Target Attribute

We need a way to enforce named autowiring. Say hello to the #[Target] attribute!

Over in LoggerRemote, add #[Target] above the LoggerInterface $logger argument. Inside, set the value to buttonLogger - the argument name (without the $) we saw in the debug:autowiring command:

40 lines | src/Remote/LoggerRemote.php
// ... lines 1 - 9
final class LoggerRemote implements RemoteInterface
{
public function __construct(
#[Target('buttonLogger')]
private LoggerInterface $logger,
// ... line 15
) {
}
// ... lines 18 - 38
}

Now, the argument name could be anything, so get creative!

Jump back to the app, refresh, press "Volume Up", and check the "Logs" profile panel. We're back to the button channel! Sweet!

Enforcing Named Autowiring

To see what I mean by enforcing named autowiring, back in config/packages/monolog.yaml, rename the channel to buttons (with an "s"):

64 lines | config/packages/monolog.yaml
monolog:
channels:
// ... line 3
- buttons
// ... lines 5 - 64

Back in the app, refresh, and... we see an error!

Cannot autowire service "LoggerRemote": argument "$logger" of method "__construct()" has "#[Target('buttonLogger')]" but no such target exists.

Yes! Normally we don't celebrate errors, but this is a good one! We now have a hard error when the named autowiring service cannot be found. This is what we want!

Fix this by going back to LoggerRemote and updating the #[Target] to buttonsLogger (with an "s"):

40 lines | src/Remote/LoggerRemote.php
// ... lines 1 - 9
final class LoggerRemote implements RemoteInterface
{
public function __construct(
#[Target('buttonsLogger')]
private LoggerInterface $logger,
// ... line 15
) {
}
// ... lines 18 - 38
}

Refresh the app and we're back in business! Press "Mute" and pop into the "Logs" profile panel. Yep, we're logging to our renamed buttons channel!

I love named autowiring, but also love that we can enforce it with the #[Target] attribute.

Next: We're going to add a new button, but this time, we'll make it conditional, so it only appears in certain environments.