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We've got two problems. First, our services are always created. What if we had a mailer service? You only need to mail something on a very small percentage of requests. With this setup, we'll spend time and memory on every request to create the mailer object, even though we don't need it. That's bananas! Especially if you have a big system.

Second, the services need to be created in order: we have to create the $streamHandler first so that it's available when we create the logger. If we reorder things, it'll blow up. With a big system where services are created in many places, this will get tricky fast.

Here's the answer: don't create the services. Instead, let's teach the container how to create them. Then it will create the objects when and if we ask for them.

This means that instead of creating a Logger, create an $loggerDefinition variable and set it to a new Definition object. For the first argument, pass it the class name - Monolog\Logger.

30 lines dino_container/roar.php
... lines 1 - 16
$loggerDefinition = new Definition('Monolog\Logger');
... lines 18 - 30

This Definition object knows everything about how to instantiate an object. We'll use it to teach the container that when I ask for the logger service, this is how you should create it. So naturally, if the class has constructor arguments, we need to configure those. Do that with $loggerDefinition->setArguments(), and this takes an array of the arguments. The first is just a string: main:

30 lines dino_container/roar.php
... lines 1 - 16
$loggerDefinition = new Definition('Monolog\Logger');
$loggerDefinition->setArguments(array(
'main',
... line 20
));
... lines 22 - 30

The second argument is an array of handler objects. So you might expect me to just pass $container->get('logger.stream_handler'). But no! That would mean that I'd still have to worry about creating the stream handler first.

Instead, we can refer to the service by its id. Create a new Reference object and pass logger.stream_handler. This tells Symfony: "Hey, this argument isn't the string logger.stream_handler, it's a service with this id.":

30 lines dino_container/roar.php
... lines 1 - 16
$loggerDefinition = new Definition('Monolog\Logger');
$loggerDefinition->setArguments(array(
'main',
array(new Reference('logger.stream_handler'))
));
... lines 22 - 30

To put this into the container, instead of calling set, use setDefinition with the nickname logger and the $loggerDefinition. Now get rid of the old lines that set the logger service directly:

30 lines dino_container/roar.php
... lines 1 - 16
$loggerDefinition = new Definition('Monolog\Logger');
$loggerDefinition->setArguments(array(
'main',
array(new Reference('logger.stream_handler'))
));
$container->setDefinition('logger', $loggerDefinition);
... lines 23 - 30

The container now knows how to create the logger service. So later, when and if we ask for it, the container will create it in the background using these instructions.

And our app doesn't know or care this is happening - it just happily asks for that service. So let's try it out:

php dino_container/roar.php
tail dino_container/dino.log

Boom! 3 log entries now.

The disadvantage is that adding service is now more abstract: instead of creating them directly, you describe them. But the up-side is huge. Services aren't created unless you need them, the container will give you clear error messages if you mess something up, and the way this is all cached will blow your mind.

Oh, and now order doesn't matter. Down near the bottom, create a new Definition and pass it the StreamHandler class. We can remove the Logger and StreamHandler use statements too, because in a second, we won't be referencing these directly anymore. Just like before, call setArguments, pass it an array, and put the one constructor argument - the log file path - inside of it. Finish it off with $container->setDefinition(), passing the service nickname as the first argument, and the Definition next:

30 lines dino_container/roar.php
... lines 1 - 19
$handlerDefinition = new Definition('Monolog\Handler\StreamHandler');
$handlerDefinition->setArguments(array(__DIR__.'/dino.log'));
$container->setDefinition('logger.stream_handler', $handlerDefinition);
... lines 23 - 30

And now that the logger.stream_handler service is being set with a Definition, we can remove the original code that set it directly.

So even though the logger service needs the stream handler service, we can describe them in any order. When we eventually ask for the logger, Symfony will go make the logger.stream_handler service first, then pass it to logger. That's why it's called a dependency injector container: it helps you manage dependencies.

But like before, our "app code" has no idea any of this is happening. So when we hit the script again, there's another log message:

php dino_container/roar.php
tail dino_container/dino.log

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