How to Make Your Services Use Tags

Several of Symfony2's core services depend on tags to recognize which services

should be loaded, notified of events, etc. For example, Twig uses the tag

``twig.extension`` to load extra extensions.

But you can also use tags in your own bundles. For example in case your service

handles a collection of some kind, or implements a "chain", in which several alternative

strategies are tried until one of them is successful. In this article I will use the example

of a "transport chain", which is a collection of classes implementing ``\Swift_Transport``.

Using the chain, the Swift mailer may try several ways of transport, until one succeeds.

To begin with, define the ``TransportChain`` class.

Now we want several of the ``\Swift_Transport`` classes to be instantiated and added to the chain

automatically using the ``addTransport()`` method. As an example we add the following transports

as services:

Notice the tags named "acme_mailer.transport". We want the bundle to recognize these transports

and add them to the chain all by itself. In order to achieve this, we need to

add a ``build()`` method to the ``AcmeMailerBundle`` class:

class AcmeMailerBundleBundle extends Bundle

You will have spotted a reference to the not yet existing ``TransportCompilerPass`` class.

This class will make sure that all services with a tag "acme_mailer.transport" will be added to

the ``TransportChain`` class by calling the ``addTransport()`` method.

The ``TransportCompilerPass`` should look like this:

The ``process()`` method checks for the existence of the ``acme_mailer.transport_chain`` service,

then looks for all services tagged "acme_mailer.transport". It adds to the definition of the

``acme_mailer.transport_chain`` service a call to ``addTransport()`` for each "acme_mailer.transport" service

it has found. The first argument of each of these calls will be the mailer transport service itself.

By convention, tag names consist of the name of the bundle (lowercase, underscores as separators),

followed by a dot, and finally the "real" name, so the tag "transport" in the AcmeMailerBundle should be:

"acme_mailer.transport".

Adding the compiler pass will result in the automatic generation of the following lines of code

in the compiled service container. In case you are working in the "dev" environment, open the file

``/cache/dev/appDevDebugProjectContainer.php`` and look for the method ``getTransportChainService()``.

It should look like this: