[DI] Added _instanceof example

Author: HeahDude

service_container/3.3-di-changes.rst

@@ -22,6 +22,8 @@ which means it's always safe to upgrade across minor versions.
 All of the new features are **optional**: they are not enabled by default, so you
 need to actually change your configuration files to use them.
 
+.. _`service-33-default_definition`:
+
 The new Default services.yml File
 ---------------------------------
 
@@ -417,7 +419,7 @@ In general, the new best practice is to use normal constructor dependency inject
 4) Auto-tagging with autoconfigure
 ----------------------------------
 
-The last big change is the ``autoconfigure`` key, which is set to ``true`` under
+The fourth big change is the ``autoconfigure`` key, which is set to ``true`` under
 ``_defaults``. Thanks to this, the container will auto-tag services registered in
 this file. For example, suppose you want to create an event subscriber. First, you
 create the class::
@@ -474,6 +476,69 @@ Many autoconfigured tags have an optional priority. If you need to specify a pri
 (or any other optional tag attribute), no problem! Just :ref:`manually configure your service <services-manually-wire-args>`
 and add the tag. Your tag will take precedent over the one added by auto-configuration.
 
+5) Auto-configure with _instanceof
+----------------------------------
+
+And the final big change is ``_instanceof``, it acts as a default definition
+template (see `service-33-default_definition`_), but only for service whose
+class matches a defined one.
+This can be very useful when many services shares some tag that cannot be
+inherited from abstract definition:
+
+.. configuration-block::
+
+    .. code-block:: yaml
+
+        # app/config/services.yml
+        services:
+            # ...
+
+            _instanceof:
+                class: AppBundle\Domain\LoaderInterface
+                public: true
+                tags: ['app.domain_loader']
+
+    .. code-block:: xml
+
+        <!-- app/config/services.xml -->
+        <?xml version="1.0" encoding="UTF-8" ?>
+        <container xmlns="http://symfony.com/schema/dic/services"
+            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+            xsi:schemaLocation="http://symfony.com/schema/dic/services
+                http://symfony.com/schema/dic/services/services-1.0.xsd">
+
+            <services>
+                <!-- ... -->
+
+                <instanceof class="AppBundle\Domain\LoaderInterface" public="true">
+                    <tag name="app.domain_loader" />
+                </prototype>
+            </services>
+        </container>
+
+    .. code-block:: php
+
+        // app/config/services.php
+        use Symfony\Component\DependencyInjection\Definition;
+
+        $domainLoaderDefinition = new Definition();
+
+        $domainLoaderDefinition->addTag('app.domain_loader');
+
+        // To use as default template
+        $definition = new Definition();
+
+        $definition
+            ->setAutowired(true)
+            ->setAutoconfigured(true)
+            ->setPublic(false)
+            ->setInstanceofConditionals(
+                'AppBundle\Domain\LoaderInterface' => $domainLoaderDefinition,
+            )
+        ;
+
+        $this->registerClasses($definition, 'AppBundle\\', '../../src/AppBundle/{Entity,Repository}');
+
 What about Performance
 ----------------------