Why Lazy Services?

In some cases, you may want to inject a service that is a bit heavy to instantiate, but is not always used inside your object. For example, imagine you have a NewsletterManager and you inject a mailer service into it. Only a few methods on your NewsletterManager actually use the mailer, but even when you don't need it, a mailer service is always instantiated in order to construct your NewsletterManager.

Configuring lazy services is one answer to this. With a lazy service, a "proxy" of the mailer service is actually injected. It looks and acts like the mailer, except that the mailer isn't actually instantiated until you interact with the proxy in some way.

Installation

In order to use the lazy service instantiation, you will need to install the symfony/proxy-manager-bridge package:

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$ composer require symfony/proxy-manager-bridge

Configuration

You can mark the service as lazy by manipulating its definition:

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# config/services.yaml
services:
    App\Twig\AppExtension:
        lazy: true

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<!-- 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
        https://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>
        <service id="App\Twig\AppExtension" lazy="true"/>
    </services>
</container>

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// config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;

use App\Twig\AppExtension;

return function(ContainerConfigurator $configurator) {
    $services = $configurator->services();

    $services->set(AppExtension::class)->lazy();
};

Once you inject the service into another service, a virtual proxy with the same signature of the class representing the service should be injected. The same happens when calling Container::get() directly.

The actual class will be instantiated as soon as you try to interact with the service (e.g. call one of its methods).

To check if your proxy works you can check the interface of the received object:

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dump(class_implements($service));
// the output should include "ProxyManager\Proxy\LazyLoadingInterface"

Interface Proxifying

Under the hood, proxies generated to lazily load services inherit from the class used by the service. However, sometimes this is not possible at all (e.g. because the class is final and can not be extended) or not convenient.

To workaround this limitation, you can configure a proxy to only implement specific interfaces.

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# config/services.yaml
services:
    App\Twig\AppExtension:
        lazy: 'Twig\Extension\ExtensionInterface'
        # or a complete definition:
        lazy: true
        tags:
            - { name: 'proxy', interface: 'Twig\Extension\ExtensionInterface' }

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<!-- 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
        https://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>
        <service id="App\Twig\AppExtension" lazy="Twig\Extension\ExtensionInterface"/>
        <!-- or a complete definition: -->
        <service id="App\Twig\AppExtension" lazy="true">
            <tag name="proxy" interface="Twig\Extension\ExtensionInterface"/>
        </service>
    </services>
</container>

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// config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;

use App\Twig\AppExtension;
use Twig\Extension\ExtensionInterface;

return function(ContainerConfigurator $configurator) {
    $services = $configurator->services();

    $services->set(AppExtension::class)
        ->lazy()
        ->tag('proxy', ['interface' => ExtensionInterface::class])
    ;
};

The virtual proxy injected into other services will only implement the specified interfaces and will not extend the original service class, allowing to lazy load services using final classes. You can configure the proxy to implement multiple interfaces by adding new "proxy" tags.

Additional Resources

You can read more about how proxies are instantiated, generated and initialized in the documentation of ProxyManager.