New in Symfony 3.4: Local service binding

Contributed by Guilhem Niot in #22187

The configuration of services in Symfony applications has been greatly simplified in recent versions. Thanks to autowiring you can create and use services without having to actually configure most of them.

However, there's an exception to that: you can't autowire scalar arguments (e.g. strings and numbers). For example, if three of your services need the value of the kernel.project_dir parameter, you need to do this:

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services:
    _defaults:
        autowire: true
        autoconfigure: true
        public: false

    App\Some\Service1:
        $projectDir: '%kernel.project_dir%'

    App\Some\Service2:
        $projectDir: '%kernel.project_dir%'

    App\Some\Service3:
        $projectDir: '%kernel.project_dir%'

In Symfony 3.4, to avoid repetitions like this, we introduced local service binding. First, this new feature allows to define the scalar arguments once and apply them to any service defined/created in that file. That's why the previous example looks like this in Symfony 3.4:

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services:
    _defaults:
        autowire: true
        autoconfigure: true
        public: false
        bind:
            $projectDir: '%kernel.project_dir%'

That's all! The Service1, Service2 and Service3 don't have to be defined explicitly because autowiring is being used and you already defined the value of the $projectDir scalar argument for all the services created/defined in this file. If you prefer XML configuration, use this notation to bind parameters:

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<services>
    <defaults autowire="true" autoconfigure="true" public="false">
        <bind key="$projectDir">%kernel.project_dir%</bind>
    </defaults>
</services>

Local binding is also useful to explicitly define the services to inject for some service arguments. This is required for example when you have multiple services related to the same class:

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# when services created/defined in this file inject 'BarInterface',
# use the '@normal_bar_service' ...
services:
    _defaults:
        bind:
            BarInterface: '@normal_bar_service'

    # ... except for this particular service, which uses a different service
    Foo:
        bind:
            BarInterface: '@special_bar_service'