How to Decorate Services

Version: 2.8
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Read the updated version of this page for Symfony 5.3 (the current stable version).

How to Decorate Services

When overriding an existing definition, the original service is lost:

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services:
    app.mailer:
        class: AppBundle\Mailer

    # this replaces the old app.mailer definition with the new one, the
    # old definition is lost
    app.mailer:
        class: AppBundle\NewMailer

Most of the time, that's exactly what you want to do. But sometimes, you might want to decorate the old one instead (i.e. apply the Decorator pattern). In this case, the old service should be kept around to be able to reference it in the new one. This configuration replaces app.mailer with a new one, but keeps a reference of the old one as app.decorating_mailer.inner:

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services:
    # ...

    app.decorating_mailer:
        class:     AppBundle\DecoratingMailer
        decorates: app.mailer
        arguments: ['@app.decorating_mailer.inner']
        public:    false

Here is what's going on here: the decorates option tells the container that the app.decorating_mailer service replaces the app.mailer service. By convention, the old app.mailer service is renamed to app.decorating_mailer.inner, so you can inject it into your new service.

Tip

Most of the time, the decorator should be declared private, as you will not need to retrieve it as app.decorating_mailer from the container.

The visibility of the decorated app.mailer service (which is an alias for the new service) will still be the same as the original app.mailer visibility.

Note

The generated inner id is based on the id of the decorator service (app.decorating_mailer here), not of the decorated service (app.mailer here). This is mandatory to allow several decorators on the same service (they need to have different generated inner ids).

You can change the inner service name if you want to using the decoration_inner_name option:

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services:
    app.decorating_mailer:
        # ...
        decoration_inner_name: app.decorating_mailer.wooz
        arguments: ['@app.decorating_mailer.wooz']

Decoration Priority

New in version 2.8

The ability to define the decoration priority was introduced in Symfony 2.8. Prior to Symfony 2.8, the priority depends on the order in which definitions are found.

When applying multiple decorators to a service, you can control their order with the decoration_priority option. Its value is an integer that defaults to 0 and higher priorities mean that decorators will be applied earlier.

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foo:
    class: Foo

bar:
    class: Bar
    public: false
    decorates: foo
    decoration_priority: 5
    arguments: ['@bar.inner']

baz:
    class: Baz
    public: false
    decorates: foo
    decoration_priority: 1
    arguments: ['@baz.inner']

The generated code will be the following:

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$this->services['foo'] = new Baz(new Bar(new Foo())));
This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.